Period 6

Period 6 of the periodic table is a remarkable and extended row containing 32 elements – the longest period of the table, tied only with the even heavier period 7. It begins at cesium (atomic number 55) and runs all the way to radon (atomic number 86). These elements fill the 6th principal energy level of electrons, and for the first time in the table, an f-block of elements (the lanthanide series) appears, making period 6 substantially longer than the previous periods. In terms of electron configuration, period 6 elements typically fill their 6s shell first, then proceed to fill the 4f, 5d, and 6p orbitals in that order. This added f–orbital layer gives rise to the lanthanides, also known as the rare earth elements, which are all part of period 6.

From a chemical perspective, period 6 is extraordinarily diverse. It includes highly reactive metals at the start, a block of 15 lanthanide metals with very similar chemistry, followed by heavy transition metals known for their strength and high value, and ends with several heavy “post-transition” metals, a metalloid, a halogen, and a noble gas. Period 6 contains both some of the most precious elements (like gold and platinum) and some of the most toxic (like thallium and mercury). Notably, lead (element 82 in period 6) is the last element in the periodic table that has at least one stable isotope; all elements after lead are radioactive. (Bismuth, element 83, is technically radioactive but has such an immensely long half-life – over a billion times longer than the age of the universe – that it can be considered effectively stable in practice.) The very end of period 6 (polonium, astatine, radon) consists of short-lived, rare radioactive elements – for example, astatine is so rare that at any given time less than a gram of it is estimated to exist on Earth.

In summary, period 6 occupies a unique place in the periodic table. It is the first period to incorporate the f-block (lanthanide) elements, contributing to its length. This period showcases a wide range of chemical behaviors: from the extremely reactive alkali metal cesium, through the technologically critical lanthanides known for their magnetic and phosphorescent properties, to the high-density transition metals and heavy p-block elements with their uses and hazards. Many of the materials and technologies we encounter daily – from electronics and medical devices to jewelry and vehicles – rely on elements from period 6.

Summary of Period 6 Elements

To help compare the period 6 elements at a glance, the table below lists each element’s name, symbol, atomic number, and one key use or notable property:

Element	Symbol	Atomic Number	Key Use / Notable Property
Cesium	Cs	55	Defines the second in atomic clocks (timekeeping)
Barium	Ba	56	BaSO₄ “barium meal” used as X-ray contrast agent in medical imaging
Lanthanum	La	57	Lanthanum oxide in high-quality camera and telescope lenses
Cerium	Ce	58	Cerium oxide in catalytic converters (cleaning car exhaust); used in lighter “flint” alloys
Praseodymium	Pr	59	Component of didymium glass in welder’s goggles (filters bright light); used with neodymium in magnets
Neodymium	Nd	60	Key ingredient in neodymium magnets, the strong permanent magnets in electronics[1]
Promethium	Pm	61	Radioactive; used in some atomic batteries and luminous paint for specialized applications
Samarium	Sm	62	Used in Samarium–Cobalt magnets (high-temperature magnets)[2]; samarium-153 is used in cancer therapy for pain relief in bone cancer
Europium	Eu	63	Europium phosphors produce the red color in TV and LED screens and are used as anti-counterfeiting markers in Euro banknotes
Gadolinium	Gd	64	Used in MRI contrast agents to enhance imaging (due to its magnetic properties); also used in neutron absorbing nuclear reactor rods
Terbium	Tb	65	Provides green phosphor in fluorescent lamps and display screens; added to magnets (with Nd/Dy) to improve high-temperature performance
Dysprosium	Dy	66	Added to neodymium-iron-boron magnets to allow them to retain strength at high temperatures; name means “hard to get” (reflecting difficulty in isolation)
Holmium	Ho	67	Has the highest magnetic moment of any element, used in specialty magnets and as a nuclear reactor “burnable poison” to absorb neutrons[3]
Erbium	Er	68	Erbium-doped fibers are used as fiber-optic signal amplifiers for long-distance internet cables; also used in some laser medical and skin treatments
Thulium	Tm	69	Rare and costly; one isotope (Tm-170) is used as a portable X-ray source in small X-ray devices
Ytterbium	Yb	70	Used in certain atomic clocks and infrared lasers; helps improve stainless steel grain structure and is used as a dopant in fiber-optic amplifiers (similar to erbium)
Lutetium	Lu	71	Used in PET scan detectors (lutetium oxyorthosilicate scintillator crystals) and as a catalyst in petroleum refining
Hafnium	Hf	72	An excellent neutron absorber, used in nuclear reactor control rods (especially in submarine reactors); also used in microprocessor chips (HfO₂ as an insulator)
Tantalum	Ta	73	Used in small tantalum capacitors found in electronics like smartphones; biocompatible metal for surgical implants (e.g. bone pins)
Tungsten	W	74	Highest melting point of any metal, used for light bulb filaments and in extremely hard tungsten carbide cutting tools
Rhenium	Re	75	Added to nickel superalloys for jet engine turbine blades to withstand high temperatures; also used in platinum–rhenium catalysts for making lead-free gasoline
Osmium	Os	76	Densest element (twice as dense as lead); used in very hard alloys (e.g. osmium–iridium tips for fountain pens) and as a staining agent in microscopy (osmium tetroxide)
Iridium	Ir	77	Extremely corrosion-resistant metal used in spark plug electrodes and crucibles. An iridium-rich layer in Earth’s geologic record provided evidence for the asteroid impact that ended the dinosaurs.
Platinum	Pt	78	A precious metal used in catalytic converters (to clean car exhaust gases) and jewelry; also used in chemical catalysts and electrodes due to its inertness
Gold	Au	79	Highly valued precious metal used in jewelry and coinage; an excellent electrical conductor that resists corrosion, used for electronics connectors and thin film coatings (for example, on astronaut visors)
Mercury	Hg	80	The only metal liquid at room temperature; once common in thermometers and barometers (now largely phased out due to toxicity). Still used in fluorescent light bulbs (vapor inside emits UV light) and some electrical switches.
Thallium	Tl	81	A highly toxic metal historically used as a rat poison (“poisoner’s poison”). Today it finds limited use in specialized electronics, infrared optical glass, and in medical imaging (a radioactive thallium isotope in heart stress tests).
Lead	Pb	82	A heavy, soft metal used in lead-acid car batteries (the primary use of lead today) and for radiation shielding (e.g. X-ray aprons). Once used in paints and gasoline, but phased out due to its toxicity (lead poisoning).
Bismuth	Bi	83	Heaviest element that is effectively stable. Non-toxic compared to lead – used in medicines (bismuth subsalicylate is the active ingredient in Pepto-Bismol for upset stomach) and in low-melting alloys for fire sprinklers and fuses. Bismuth’s colorful hopper crystals are popular with element collectors.
Polonium	Po	84	A rare, highly radioactive metalloid discovered by Marie Curie. It emits intense alpha radiation and has been used in devices to eliminate static charge in industrial processes. Polonium-210 also served as a heat source in early space equipment, but its short half-life limits uses. (Its intense radioactivity infamously made it a poison in the 2006 assassination of ex-spy Alexander Litvinenko.)
Astatine	At	85	The rarest naturally occurring element on Earth (all isotopes are radioactive and short-lived). No significant commercial uses due to its scarcity and radioactivity, but researchers are investigating astatine-211 in targeted cancer therapy (it emits alpha particles that can destroy tumor cells).
Radon	Rn	86	A radioactive noble gas that seeps from the ground (from natural uranium decay) and can accumulate in buildings. Radon is colorless and odorless, and it poses a health hazard as a leading cause of lung cancer when inhaled over time. Historically it was used in some cancer treatments and even “health spa” therapies, but today radon is chiefly a concern for indoor air safety.

