Groups and periods organize elements on the periodic table of the elements. A group is a vertical column down the periodic table, while a period is a horizontal row across the table. Both groups and periods reflect the organization of electrons in atoms. Element atomic number increases as you move down a group from top to bottom or across a period from left to right.
- An element group is a vertical column on the periodic table. Atoms in a group share the same number of valence electrons. There are 18 element groups.
- An element period is a horizontal row on the periodic table. Atoms in a period have the same number of electron shells. There are 7 element periods.
Element Groups
Elements within the same group share the same number of valence electrons. The number of valence electrons depends on the octet rule. For example, elements in group 1 have 1 valence electron, elements in groups 3-12 have a variable number of valence electrons, and elements in group 17 have 7 valence electrons. The lanthanides and actinides, located below the main table, all fit within group 3.
There are 18 element groups. Elements in the same group share common chemical and physical properties. For example, the group 1 elements are all soft, reactive metals. The group 17 elements are highly reactive, colorful nonmetals.
| IUPAC Name | Common Name | Family | Old IUPAC | CAS | notes |
| Group 1 | alkali metals | lithium family | IA | IA | sometimes excludes hydrogen |
| Group 2 | alkaline earth metals | beryllium family | IIA | IIA | |
| Group 3 | transition metals | scandium family | IIIA | IIIB | |
| Group 4 | transition metals | titanium family | IVA | IVB | |
| Group 5 | transition metals | vanadium family | VA | VB | |
| Group 6 | transition metals | chromium family | VIA | VIB | |
| Group 7 | transition metals | manganese family | VIIA | VIIB | |
| Group 8 | transition metals | iron family | VIII | VIIIB | |
| Group 9 | transition metals | cobalt family | VIII | VIIIB | |
| Group 10 | transition metals | nickel family | VIII | VIIIB | |
| Group 11 | coinage metals | copper family | IB | IB | |
| Group 12 | volatile metals | zinc family | IIB | IIB | |
| Group 13 | icoasagens | boron family | IIIB | IIIA | |
| Group 14 | tetrels, crystallogens | carbon family | IVB | IVA | tetrels from the Greek tetra for four |
| Group 15 | pentels, pnictogens | nitrogen family | VB | VA | pentels from the Greek penta for five |
| Group 16 | chalcogens | oxygen family | VIB | VIA | |
| Group 17 | halogens | fluorine family | VIIB | VIIA | |
| Group 18 | noble gases, aerogens | helium family or neon family | Group 0 | VIIIA |
Alternate Group Classification System
Sometimes chemists classify element groups according to shared properties, which do not strictly adhere to individual columns. These groups go by the names alkali metals, alkaline earth metals, transition metals, basic metals, nonmetals, halogens, noble gases, lanthanides, and actinides. Under this system, hydrogen is a nonmetal. The nonmetals, halogens, and noble gases are all types of nonmetals. The metalloids have properties intermediate between metals and nonmetals. The alkali metals, alkaline earths, lanthanides, actinides, transition metals, and basic metals are all groups of metals.
Element Periods
Elements within a period share the same number of electron shells and the same highest unexcited electron energy level. Elements within a period display periodic table trends, moving from left to right, involving atomic and ionic radius, electronegativity, There are seven element periods. Some periods contain more elements than others because the number of included elements depends on the number of electrons allowed in an energy sublevel. Note that the lanthanides are within period 6 and the actinides are in period 7.
- Period 1: H, He (does not follow the octet rule)
- Period 2: Li, Be, B, C, N, O, F, Ne (involves s and p orbitals)
- Period 3: Na, Mg, Al, Si, P, S, Cl, Ar (all have at least 1 stable isotope)
- Period 4: K, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ga, Ge, As, Se, Br, Kr (first period with d-block elements)
- Period 5: Rb, Sr, Y, Zr, Nb, Mo, Tc, Ru, Rh, Pd, Ag, Cd, In, Sn, Sn, Te, I, Xe (same number of elements as period 4, same general structure, and includes the first exclusively radioactive element, Tc)
- Period 6: Cs, Ba, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Hf, Ta, W, Re, Os, Ir, Pt, Au, Hg, Tl, Pb, Bi, Po, At, Rn (first period with f-block elements)
- Period 7: Fr, Ra, Ac, Th, Pa, U, Np, Pu, Am, Cm, Bk, Cf, Es, Fm, Md, No, Lr, Rd, Db, Sg, Bh, Hs, Mt, Ds, Rg, Cn, Nh, Fl, Mc, Lv, Ts, Og (all elements are radioactive; contains heaviest natural elements and many synthesized elements)
References
- Fluck, E. (1988). “New Notations in the Periodic Table” . Pure Appl. Chem. IUPAC. 60 (3): 431–436. doi:10.1351/pac198860030431
- Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. ISBN 978-0-08-037941-8.
- Scerri, E. R. (2007). The periodic table, its story and its significance. Oxford University Press. ISBN 978-0-19-530573-9.