There are currently 118 known chemical elements exhibiting many different physical and chemical properties. Amongst this diversity, scientists have found it useful to use names for various sets of elements, that illustrate similar properties, or their trends of properties. Many of these sets are formally recognized by the standards bodyIUPAC. The following collective names are recommended by IUPAC:
Alkali metals – The metals of group 1: Li, Na, K, Rb, Cs, Fr.
Alkaline earth metals – The metals of group 2: Be, Mg, Ca, Sr, Ba, Ra.
Pnictogens – The elements of group 15: N, P, As, Sb, Bi.
Chalcogens – The elements of group 16: O, S, Se, Te, Po.
Halogens – The elements of group 17: F, Cl, Br, I, At.
Noble gases – The elements of group 18: He, Ne, Ar, Kr, Xe, Rn.
Lanthanides – Elements 57–71: La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu.
Actinides – Elements 89–103: Ac, Th, Pa, U, Np, Pu, Am, Cm, Bk, Cf, Es, Fm, Md, No, Lr.
Rare-earth metals – Sc, Y, plus the lanthanoids.
Transition elements – Elements in groups 3 to 11 or 3 to 12.
Another common classification is by degree of metallic - metalloidal - nonmetallic behaviour and characteristics. There is no general agreement on the name to use for these sets: in this English Wikipedia, the name used is "category". Very often these categories are marked by a background color in the periodic table. Category names used here, without any claim to universality, are:
Lanthanoids and actinoids are instead referred to as lanthanides and actinides respectively.
Rare-earth elements, pnictogens, chalcogens, and halogens are not used as category names, but the latter three are valid as group names.
Additional element category names used:
*Post-transition metals – The metals of groups 12–18: Zn, Cd, Hg, Al, Ga, In, Tl, Sn, Pb, Bi, Po. The period 7 elements Nh, Fl, Mc, Lv, and Ts are additionally predicted to be post-transition metals, and Og may be one as well.
*Metalloids – Elements with properties intermediate between metals and non-metals: B, Si, Ge, As, Sb, Te, At.
*Reactive nonmetals – Nonmetals that are chemically active : H, C, N, P, O, S, Se, F, Cl, Br, I
*Superactinides – Hypothetical series of elements 121 to 157, which includes a predicted "g-block" of the periodic table.
Many other names for sets of elements are in common use, and yet others have been used throughout history. These sets usually do not aim to cover the whole periodic table. Some examples:
Precious metals – Variously-defined group of non-radioactive metals of high economical value.
Noble metal – Variously-defined group of metals that are generally resistant to corrosion. Usually includes Ag, Au, and the platinum-group metals.
Heavy metals – Variously-defined group of metals, on the base of their density, atomic number, or toxicity.
Native metals – Metals that occur pure in nature, including the noble metals and others such as Sn and Pb.
Earth metal – Old historic term, usually referred to the metals of groups 3 and 13, although sometimes others such as beryllium and chromium are included as well.
Transuranium elements – Elements with atomic number greater than 92.
Transactinide elements – Elements after the actinides.
Transplutonium elements – Elements with atomic number greater than 94.
Minor actinides – Actinides found in significant quantities in nuclear fuel, other than U and Pu: Np, Am, Cm.