A large sealed ampoule containing about 1.3 kg of cesium metal. Cesium is an extremely reactive alkali metal; it must be kept in an inert atmosphere or sealed container because it reacts violently with water and air. Notably, cesium has a low melting point (28 °C), so this solid metal ampoule will liquefy to a golden-yellow liquid if gently warmed in one’s hand.

The Elements of Period 6, In Sequence

Let us now explore each of the period 6 elements in order, noting their key characteristics, interesting facts, and roles in science, technology, or everyday life. From the explosively reactive cesium to the inert radioactive gas radon, period 6 truly runs the gamut of chemical variety.

Alkali and Alkaline Earth Metals (Group 1–2)

Cesium (Cs, Z=55) – An Atomic Clock Standard

Main article: Periodic table/Alkali metals/Caesium

Cesium is a silvery-golden alkali metal so soft it can be cut with a knife. Uniquely, it melts at only about 28 °C (83 °F), meaning it can be liquid on a warm day. Cesium is incredibly reactive – it ignites in air and explodes in water (even ice!) due to violent reaction with water to produce hydrogen gas and heat. Because of this reactivity, cesium is stored in sealed glass ampoules under an inert gas or vacuum (as shown in the image above).

One of cesium’s most famous applications is in atomic clocks. In 1967, the international definition of the second was based on the precise frequency of microwave radiation emitted by electrons transitioning in the cesium-133 atom. Cesium atomic clocks are extraordinarily accurate and form the timekeeping heart of GPS satellites and global time standards. In industry, cesium compounds (like cesium formate) are used in oil drilling fluids; a dense cesium formate brine helps lubricate drill bits and carry rock cuttings to the surface. The metal has specialized uses in photoelectric cells and vacuum tubes as a “getter” to scavenge oxygen. Cesium’s radioactive isotope Cs-137 (produced from nuclear fission) is used in medical radiotherapy and industrial gauges, though it is also a major contaminant in nuclear fallout. Despite its high reactivity, pure cesium is only mildly toxic chemically – but its intense radioisotopes and violent reactions make it a hazardous substance to handle.

Barium (Ba, Z=56) – From Fireworks to Medicine

Main article: Periodic table/Alkaline earth metals/Barium

Barium is a soft, silvery alkaline earth metal. Like other group 2 elements, it oxidizes quickly and is never found in nature as a free metal. In fact, the name “barium” comes from the Greek barys, meaning “heavy,” inspired by the high density of barium ores. Barium’s most familiar compounds are barium sulfate and barium carbonate, which are very insoluble – this is important because soluble barium salts are quite poisonous.

