The s-Block Elements

Question

The alkaline earth metals are denser and harder than the corresponding alkali metals. Explain ?

Answer

The atoms of alkaline earth elements have smaller size and stronger intermetallic bonds, as compared to alkali metals. Therefore, they are more closely packed in their crystal lattices which account for high density and hardness of these elements.
In general, the density first decreases up to calcium and then increases up to radium. The case for a decrease in density up to calcium is presumably due to less good packing arrangement of constituents in the solid lattice.

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