Groups and periods of the periodic table

Groups: the columns

Groups are numbered 1 to 18 from left to right. In the main groups, every element in a column (helium aside) has the same number of valence electrons, so they react in similar ways: sodium and potassium both react with water and both form +1 ions. Properties change gradually down a group as the atoms get larger. In the United States you may also see an older numbering, in which groups 1, 2 and 13 to 18 are called 1A to 8A and the transition metals form the B groups.

The main families by group

Group 1 holds the alkali metals (hydrogen sits at the top but is a nonmetal), and group 2 the alkaline earth metals. Groups 3 to 12 are the transition metals; zinc, cadmium and mercury in group 12 are counted here, although some chemists leave them out because their d subshell is full. Group 13 is the boron group, group 14 the carbon group, group 15 the nitrogen group or pnictogens, and group 16 the oxygen group or chalcogens. Group 17 contains the halogens and group 18 the noble gases.

Groups 13 to 16 each contain a mix of element types, so their cells are colored by category rather than by family. In group 14, for example, carbon is a nonmetal, silicon and germanium are metalloids, and tin and lead are post-transition metals. Click a category in the legend to highlight it.

Periods: the rows

The seven periods are numbered from top to bottom. With one exception (palladium), the period number equals the number of electron shells an atom has: sodium, in period 3, has electrons in three shells. Periods grow longer as more subshells become available: period 1 has 2 elements, periods 2 and 3 have 8, periods 4 and 5 have 18, and periods 6 and 7 have 32.

From period 2 onward, properties change step by step across each row, from reactive metals on the left through metalloids and nonmetals to a noble gas on the right. The lanthanides and actinides, shown in two rows at the bottom, belong to periods 6 and 7; they are placed there only so the table is not too wide.

Worked example: chlorine

Chlorine is in group 17 and period 3. Group 17 tells you it is a halogen with 7 valence electrons, so it gains one electron to form the Cl⁻ ion and behaves like fluorine and bromine. Period 3 tells you its 17 electrons occupy three shells, arranged 2, 8, 7.