Barium has some striking uses. If you’ve ever seen a bright green color in fireworks, that’s due to barium compounds imparting a green flame. Barium metal itself was once used as a “getter” in vacuum tubes to remove the last traces of air. In medicine, a suspension of barium sulfate (the “barium meal”) is given to patients undergoing X-ray imaging of the stomach and intestines. The heavy barium sulfate is opaque to X-rays, coating the digestive tract so that it shows up clearly on X-ray scans. Because BaSO₄ is so insoluble, it passes through the body without being absorbed – a safe use, despite barium’s toxicity in other forms. Another everyday use of barium is in drill rig muds (similar to cesium formate brines): barite (barium sulfate) is added to drilling fluids to increase their density. Barium is also a component in some high-temperature superconductors and electronic ceramics. Given its toxicity, new applications are explored cautiously, but its ability to add density and distinctive color makes barium useful in industry and art.

The Lanthanides: Rare Earth Metals (La to Lu, Z=57–71)

Main article: Periodic table/Lanthanides

Lanthanum and the 14 elements after it up to lutetium are collectively known as the lanthanides, or rare earth elements. All are metallic, typically silvery-white, and are chemically similar – they usually occur together in the same mineral ores. The lanthanides are often shown as a separate row below the main periodic table. In period 6, lanthanum (La) is actually a d-block element in terms of electron configuration, but due to its chemistry it is included with the lanthanides. The filling of 4f orbitals across this series leads to the famous “lanthanide contraction,” a gradual decrease in atomic size with increasing atomic number.

Though called “rare,” many lanthanides are relatively abundant in Earth’s crust (cerium, for instance, is more common than copper). However, they are hard to separate from one another, which historically made them seem rare. In modern technology, lanthanides are invaluable: they enable high-strength magnets, efficient lighting, lasers, and much more. A notable characteristic is that most lanthanides are paramagnetic (attracted to magnets) and some have exceptionally high magnetic moments. Many can exhibit vibrant colors in compounds and produce sharp spectral lines – useful in lighting phosphors.

Transition Metals of Period 6 (Hf to Au)

Following lutetium, period 6 enters the realm of heavy transition metals – many of which are familiar due to their uses as structural materials, electrical contacts, or precious metals. These elements fill the 5d orbital as we move from hafnium to gold.

Hafnium (Hf, Z=72)

Main article: Periodic table/Transition metals/Hafnium

Hafnium is a lustrous, silver-gray transition metal that is often found intertwined with zirconium in minerals (due to their chemical similarity). A remarkable property of hafnium is its ability to absorb neutrons. Hafnium has a very high thermal neutron capture cross-section, which makes it ideal for use in nuclear reactor control rods. In fact, hafnium is commonly used in the control rods of nuclear submarines and other reactors to help regulate the fission process by soaking up excess neutrons. Another critical use of hafnium is in the semiconductor industry: hafnium(IV) oxide (HfO₂) is employed as a high-k dielectric material in modern computer chips. Starting around the 45-nanometer technology node (late 2000s), Intel and other companies introduced HfO₂ to replace silicon dioxide as the gate insulator in transistors, because HfO₂ can reduce leakage current while allowing further miniaturization – effectively “making microchips smaller and faster”. Hafnium’s high melting point and compatibility with uranium dioxide also make it useful in aerospace superalloys and in plasma cutting tips. Fun fact: hafnium carbide is one of the most refractory compounds known, with a melting point around 3900 °C, investigated for use in rocket nozzle linings. Though not common in consumer items by name, hafnium’s contributions are embedded in nuclear safety and the brains of our electronics.

Tantalum (Ta, Z=73)

Main article: Periodic table/Transition metals/Tantalum

Tantalum is a hard, bluish-gray metal named after Tantalus (of Greek myth) – appropriate since it sits below niobium (named for Tantalus’s daughter Niobe). Tantalum is highly resistant to corrosion and chemically inert to bodily fluids, which is why it’s used in surgical implants and bone repair. But its most widespread use is in electronics: tantalum powder is used to make tantalum capacitors, which are small components that store charge in virtually all types of electronic devices. If you have a smartphone, laptop, or camera, chances are it contains multiple tantalum capacitors – prized for their high capacitance in a small size. The electronics industry consumes a large portion of the world’s tantalum for this purpose. Tantalum’s ability to form a stable oxide layer also makes it useful in high-power resistors and electron tube parts. Historically, tantalum was used for very fine wire in light bulbs (early filaments) before tungsten took over. In metallurgy, small additions of tantalum give alloys high-temperature strength (jet engine turbine blades may contain tantalum as a minor ingredient). Tantalum oxide is used in specialized high-refractive index glass for camera lenses too. Because it doesn’t react with the human body, tantalum can be used for skull plate implants or surgical clips that remain in patients. In short, tantalum’s presence is felt every time we turn on a digital device – it’s a hidden enabler of our miniaturized electronics age.

