Rows and columns: periods and families

A table, not a jumble of elements

The periodic table is not a random list: each chemical element occupies a specific position, chosen because its properties resemble those of its neighbours. This organisation into rows and columns is the brilliant idea that transformed chemistry into a predictable science.

The rows: the periods

Each horizontal row of the table is called a period. As you move across a period from left to right, electrons are gradually added to the same electron shell. There are 7 periods; the first contains only 2 elements (H and He), as the shell closest to the nucleus can hold only 2 electrons.

The columns: the groups

Each vertical column is called a group. Elements in the same group have the same number of electrons in their outermost shell, which gives them very similar chemical properties. For example, lithium (Li), sodium (Na) and potassium (K) all belong to the alkali metal group.

A quick guide

	Column 1	Column 17	Column 18
Group	Alkali metals	Halogens	Noble gases
Example	Na, K	Cl, Br	He, Ne, Ar

Common misconception

Many pupils think that an element’s position in the periodic table is arbitrary, a bit like an alphabetical order. This is incorrect: the position directly reflects the electron configuration of the atom. Two neighbouring elements on the same row may have very different properties, whilst two elements that are far apart on the periodic table but in the same column are chemically similar.