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

Explanation

Due to high ionisation enthalpies of Be and Mg, the valence electrons in these elements are too strongly bound to get excited by a Bunsen burner flame. Hence, these elements do not impart any colour to the flame. The ionisation enthalpies of other alkaline earth metals are smaller and their electrons are easily excited to higher energy levels by the flame. Therefore, they give characteristic colours to the flame.