Tungsten (W, Z=74)

Main article: Periodic table/Transition metals/Tungsten

Tungsten (from the Swedish tung sten, “heavy stone”), also known as wolfram, is famous for having the highest melting point of any metal at 3422 °C. It’s incredibly dense (similar to gold) and hard. For much of the 20th century, tungsten’s most visible role was in the filament of incandescent light bulbs – a coiled tungsten wire that glows white-hot when electricity passes through it. Tungsten’s high melting point and strength at red heat made it ideal for lightbulbs (and indeed “hot” things like old vacuum tube heaters and X-ray tube targets). While incandescent bulbs are being phased out, tungsten still shines in specialty lamps and household oven lightbulbs. Another huge use of tungsten is in tungsten carbide – a super-hard material made by combining tungsten with carbon and typically cemented with cobalt. Tungsten carbide is used to make cutting tools, drill bits, mining machinery, and armor-piercing ammunition. Those tough gray tips on saw blades or the burrs on drill bits are often tungsten carbide. Tungsten’s density also finds use in applications where weight and compactness are needed: balance weights in race cars, counterweights in aircraft control surfaces, as ballast in yachts, and in the military (for kinetic energy penetrators as a substitute for depleted uranium). Additionally, tungsten’s high temperature stability makes it useful for rocket nozzle throats, heating elements in high-temp furnaces, and electrodes in TIG welding torches. On a more exotic front, tungsten has been used in jewelry (tungsten carbide rings are very scratch-resistant) and even for making artificial gemstones (calcium and magnesium tungstate can mimic diamonds). Whether lighting the world or cutting through rock, tungsten is the definition of toughness and heat-resistance.

Rhenium (Re, Z=75)

Main article: Periodic table/Transition metals/Rhenium

Rhenium is a rare, silvery metal that is one of the last stable elements discovered (not found until 1925). It has the third-highest melting point of any element (behind tungsten and carbon) and retains strength at high temperatures, which makes it extremely valuable in superalloys for jet engines. In fact, rhenium is alloyed in the nickel-based turbine blades of jet engines (and industrial gas turbines) to allow them to operate at hotter temperatures for better efficiency. A modern single-crystal turbine blade may contain 3–6% rhenium; this use accounts for the majority of rhenium demand. Rhenium is also used as a catalyst in the petroleum industry: rhenium with platinum on alumina (a Pt-Re catalyst) is used for catalytic reforming in refineries to produce high-octane gasoline and lead-free fuels. Before leaded gasoline was phased out, these catalysts were crucial to maintain gasoline quality without tetraethyl lead. Rhenium catalysts are also involved in making high-purity olefins for plastics and chemicals. Another niche use: rhenium filaments in mass spectrometers and ion gauges (taking advantage of rhenium’s ability to stay stable when hot). Rhenium wire is used in flash lamps for photography and in some medical probes. Because rhenium is so scarce (annual production is only tens of tons worldwide) and expensive, its applications are all high-value. It truly is a “stealth” metal – you likely won’t see it, but if you’ve flown on a jet aircraft, rhenium was in the engines that carried you.

Osmium (Os, Z=76)

Main article: Periodic table/Transition metals/Osmium

Osmium is a bluish-gray metal that is exceedingly dense – it vies with iridium for the title of densest element, at about 22.59 g/cc. Osmium is hard but also very brittle in pure form. One of its early uses (and origin of its name, from Greek osme meaning smell) is osmium tetroxide, a compound that has a pungent odor and is used as a biological staining agent (e.g. staining fats in microscope slides) and as an oxidizing reagent in chemistry. Metallic osmium’s most important applications come from its hardness and density when alloyed. Osmium alloyed with iridium (forming “osmiridium” or osmium-iridium alloy) is used for extremely durable tips and bearings. For example, the tips of old fountain pen nibs and phonograph needles (record player stylus) were often made of osmium alloy, because it can write on paper for years without wearing down. Similarly, electrical contacts that endure wear, and pivot bearings of scientific instruments (like the Beaumont balance pivot) have used osmium alloys. Osmium’s density also made it a candidate for penetrating projectiles, though tungsten heavy alloys are more common. In chemical manufacturing, osmium tetroxide is used in tiny quantities to catalyze dihydroxylation of alkenes (the Milas hydroxylation or Sharpless dihydroxylation in synthesis). A word of caution: osmium tetroxide is highly toxic, so osmium must be handled carefully to avoid that oxide forming. Because osmium is rare and pricey, its applications tend to be specialized – but its extreme properties (hard, dense, and a powerful oxidizer in tetroxide form) make it invaluable in those niches.

