Periodic Table Groups and Periods

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.