Taken together, the alkali metals and the alkaline earth metals are known as the s-block metals, in allusion to the fact that only their s-electron orbitals are being filled. They are also known as the reactive series, in allusion to their marked reactivity.
This reactivity of the s-block elements is their most striking characteristic. Most of them react readily with non-metals, and the reactions of the alkali metals in particular can be extremely vigorous. Caesium, for example, explodes when dropped in water, and bursts into flame when exposed to air. Most dissolve in water to give alkaline solutions, hence the group names. All of the S-block metals are too reactive to be found uncombined in nature.
In comparison with the transition metals, the s-block metals have relatively low specific gravities. Specific gravity tends to rise steadily from left to right along the fourth, fifth and sixth periods, so potassium, for example, with a specific gravity of 0.862, is far less dense than vanadium (6.11) or cobalt (8.9). Lithium is the least dense of all the metals, so much so that it can float on the inert oil in which it is stored.
The s-block metals tend also to have lower melting and boiling points than the transition elements do. To take the fifth period as an example: rubidium, an alkali metal, has a meltng point of 39.48 degrees Celsius; strontium, an alkaline earth metal, melts at 769 degrees Celsius; and yttrium, a transition metal, melts at 1522 degrees Celsius. Caesium has the lowest melting point of the s-block elements, at 28.39 degrees Celsius.
They are characterized also by constant oxidation states. Whereas transition metals display variable valency, s-block metals tend to adhere to their group oxidation state.