Iridium (Ir, Z=77)

Main article: Periodic table/Transition metals/Iridium

Iridium is a lustrous, very hard, and corrosion-proof metal – so much so that pure iridium metal will resist attack from acids, aqua regia, or even direct chlorine gas. It’s one of the noblest metals and also extremely dense (just slightly less dense than osmium). One of iridium’s historical claims to fame is the international standard meter and kilogram prototypes: the standard meter bar (in the late 19th to mid-20th century) and the kilogram mass (until 2019) were made of a platinum-10% iridium alloy for stability and corrosion resistance. Iridium’s corrosion resistance and high melting point make it ideal for devices that face harsh conditions. For example, many spark plugs in modern cars have iridium-tipped electrodes – these fine tips last much longer than conventional ones due to iridium’s durability (it can handle the high-temperature sparks without eroding quickly). Iridium crucibles are used to grow high-purity single crystals (like synthetic garnets or semiconductor crystals) because iridium can withstand hot oxidizing conditions that would destroy other metals. Iridium coatings on engine spark emitters and deep oil well pipes protect against heat and corrosion. In the chemical industry, iridium complexes are catalysts (e.g. the Cativa process for making acetic acid uses an iridium catalyst). For jewelry, solid iridium is too hard to work, but as an alloy with platinum it adds hardness (this is why most platinum jewelry is 90–95% Pt with a bit of Ir).

Iridium also has a cosmic distinction: the thin clay layer marking the Cretaceous-Paleogene (K-Pg) boundary (the time of the dinosaurs’ extinction ~66 million years ago) is enriched in iridium worldwide. This iridium anomaly was key evidence leading scientists (Luis and Walter Alvarez and team) to propose that a massive asteroid impact caused the mass extinction. Asteroids are relatively richer in iridium than the Earth’s crust, so the discovery of excess iridium in that geologic layer strongly pointed to an extraterrestrial impact event. The ensuing theory – now widely accepted – was confirmed by finding the Chicxulub impact crater in Mexico. Thus, iridium literally marks a pivotal moment in Earth’s history.

Platinum (Pt, Z=78)

Main article: Periodic table/Transition metals/Platinum

Platinum is a precious metal known for its beautiful silver-white luster and remarkable resistance to tarnish. It’s dense, ductile, and conducts electricity well. Platinum’s name comes from “platina,” meaning “little silver,” but its value is often greater than gold. Jewelry is a well-known use – platinum rings and watches signify luxury and durability. However, platinum’s biggest industrial use is in catalysis. If you own a gasoline-powered car, the catalytic converter in its exhaust likely contains platinum (along with palladium and rhodium). Platinum catalysts facilitate the conversion of harmful exhaust gases (carbon monoxide, hydrocarbons, NOx) into less harmful carbon dioxide, water, and nitrogen. Platinum is also used as a catalyst in chemical production, such as in making silicones and in refining processes like naphtha reforming (though platinum-rhenium combos are common there). In the lab, platinum dishes and electrodes are prized because they don’t react or corrode easily – e.g., a platinum electrode is used in the standard hydrogen electrode reference. Historically, platinum has been used for important scientific instruments: the early 19th century standard meter bars were platinum, and the standard kilogram was a Pt-Ir cylinder. In medicine, certain platinum compounds have been revolutionary: cisplatin, carboplatin, and oxaliplatin are platinum-based chemotherapy drugs that treat various cancers by binding to DNA in cancer cells. So in a way, platinum helps save lives. Another everyday use: hard disks in some computers have a platinum coating in the magnetic layer to improve performance. And if you look at a high-end spark plug or O₂ sensor, you might find platinum there as well (for stable electrical behavior at high temps). With its blend of beauty and chemical utility, platinum straddles the world of luxury and the world of industry.

Gold (Au, Z=79)

Main article: Periodic table/Transition metals/Gold

Gold hardly needs introduction – it has captivated humans for millennia. It is a soft, bright yellow metal that is famously inert (it doesn’t tarnish or corrode in air or water). These properties made gold a natural choice for money and jewelry throughout history. Beyond adornment and coinage, gold’s excellent electrical conductivity and resistance to oxidation make it crucial in electronics. Tiny quantities of gold are used for electrical contacts, bond wires, and plating in connectors, from the SIM card and memory card contacts in your phone to high-reliability connectors in aerospace hardware. Gold plating ensures that contacts remain conductive over time without rusting. Gold is also used in certain high-end audio and video cables (though the benefit there is sometimes more marketing than necessity). In dentistry, gold alloys have been used for crowns and fillings due to gold’s biocompatibility and workability (though porcelain and other metals are common now). Gold’s reflectivity of infrared radiation led to its use as a thin film on astronaut visors and satellites (a gold-coated visor protects astronauts’ eyes by reflecting harmful solar rays, and gold foils on satellites help with thermal control). Chemically, gold is interesting in nanotechnology: tiny gold nanoparticles are used in biomedical assays (like some pregnancy tests where colloidal gold produces a red line) and being researched for targeted drug delivery and photothermal therapy in cancer. Although gold is much too costly to use in bulk applications, its unique blend of nobility and conductivity ensures it remains indispensable in small, critical roles. And of course, the cultural and financial significance of gold endures – from Olympic gold medals to central bank reserves. Few elements have a hold on the human imagination like gold does.

