The similarity between Li and Mg is because of their similar atomic radii (Li = 152 pm; Mg = 160 pm) and ionic radii. (Li+ = 76 pm,  Mg2+ = 72 pm).These two elements resemble each other in the following properties:(i) Both Li and Mg decompose water very slowly with the liberation of hydrogen.(ii) Both Li and Mg form nitrides  - Li directly and Mg on burning in nitrogen.(iii) LiOH and Mg(OH)2 are weak bases. (iv) Nitrates of both decompose on heating to give oxides.                  (v) Both LiCl and MgCl2 are soluble in ethanol. (vi) Both LiCl and MgCl2 are deliquescent and crystalline from aqueous solution as hydrates, LiCl2. 2H2O and MgCl2.8H2O.