Group 1 of the periodic table consists of the elements : lithium, sodium, potassium, rubidium, caesium and francium. The elements of Group 2 include beryllium, magnesium, calcium, strontium, barium and radium.
Hydration Enthalpy : The hydration enthalpies of alkali metal ions decrease with increase in ionic sizes. Li+ has maximum degree of hydration and for this reasons lithium salts are mostly hydrated e.g., LiCI . 2H2O
Physical properties : All the alkali metal are silvery white, soft and light metals. Because of the larger size, these element have low density. The melting and boiling point of the alkali metals are low indicating weak metallic bonding alkali metals and their salts impart characteristic colour to an oxidizing flame.
Chemical Properties: The alkali metal are highly reactive due to their larger size and low ionization enthalpy.
ANOMALOUS PROPERTIES OF LITHIUM
(i) exceptionally small size of its atom and ion, and (ii) high polarising power (i.e., charge/ radius ratio). The similarity between lithium and magnesium is particularly striking and arises because of their similar size : atomic radii, Li = 152 pm, Mg = 160 pm; ionic radii: Li+ = 76 pm, Mg2+ = 72 pm.
GROUP 2 ELEMENTS : ALKALINE EARTH METALS
The first element beryllium differs from the rest of the member and shows diagonal relationship to aluminium.
Hydration enthalpies of alkaline earth metal ions. Be2+ > Mg2+ > Ca2+ > Sr2+ > Ba2+. The hydration enthalpies of alkaline earth metal ions are larger than those of alkali metal ions. Thus, compounds of alkaline earth metals are more extensively hydrated than those of alkali metals , e.g., MgCI2 and CaCI2 exist as MgCI2.6H2O and CaCI2. 6H2O while NaCI and KCI do not form such hydrates.
The alkaline earth metals, in general, are silvery white, lustrous and relatively soft but harder than the alkali metals. The melting and boiling point of these metals are higher due to smaller sizes. Because of the low ionisation enthalpies they are strongly electropositive in nature. The electrons in beryllium and magnesium are too strongly bound to get excited by flame. Hence these elements do not impart any colour to the flame. Calcium, strontium and barium impart characteristic colour to the flame.
ANOMALOUS BEHAVIOUR OF BERYLLIUM
Beryllium the first member of the Group 2 metals, shows anomalous behaviour as compared to magnesium and rest of the members. Further, it shows diagonal relationship to aluminium.
Diagonal Relationship between Beryllium and Aluminium
The ionic radius of Be2+ is estimated to be 31 pm; the charge/radius ratio is nearly the same as that of the Al3+ ion. Hence beryllium resembles aluminium in some ways.
Compounds of s-block elements :