There are currently 118 known chemical elements, exhibiting a large number of different physical and chemical properties. Amongst this diversity, scientists have found it useful to use names for various groupings of elements, that illustrate similar properties, or their trends of properties. Many of these groupings are formally recognized by the standards body IUPAC.
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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. (Mc had not yet been named when the 2005 IUPAC Red Book was published, and its chemical properties are not yet experimentally known.)
Chalcogens – The elements of group 16: O, S, Se, Te, Po. (Lv had not yet been named when the 2005 IUPAC Red Book was published, and its chemical properties are not yet experimentally known.)
Halogens – The elements of group 17: F, Cl, Br, I, At. (Ts had not yet been named when the 2005 IUPAC Red Book was published, and its chemical properties are not yet experimentally known.)
Noble gases – The elements of group 18: He, Ne, Ar, Kr, Xe, Rn. (Og had not yet been named when the 2005 IUPAC Red Book was published, and its chemical properties are not yet experimentally known.)
Lanthanoids – Elements 57–71: La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu.
Actinoids – Elements 89–103: Ac, Th, Pa, U, Np, Pu, Am, Cm, Bk, Cf, Es, Fm, Md, No, Lr.
Rare earth elements – Sc, Y, and the lanthanoids.
Transition metals – Elements in groups 3 to 11 or 12.
This English Wikipedia uses the below hybrid system in its periodic table subgroupings. These groupings are called category in this Wikipedia (without any claim to universality):
Same as the IUPAC system above for alkali metals, alkaline earth metals, noble gases, and transition metals.
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 (column) names.
Additional element groupings used:
Post-transition metals – The metals of groups 13–17: Al, Ga, In, Tl, Sn, Pb, Bi, Po, Fl. Nh, Mc, Lv, and Ts are additionally predicted to be post-transition metals.
Metalloids – Elements with properties intermediate between metals and non-metals: B, Si, Ge, As, Sb, Te, At.
Polyatomic nonmetals – Nonmetals distinguished by polyatomic bonding in their standard states, in either discrete or extended molecular forms: C, P, S and Se.
Diatomic nonmetals – Nonmetals that exist as diatomic molecules in their standard states: H, N, O, 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 groups of elements are in common use, and yet others have been used throughout history. Some examples include:
Precious metal – Variously-defined group of non-radioactive metals of high economical value.
Coinage metals – Various metals used to mint coins, primarily the group 11 elements of Cu, Ag, and Au.
Platinum group – Ru, Rh, Pd, Os, Ir, Pt.
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 (atomic number greater than 103).
Transplutonium elements – Elements with atomic number greater than 94.
Minor actinides – Actinides found in nuclear fuel, other than U and Pu: Np, Am, Cm.
Heavy atom – term used in computational chemistry to refer to any element other than hydrogen and helium.
Any periodic table group can also be used in this sense. Sometimes the group number is used, as in group 14 element, and sometimes the name of the first element in the group is used, as in carbon group or carbon family.
Relativistic Dirac equation
The relativistic Dirac equation has problems for elements with more than 137 protons. For such elements, the wave function of the Dirac ground state is oscillatory rather than bound, and there is no gap between the positive and negative energy spectra, as in the Klein paradox.More accurate calculations taking into account the effects of the finite size of the nucleus indicate that the binding energy first exceeds the limit for elements with more than 173 protons. For heavier elements, if the innermost orbital (1s) is not filled, the electric field of the nucleus will pull an electron out of the vacuum, resulting in the spontaneous emission of a positron;however, this does not happen if the innermost orbital is filled, so that element 173 is not necessarily the end of the periodic table.
The many different forms of periodic table have prompted the question of whether there is an optimal or definitive form of periodic table. The answer to this question is thought to depend on whether the chemical periodicity seen to occur among the elements has an underlying truth, effectively hard-wired into the universe, or if any such periodicity is instead the product of subjective human interpretation, contingent upon the circumstances, beliefs and predilections of human observers. An objective basis for chemical periodicity would settle the questions about the location of hydrogen and helium, and the composition of group 3. Such an underlying truth, if it exists, is thought to have not yet been discovered. In its absence, the many different forms of periodic table can be regarded as variations on the theme of chemical periodicity, each of which explores and emphasizes different aspects, properties, perspectives and relationships of and among the elements.
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