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

Step 1: How flame colouration works

When a compound is heated in a flame, thermal energy can excite a loosely-held valence electron to a higher energy level. As the electron falls back to the ground state, it emits energy as light; if this falls in the visible range, the flame shows a characteristic colour.

Step 2: Be and Mg — too small, too tightly bound

Be and Mg are the smallest members of Group 2 and have comparatively high ionisation enthalpies, so their valence electrons are held very strongly by the nucleus. The energy available from an ordinary flame is not enough to excite these electrons to a level from which the electron, on falling back, would emit light in the visible region.

Step 3: Ca, Sr, Ba — larger, lower ionisation enthalpy …

Link flame colour to the ease of exciting the valence electron: small size $\Rightarrow$ high ionisation enthalpy for Be/Mg means flame energy is insufficient, while the larger …

• Saying Be and Mg 'don't have valence electrons that can be excited' without linking this specifically to their small size and high ionisation enthalpy.
• Confusing flame colouration with the blue colour of alkali-metal/ammonia solutions (Q10) — these are unrelated phenomena. …