Structure 3.1.2 - the period number shows the outer energy level that is occupied by electrons
see 3.1.1 periods, groups and blocks
elements in a group have a common number of valence electrons
group 1: alkali metals
group 2: alkali earth metals
group 15: pnictogens
group 16: chalcogens
group 17: halogens
group 18: noble gases
the period of an element gives the number of occupied energy levels