Beryllium and magnesium to not give colour to flame whereas other alkaline earth metals do so. Why ?

Explanation

Because of the small size, the lonization enthalpies of Be and Mg are much higher than those of other alkaline earth metals. Therefore, they need large amount of energy for excitation of their valence electrons to higher energy levels. Since such a large amount of energy is not available in Bunsen flame, therefore, these metals do not impart any colour to the flame.