The elements of the first two groups of the periodic table; i.e. groups 1 (Li, Na, K, Rb, Cs, Fr) and 2 (Be, Mg, Ca, Sr, Ba, Ra). The outer electronic configurations of these elements all have inert-gas structures plus outer ns 1 (group 1) or ns 2 (group 2) electrons. The term thus excludes elements with incomplete inner d-levels (transition metals) or with incomplete inner f-levels (lanthanoids and actinoids) even though these often have outer ns 2 or occasionally ns 1 configurations. Typically, the s-block elements are reactive metals forming stable ionic compounds containing M+ or M2+ ions. See alkali metals; alkaline-earth metals.