Alkali Earth Metal, Alkali earth metals have low density
Alkali Earth Metal
Reason: Alkali earth metal crystallize with body centered cubic lattice. So they do not have close packed structure. Further the size of alkali earth metal atoms are large. As a result bonding in alkali metals is weak.
Alkali earth metal variation of density
Alkali metals have only one electron per atom available for metallic bonding. The increase in volume with the addition of new shell does not compensate the increase in the nuclear mass. So mass of alkali earth metal atom increases faster than the volume. So the density increases down the group. The density of Li, Na and K are less than that of water. As an exception potassium is found to be lighter than sodium.
Alkali earth metal melting and boiling points
Alkali metals have only one electron per atom available for metallic bonding. So the metallic bond in alkali metals is weak. This accounts for low melting and boiling points of alkali metals With the increase is atomic size down the group the bonding becomes weaker and hence the melting point decreases from Li to Fr.
Alkali earth metal ionization energy
In alkali metals because of large size the valence electron iS loosely bound to the nucleus. Further the effective nuclear charge for all the elements of the 1stgroup is only +1 since increase in the positive charge in the nucleus down the group is nullified by the increase in the number of screening electrons. So the valence electron can be easily removed. Hence alkali metals have low ionization potential.
Variation of ionization potential down the group: Down the group from Li to Cs the size increases. So the electrons of the Outershell get less and less tightly bound. This causes a steady decrease in the ionizationpotential value down the group.
Note: After the first ionization all the alkali metal ions attain the electronic configuration of the previous inert gas. So the second ionization energy of all the alkali metals is very high. The second ionization energy decreases down the group.