You Gotta Know These Chemical Elements
- Hydrogen (atomic symbol H, atomic number 1) is the first element on the periodic table and, by far, the most common element in the Universe. In addition to the main isotope (also called protium), there are two other significant isotopes of hydrogen: deuterium (2H or D), which has one neutron, and tritium (3H or T), which has two neutrons. It naturally exists as a diatomic gas (H2), which was discovered by the British chemist Henry Cavendish. Hydrogen is highly flammable when exposed to high temperatures or electric current, a fact demonstrated by the Hindenburg disaster. It can react with nonmetals by losing an electron to form the H+ ion, or react with metals to form the hydride ion, H–.
- Helium (He, 2) is the lightest noble gas and the second most abundant element in the Universe (after hydrogen). Discovered by Sir William Ramsey, Pierre Janssen, and Norman Lockyer, it has two stable isotopes, helium-3 and helium-4, with helium-4 by far the more common. Because of their different quantum properties (the helium-3 nucleus is a fermion, while the helium-4 nucleus is a boson), the isotopes of helium actually have significantly different physical properties. Helium-4 can exist in a zero-viscosity state known as superfluidity when its temperature drops below the lambda point. Helium has the lowest boiling point of any element; liquid helium is used for devices that need intense cooling, such as MRI machines. Most helium on Earth results from radioactive decay, since the helium nucleus is equivalent to an alpha particle.
- Oxygen (O, 8) is, by mass, the most common element in Earth’s crust. It was discovered independently by Carl Scheele and Joseph Priestley; Priestley originally called it “dephlogisticated air.” Oxygen normally exists in elemental form as a diatomic gas (O2), but it can also exist in a triatomic form, ozone (O3), which is known for its role in blocking UV rays in Earth’s stratosphere. Diatomic oxygen is — despite having an even number of electrons — paramagnetic, meaning it has unpaired electrons. This points out a problem with traditional valence bond theories, which predict that oxygen should be diamagnetic; molecular orbital theory correctly explains this behavior. Because oxygen is easily capable of accepting electrons, reactions in which a species gives up electrons are known as oxidation reactions.
- Nitrogen (N, 7) is the most abundant element in Earth’s atmosphere. Nitrogen, which was first isolated as “noxious air” by Daniel Rutherford, exists primarily as a diatomic molecule containing two triple-bonded nitrogen atoms (N2). Because nitrogen gas is extremely stable, N2 is unusable for many biological and chemical purposes. To make it useful, it often undergoes fixation to convert it into usable nitrogen species such as the ammonium ion (NH4+) — as it is by bacteria in the root nodules of legume plants—or ammonia gas (NH3), as is done industrially in the Haber-Bosch process. Conversely, its stability makes it useful in preventing unwanted combustion reactions. It also has a relatively low boiling point (–196°C), which makes liquid nitrogen useful as a refrigerant.
- Mercury (Hg, 80) is one of just two elements that is a liquid at standard temperature and pressure (the only other one is bromine). It has been known since antiquity, and is found in ores such as cinnabar. Older names for it, reflecting its liquid nature, include hydrargyrum (the source of its symbol) and quicksilver. Because it is a very dense liquid, it is commonly used in barometers to measure atmospheric pressure; the pressure exerted by the atmosphere equals the pressure exerted by a column containing 760 millimeters of mercury. Alloys of mercury with other metals are called amalgams, some of which have been used as dental fillings. Chronic exposure to mercury can cause psychological problems; its use in hatmaking led to the expression “mad as a hatter.” More recently, concerns about mercury exposure have led to the banning of mercury in thermometers.
