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

Explanation

Due to smaller size, valence electrons of Be and Mg are more tightly held by the nucleus. Therefore, they need a large amount of energy for the excitation of their valence electrons to higher energy levels. Since such a large amount of energy is not available from bunsen flame, these two metals do not impart any colour to the flame.