As we move along the period the radii of atoms decrease, i.e. radii of alkaline earth metals are smaller than the radii of alkali metals. The smaller the radius of an atom/ion the greater the polarizing power. The alkaline earth metals have greater polarizing power, and therefore have higher ability to distort the oxygen cloud compared to alkali metals(fajan's rule). They will harder shear away the electrons from oxygen. Therefore slight heating leads to break of a bond between oxygen and metal's atom. This is the reason of the following trend in the thermal stability of the oxosalts of alkali and alkaline earth metals: thermal stability of alkali metal's oxosalts is higher than that of alkaline earth metals.