- Sulfur (S, 16) was widely known in the ancient world, and is referred to in the Bible as brimstone. Its nature as an element was first recognized by Antoine Lavoisier. Its most stable allotrope is an eight-membered ring that exists as a yellow solid. It is most often isolated by injecting superheated steam into the ground in the Frasch process. As an element, it is used in the vulcanization process to cross-link the polymer strands of rubber to increase strength; similarly, sulfur-sulfur bonds hold many proteins together. Industrially, though, the majority of sulfur is used to make sulfuric acid, H2SO4 (in fact, sulfuric acid is the most widely produced chemical in the chemical industry). Sulfur compounds are noted for their strong and unpleasant odors; small quantities of hydrogen sulfide, H2S, are frequently added to natural gas — which is normally odorless — to help people notice gas leaks.
- Iron (Fe, 26) is the most common metal in the Earth, and one of the major components of the Earth’s core. Iron was known to the ancients; its atomic symbol comes from the Latin name ferrum. Iron is the namesake of ferromagnetism; one of its ores is magnetite, Fe3O4, which contains iron in both of its most common oxidation states, 2+ and 3+. Iron(II) sulfide, FeS2, is formally known as pyrite, but because of its appearance has long been known as fool’s gold. Iron can react with oxygen in the air to form iron(III) oxide, Fe2O3, in a relatively slow but exothermic process; this process is used in “all-day” heat patches. Hydrated iron(III) oxide is better known as rust; rust only forms when iron is exposed to both oxygen and water. Its isotope 56 is “doubly magic” in that its nucleus has 28 protons and 28 neutrons; 28 is a “magic number” that carries special stability. As a result, iron-56 is one of the most stable of all nuclei, and it is the heaviest nucleus that is normally produced during stellar nucleosynthesis. The largest use of iron is in steel.
- Carbon (C, 6) is found, by definition, in all organic compounds. It is the fourth most abundant element in the Universe. It has three major isotopes: isotope 12, which is stable; isotope 13, which is used in NMR spectroscopy; and isotope 14, which is radioactive and is the basis of carbon dating. Carbon’s ability to form four chemical bonds means that it has many different allotropes. The best-characterized natural isotopes are diamond, which consists of a tetrahedral network of carbon atoms, and graphite, which consists of planes of carbon atoms arranged in hexagons. Fullerenes such as buckyballs and carbon nanotubes, on the other hand, are generally produced synthetically; buckyballs are roughly spherical. More recently, graphene, which is a single layer of atoms shaped like graphite, has proven to have remarkable properties; for example, it is nearly transparent while being about 200 times stronger than an equivalent mass of steel.
- Aluminum (Al, 13) is the most common metal in Earth’s crust, and the first metal in the p block of elements. First isolated by Hans Christian Ørsted, its primary ore is bauxite, from which it is refined using large amounts of electric current, via electrolysis, through the Bayer and Hall-Héroult processes. (Because aluminum exists only in a +3 oxidation state, it takes three moles of electrons to produce one mole of aluminum; as a result, it has been estimated that 5% of all electricity in the U.S. goes to purifying aluminum.) It is found in the mineral corundum, which is found in many gems, including sapphires and rubies; the specific impurities found in a gem determine its color. It is also found in aluminosilicates, such as feldspar.
- Gold (Au, 79) was known to the ancients as a relatively inert metal. Its atomic symbol comes from its Latin name, aurum. It is resistant to attack by most acids, but it (along with platinum) will dissolve in aqua regia, a mixture of concentrated nitric acid and hydrochloric acid. Among all metals, it has the highest electronegativity and electron affinity; it occasionally is found in a –1 oxidation state as Au–. Widely used in jewelry, it also has a number of scientific uses. Ernest Rutherford’s gold foil experiment demonstrated the existence of a positively charged nucleus. Scanning electron microscopy (SEM) often requires that specimens be “sputtered,” or thinly coated, with gold atoms to allow imaging. Suspensions of gold compounds have been used to treat rheumatoid arthritis.
NAQT uses the American spellings of element names. In particular, NAQT uses “aluminum,” “cesium,” and “sulfur” instead of “aluminium,” “caesium,” and “sulphur.” The latter names are all acceptable alternatives. (The official IUPAC names are “aluminium,” “caesium,” and “sulfur.”)
This article was contributed by former NAQT writer Adil Khan.