Mercury (Hg, Z=80)

Main article: Periodic table/Transition metals/Mercury

Mercury, often called quicksilver, is notable as the only metallic element that is liquid at room temperature. Its shiny, fluid appearance has fascinated people for ages. Mercury easily forms alloys (amalgams) with many metals, which historically made it useful in gold and silver mining (to extract those metals), as well as in processes like gilding. Perhaps the most familiar use of mercury was in thermometers and blood-pressure monitors (sphygmomanometers) – the expansion of mercury with temperature provided a direct measure on a calibrated glass tube. Mercury was also commonly found in barometers, vacuum pumps, and float valves. However, due to mercury’s toxicity, these uses are being phased out in favor of safer alternatives (digital thermometers, aneroid gauges, etc.). Mercury vapor, on the other hand, is used in fluorescent lamps and neon signs: an electric discharge in mercury vapor emits UV light, which strikes a phosphor coating to produce visible light in fluorescent bulbs. Compact fluorescent lamps (CFLs) also rely on a tiny dose of mercury – that’s why they should be disposed of properly. Beyond lighting, mercury’s electrical conductivity and liquid nature had niche uses: old thermostats and tilt switches often used little mercury pools as switches (the mercury would flow and complete a circuit). In dentistry, “silver” amalgam fillings contained mercury mixed with silver and other metals (these are durable, but their use is decreasing over health and environmental concerns). Industry uses mercury in the production of chlorine and caustic soda (the mercury cell process, also being phased out) and historically in felt-making (hence the term “mad as a hatter,” from mercury poisoning in hat makers). Organomercury compounds were used as preservatives (like thimerosal in some vaccines) and agricultural fungicides, though most such uses have been curtailed. Mercury is a potent neurotoxin – exposure can lead to tremors, cognitive dysfunction, and other symptoms (as tragically depicted in Minamata disease from industrial mercury dumping). As such, the world is moving to restrict mercury use under the Minamata Convention. Nevertheless, mercury’s unique liquid behavior keeps it in some specialized scientific instruments and applications even today.

Post-Transition Metals and Others (Tl to Rn)

The final section of period 6 includes a mix of metals that are softer or more volatile, a famously toxic metalloid, a rare halogen, and a radioactive noble gas. These elements often have reputations for toxicity or rarity, but each has its interest.

Thallium (Tl, Z=81)

Main article: Periodic table/Boron group/Thallium

Thallium is a soft, lead-gray metal that tarnishes quickly to a bluish hue. It was discovered via spectroscopy (its name comes from thallos, Greek for “green shoot,” referring to a green spectral line). Thallium gained notoriety as a poison – odourless and tasteless, it was historically used in rat poisons and insecticides and unfortunately in a number of murders (earning nicknames like “the poisoner’s poison”). Due to safety, such uses are banned in many countries now. In technology, thallium has a few niche but important uses. Thallium sulfate was used in some electro-optical glass because it increases the refractive index and density of glass – thallium-containing glasses can have very low melting points and transmit infrared light, making them useful in special IR lenses and optics. Thallium bromide-iodide crystals (KRS-5) are used in infrared detectors and optics; they have the unusual property of being transparent to infrared but not water-soluble. Thallium’s semiconductor properties have been used in photoresistors: thallium sulfide and thallium selenide have been used in IR sensitive photocells and bolometers (heat detectors). In medicine, a radioactive isotope, thallium-201, is used in nuclear medicine scans (myocardial perfusion imaging) to visualize heart tissue blood flow during stress tests – essentially acting as a radiotracer that behaves like potassium in the body, highlighting areas of poor blood supply. This thallium stress test has been largely supplanted by technetium-99m tracers now, but was common in past decades. Thallium is also found in high-temperature superconducting compounds (like the TBCCO family of cuprate superconductors) – though these are mostly of research interest. Because thallium is highly toxic, handling it requires great care (even skin contact with soluble thallium compounds can be dangerous, as thallium can be absorbed). It famously causes hair loss and nerve damage in poisoning. With increased awareness, thallium’s use is restricted to those specialized areas where alternatives are not available, and even there, recycling and containment are essential.

Lead (Pb, Z=82)

Main article: Periodic table/Carbon group/Lead

Lead is a heavy, soft, blue-gray metal known since antiquity. Its Latin name plumbum gives us the plumbing term (Romans used lead pipes) and the abbreviation Pb. Lead’s hallmark properties are high density, softness, low melting point, and corrosion resistance (it forms protective oxide/carbonate layers). Historically, lead was used ubiquitously – in water pipes, paints, gasoline additives, solders, and more – which unfortunately led to widespread lead exposure and poisoning. Today we have drastically reduced these uses for health reasons (lead is a neurotoxin, especially harmful to children’s development). Still, lead remains important in certain applications. The single biggest use is in lead-acid batteries, such as car batteries. Each car battery cell has lead dioxide and spongy lead plates with sulfuric acid; despite newer battery types, lead-acid batteries remain common for vehicles due to their reliability and low cost (and they are recycled at high rates). Lead’s density and ability to absorb radiation make it the go-to material for radiation shielding. The aprons worn during X-rays or the walls of radiology rooms often contain lead sheets to block X-ray and gamma radiation. Lead blocks or containers are used to transport radioactive materials safely. In the shooting sports, lead bullets and shotgun pellets have been traditional (though environmental concerns are leading to alternatives in some cases). In electronics, the classic tin-lead solder (usually 60/40 Sn/Pb) was once everywhere connecting components – it’s easy to melt and forms reliable joints. However, recent regulations (RoHS) have pushed “lead-free” solders to reduce environmental lead. Still, some high-reliability electronics (like aerospace or military) may use leaded solder for its superior properties. Lead has also been used in alloys like pewter (historically) and in organ pipes (a lead-tin alloy). In chemical industries, lead dioxide is used in some oxidation processes and historically in matches and explosives manufacturing. One positive aspect: metallic lead, when not in an acidic environment, is not very soluble, which is why lead pipes can last long (with patina) and why lead artifacts from ancient Rome still exist. But in acidic water or as dust, lead is highly toxic. The story of lead in the modern era is one of finding balance – leveraging its useful properties in closed, recycled systems (like batteries) while eliminating it from products that could contaminate our environment or bodies (like paint, fuel, and food containers).

