`Be` and `Mg` atoms do not impart colour to the flame.

Explanation

All alkaline earth metals (except Be and Mg) impart a charactertistic colour to the Bunsen flame. The different colours arise due to different energies required for electronic excitation and de-excitation. Be and Mg atoms due to their small size, bind their electrons more stronly (because of higher effective nuclear charge). Hence, require high exciation energy and are not excited by the energy of the flame with the result that no flame colour is shown by them.