The Difference Between an Element Group and Period

Groups and periods are two ways of categorizing elements in the periodic table. Periods are horizontal rows (across) the periodic table, while groups are vertical columns (down) the table. Atomic number increases as you move down a group or across a period.

Element Groups

Elements in a group share a common number of valence electrons. For example, all of the elements in the alkaline earth group have a valence of two. Elements belonging to a group typically share several common properties.

The groups in the periodic table go by a variety of different names:

Another way to group elements is based on their shared properties (in some cases, these groupings do not correspond to the columns in the periodic table). Such groups include alkali metals, alkaline earth metals, transition metals (including rare earth elements or lanthanides and also actinides), basic metals, metalloids or semimetals, nonmetals, halogens, and noble gases. Within this classification system, hydrogen is a nonmetal. The nonmetals, halogens, and noble gases are all types of nonmetallic elements. The metalloids have intermediate properties. All of the other elements are metallic.

Element Periods

Elements in a period share the highest unexcited electron energy level. There are more elements in some periods than others because the number of elements is determined by the number of electrons allowed in each energy sub-level.

There are seven periods for naturally occurring elements:

• 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 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, Uut, Fl, Uup, Lv, Uus, Uuo (all elements are radioactive; contains heaviest natural elements)