Bismuth (Bi, Z=83)

Main article: Periodic table/Pnictogens/Bismuth

Bismuth is a brittle, silver-white metal with a faint pink tinge. Interestingly, bismuth sits next to lead in the table and was long confused for it, yet bismuth is much less toxic – in fact, bismuth is considered the least toxic heavy metal and some of its compounds are safe enough to ingest. A famous example is bismuth subsalicylate, the active ingredient in Pepto-Bismol and similar stomach remedies, used to treat indigestion, nausea, and diarrhea. So unlike its neighbors thallium or lead, bismuth is actually medicinal! Another notable aspect: bismuth-209 was for a long time considered stable, but it was discovered to be very mildly radioactive with an astronomically long half-life (over $10^{19}$ years), meaning effectively stable for any practical purpose. Bismuth is widely used in low-melting alloys. For instance, Wood’s metal and Rose’s metal, which melt in boiling water or even hand-hot water, contain bismuth along with lead, tin, and cadmium. These alloys are used in fire sprinkler links (melting to release a plug when a fire heats them), in fuse plugs for boilers, and for field soldering or bending of pipes. Bismuth’s expansion upon solidifying (it’s one of few metals that expand when they freeze, like water does) makes it useful in alloys for casting sharp details (it compensates shrinkage of other components). Environmentally, bismuth is emerging as a safer replacement for lead in many applications: e.g., lead-free solders often contain a few percent bismuth; some “green” bullets use bismuth-tin or bismuth alloys instead of lead; fishing sinkers and shotgun pellets can be made of bismuth alloys to avoid lead pollution in wetlands. Bismuth oxychloride is used as a shiny pearlescent pigment in cosmetics like eyeshadows and nail polish. When you see a certain silvery shimmer in makeup, that could be bismuth. Moreover, bismuth’s colorful chemistry produces the stunning bismuth crystal hopper form – lab-grown bismuth crystals with rainbow oxide layers are popular among mineral collectors (you may have seen these iridescent, geometric stepped crystals for sale as curios). In nuclear medicine, a radioactive bismuth isotope (Bi-213) is used in targeted cancer therapy research (as part of radio-immunotherapy for leukemia). Bismuth truly stands out as a heavy metal that’s not so heavy on the toxicity, and its versatility ranges from the medicine cabinet to the craft bench and beyond.

Polonium (Po, Z=84)

Main article: Periodic table/Chalcogens/Polonium

Polonium is a very rare, highly radioactive metal. It was discovered by Marie Curie in 1898 (and named after her homeland, Poland). In the periodic table, polonium sits in the same group as oxygen, sulfur, selenium, and tellurium, but as a metal it shows metallic behavior. Polonium has several isotopes, all radioactive; the most accessible one is polonium-210, which emits alpha particles. Polonium-210 is extremely radioactive: just a microgram emits a fatal dose of radiation if ingested. This isotope famously gained notoriety when used to poison ex-Russian agent Alexander Litvinenko in 2006, illustrating its lethality. Despite the danger, polonium has had a couple of specialized uses. Static eliminators in industries (for removing dust from photographic film, for example) used polonium sources – the alpha radiation from polonium ionizes air, allowing static charges to dissipate. Polonium-based devices were once strapped onto record player arms to neutralize static in vinyl records! Polonium-210 has also been used as a lightweight heat source in space probes. The Soviet Luna 9 and 13 missions and Lunokhod rovers in the 1960s used polonium-210 in isotopic heat generators to keep their electronics warm during the lunar night. The short half-life of Po-210 (138 days) meant it delivered a lot of heat over a short mission duration. Before modern neutron sources, polonium-210 mixed with beryllium was used to make compact neutron sources (the combination emits neutrons). In fact, early nuclear bomb initiators (in the Manhattan Project’s implosion bombs) used a “urchin” of polonium-beryllium to provide a shower of neutrons at the moment of detonation. Polonium’s intense radioactivity also finds use in research as a source of alpha particles (for example, in physics experiments exploring fundamental particles or in anti-static brushes mentioned). Polonium is so radiotoxic that even in milligram amounts it self-heats (a capsule of Po might glow blue from ionizing the surrounding air). Because of this, outside of highly controlled environments, polonium isn’t encountered. It’s a fascinating but dangerous element that mostly lives in the realm of nuclear science and history books.

