s-block

The s-block gathers the elements whose ns orbital is being filled: IUPAC groups 1 and 2. Their single or double outermost valence electron makes them strongly electropositive and very reactive, especially in the heavier periods (Cs, Fr). The chemistry of alkali and alkaline earth metals is dominated by the easy loss of these electrons, producing very stable ions. By convention, hydrogen and helium are classified in the s-block because their electrons populate the 1s subshell, even though their chemical behaviour is singular.