Periodic table

Groups

The modern periodic table is divided into 18 columns called groups or families (see illustration). Elements in each family tend to have similar properties. In column 1, each alkali metal is soft, relatively low-melting, and highly reactive toward air and water. Column 2 contains the alkaline-earth metals, which have higher melting points and are less reactive. Columns 3–12 are filled by the transition metals, which are shiny and good conductors of both heat and electricity. Columns 13–18 are often discussed along with columns 1 and 2, and, collectively, the elements in these columns are known as the main group or representative elements because these elements are the most abundant on Earth and in the universe overall. Column 15, headed by nitrogen, is known as the pnictogens; column 16, beginning with oxygen, as the chalcogens; column 17, starting with fluorine, as the halogens; and column 18, starting with helium, as the noble gases. See also: Alkaline-earth metals; Transition elements

Periods

The horizontal rows of the periodic table are called periods. Atomic mass generally increases from left to right across a period because elements are organized by atomic number,meaning that proton and neutron numbers will generally increase from left to right and top to bottom. However, despite the increase in mass, atomic size generally decreases from left to right because, as the number of protons increases, the consequent augmented overall positive charge causes the valence electron “cloud” to contract. IDspite their much smaller mass, the spreading out of electrons in orbitals is the main determinants of an atom’s relative size. Other elemental properties follow periodic trends, including the ionization potential (the energy needed to remove an electron), electron affinity (the energy released upon accepting an electron), and electronegativity (the ability of an atom in a compound to attract electron density). See also: Atomic structure and spectra; Electron affinity; Electronegativity; Ionization potential

After element 57 (lanthanum) comes a series of 14 metallic elements numbered 58–71 with closely related properties. These elements originally were named the rare earths because the elements have similar chemical reactivity and are difficult to separate. However, the elements are not actually rare and are now more appropriately called the lanthanides. Technically, the lanthanides should be placed between elements 57 (lanthanum) and 72 (hafnium). Since this placement would nearly double the width of the periodic table, the lanthanides are usually placed below all the other elements. Keeping in mind that size decreases left to right across a period, hafnium (72) is much smaller than its neighbor lanthanum (57). In fact, hafnium is essentially the same size as the element above it, zirconium (40). With comparable chemical reactivity and size, zirconium and hafnium are difficult to separate. This similarity also suggests that the second and third rows of the transition metals will possess many common chemical features, and they do. The decrease in size due to the 14 elements between lanthanum and hafnium is called the lanthanide contraction. Below the lanthanides are 14 more metallic elements (90–103) called the actinides. See also: Actinide elements; Lanthanide contraction; Rare-earth elements

Elements

Each cell in the periodic table contains a one- or two-letter symbol representing a different element such as C for carbon (6) or Sg for Seaborgium (106, see illustration). Hypothetical elements are given provisional three-letter designations that are then replaced by formal names. For example, element 119 is called ununennium and has the symbol Uue. The number in the upper left corner of each cell in the periodic table is the atomic number, which as mentioned earlier is indicative of how many protons are in the atom's nucleus. The atomic mass generally appears below the symbol and indicates the average mass observed for that element. For example, most carbon (99%) contains 6 protons and 6 neutrons, leading to a mass of 12. However, because about 1% of carbon has an extra neutron, the average mass of carbon as given in the periodic table is 12.011. If an element has no stable isotopes (forms of the same element that contain equal numbers of protons but different numbers of neutrons), then the mass of the longest-lived isotope is given in parentheses. More complex periodic tables often include information on density, melting points, and boiling points. Separate tables are also available, indicating crystal structures, magnetic properties, radioactive decay patterns, and other properties. See also: Isotope

All of the elements in the periodic table have been officially ratified by the International Union of Pure and Applied Chemistry (IUPAC). When a newly discovered element has been independently verified, the original discoverer (often a team) earns the right to propose a name to IUPAC. The elements beyond 92 (uranium) do not occur naturally and are produced using nuclear reactions. Elements beyond 100 are not particularly useful because they generally undergo rapid nuclear decay by emitting radiation. See also: Radioactivity; Transuranium elements

Other properties

Many periodic tables include a stair-step line separating metals from the metalloids and nonmetals. Most elements are metals and generally have physical properties that include luster (high reflectivity), good conductivity for both electricity and heat, high density, usually high melting points, ductility (the ability to be drawn into a wire), and malleability (the ability to be hammered into thin sheets). The chemical properties of most metals include corrosivity, such as iron rusting and silver tarnishing, as well as the ability to give up electrons. Nonmetals are found to the right of the metals and their characteristics are the inverse, meaning most of these elements have no luster (appearing dull), are poor conductors of electricity and heat, have low density, low melting points, and are brittle. Chemically, nonmetals like to gain electrons and often react with metals to produce salts. For example, combining an alkali metal that has one valence electron with a halogen that needs one electron to complete its valence shell produces an alkali halide salt such as sodium chloride, or common table salt. Metalloids straddle the stair-step line and often have properties in between metals and nonmetals. With intermediate conductivities, elements such as silicon form important semiconductors used in computer chips and solar cells. See also: Metal ; Metalloid; Nonmetal ; Semiconductor ; Solar cell