Astatine (At, Z=85)

Main article: Periodic table/Halogens/Astatine

Astatine is often cited as the rarest naturally occurring element on Earth (apart from synthetic elements). It is estimated that at most a few tens of grams of astatine exist in Earth’s crust at any time, as it appears only fleetingly as part of decay chains of heavier elements. Astatine is a halogen (down the group from iodine), but due to its radioactivity and short half-lives (the longest-lived isotope, At-210, has a half-life of about 8.1 hours), its chemistry is not well-characterized. It likely behaves somewhat like a heavier iodine – for instance, it can be prompted to form astatide (At⁻) analogous to iodide, and it might concentrate somewhat in the thyroid like iodine does (astatine’s name comes from Greek astatos meaning “unstable”). Because we cannot gather astatine in any appreciable quantity, it has almost no practical uses, but it has a tantalizing potential in medicine. Scientists are researching astatine-211 for targeted alpha therapy (TAT) in cancer treatment. At-211 emits high-energy alpha particles that can kill cancer cells effectively if delivered to a tumor site. The idea is to attach At-211 to an antibody or molecule that guides it to cancer cells; once there, the astatine delivers a lethal punch to those cells over a very short range (alpha particles don’t travel far). This could minimize damage to surrounding healthy tissue. Some early clinical trials have explored At-211 in treating certain cancers like recurrent brain tumors and small metastatic cancers. The results have been promising, though producing enough astatine (via nuclear reactors or accelerators by bombarding bismuth) and handling its short life are challenges. Outside of medicine, astatine’s uses are purely academic – filling a gap in understanding periodic trends. For instance, chemists have created a few astatine compounds in minuscule amounts (like astatine monochloride) to probe how a “super-heavy halogen” behaves. Astatine’s extreme rarity and radioactivity ensure it will remain the least encountered period 6 element for the general public.

Radon (Rn, Z=86)

Main article: Periodic table/Noble gases/Radon

Radon is a noble gas, one of the inert gases like helium and neon, but with a twist – radon is radioactive. It’s a colorless, odorless gas (under standard conditions) and is the heaviest noble gas. Radon forms from the natural decay of uranium and thorium in the Earth’s crust, specifically via radium decay (radium-226 decays to radon-222). Because it’s a gas, radon can seep through soil and cracks in rocks and accumulate in enclosed spaces like basements. The fact that radon is naturally generated and can concentrate in homes is a major health concern: radon decay products (polonium, lead, bismuth isotopes) can attach to dust and be inhaled, irradiating lung tissue. Radon exposure is a leading cause of lung cancer among non-smokers, contributing to thousands of deaths annually. This is why home radon testing and mitigation (ventilation or sub-slab systems) are important in regions with uranium-rich soil. Historically, before the dangers were fully understood, radon (or rather radon-rich air) was used in medical therapies – radon spas and health mines in the early 20th century saw patients bathing in or inhaling radon, under the notion it could treat arthritis or other ailments. Indeed, some “health resorts” built in natural radon emanation areas still exist, though they are controversial. Radon was also once used in radiotherapy for cancer: small seeds of radon gas in gold needles were implanted into tumors (brachytherapy), a practice replaced now by safer isotope sources. In the 1910s–1920s, radon-filled bulbs (radon emanators) were sold as a sort of curative – a radioactive quackery similar to radium waters of that era. Those have long since fallen out of favor as the hazards became clear. In scientific research, radon’s short-lived isotopes (beyond Rn-222’s 3.8 day half-life, there are even shorter ones like Rn-220 and Rn-219 in thorium/actinium decay chains) are used to probe phenomenon in nuclear physics and to track air mass movements (radon as a tracer). Geologically, radon monitoring is sometimes used in earthquake prediction research (as stressed rocks may release more radon). But radon has no industrial applications – it’s too radioactive and scarce to bottle for any routine use. The focus with radon is largely on mitigation: how to prevent this unseen gas from building up in homes (simple solutions include better foundation sealing and active ventilation). Radon serves as a reminder that even the noble gases, usually so aloof, can have a dark side when radioactivity is involved.

In conclusion

Period 6 is a rich tapestry of elements that have revolutionized technology and challenged scientists. From cesium’s role in timekeeping to the lanthanides that enable our modern electronics and green energy solutions, from the noble and precious metals to the notorious poisons like thallium and radon – this period spans a tremendous range of chemistry and utility. Many of these elements touch our lives daily in hidden ways: the neodymium in our earbuds, the gadolinium in an MRI scan, the tungsten in a lamp filament, the platinum in a car’s exhaust, the bismuth in a stomach remedy. Period 6 encapsulates the incredible breadth of the periodic table, demonstrating how elements as different as a soft reactive metal and an inert gas can belong to the same row, each contributing uniquely to the human story. In understanding these elements, we appreciate not only the materials that make up our world, but also the ingenuity that humans have applied to harness their properties – for beauty, for health, for progress, and occasionally for harm. As science and technology advance, the elements of period 6 will no doubt continue to play leading roles, whether in cutting-edge research (like astatine in cancer therapy or ytterbium in quantum clocks) or in everyday essentials (like safe batteries and cleaner engines). Period 6 is where the periodic table truly expands its scope, and exploring it provides a grand tour of chemical and physical possibilities.