Group
A vertical column of the periodic table whose members share the same valence electron configuration.
Unit
Dimensionless
In depth
Because chemistry is dictated by valence electrons, group members show closely related formulae and reactions. Descending a group, atoms gain shells, so radius grows while ionisation energy and electronegativity fall, making metals more reactive and non-metals less so.
Examples in the real world
Group 1 metals all form M⁺ ions and react with water; group 17 halogens all form X⁻ ions and diatomic molecules.