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

Explanation

Because of the small size, the ionisation 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 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.