The Difference Between an Element Group and Period

Table of elements
Element columns are groups; element rows are periods. Jaap Hart/Getty Images

Groups and periods are two ways of categorizing elements on the periodic table. Here is how to tell them apart and how they related to periodic table trends.

Periods are horizontal rows (across) the periodic table, while groups are vertical columns (down) the table. Atomic number increases both 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 2. Elements belonging to a group typically share several common properties.

The groups are columns in the periodic table, but they go by a variety of different names:

IUPAC NameCommon NameFamilyOld IUPACCASnotes
Group 1alkali metalslithium familyIAIAexcluding hydrogen
Group 2alkaline earth metalsberyllium familyIIAIIA 
Group 3 scandium familyIIIAIIIB 
Group 4 titanium familyIVAIVB 
Group 5 vanadium familyVAVB 
Group 6 chromium familyVIAVIB 
Group 7 manganese familyVIIAVIIB 
Group 8 iron familyVIIIVIIIB 
Group 9 cobalt familyVIIIVIIIB 
Group 10 nickel familyVIIIVIIIB 
Group 11coinage metalscopper familyIBIB 
Group 12volatile metalszinc familyIIBIIB 
Group 13icoasagensboron familyIIIBIIIA 
Group 14tetrels, crystallogenscarbon familyIVBIVAtetrels from Greek tetra for four
Group 15pentels, pnictogensnitrogen familyVBVApentels from Greek penta for five
Group 16chalcogensoxygen familyVIBVIA 
Group 17halogensfluorine familyVIIBVIIA 
Group 18noble gases, aerogenshelium family or neon familyGroup 0VIIIA 

Another way to describe element groups follows properties of the elements and isn't strictly tied to columns, in some cases. These groups are alkali metals, alkaline earth metals, transition metals (which includes rare earth elements or lanthanides and also actinides), basic metals, metalloids or semimetals, nonmetals, halogens, and noble gases.

In this classification, 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 a 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 sublevel.

There are 7 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)