Beryllium and magnesium do not give colour to flame whereas other alkaline earth metals do so. Why?
Beryllium and magnesium do not give colour to flame because they (Be and Mg) have a small size and very high ionisation enthalpies. The enthalpy of the flame is not sufficient to excite their electrons to higher energy levels. Other alkaline earth metals impart colour to the flame because they have fairly low ionisation enthalpies, so electrons in their atoms can be easily excited by the enthalpy of the flame. The excited electrons on returning to their original positions lose enthalpy in the form of coloured light.
How will you explain the reducing character of alkaline earth metals?
Except beryllium, the alkaline earth metals have a fairly strong tendency to lose two electrons to form dipositive ions because of their low ionisation enthalpies and high negative value of standard electrode potentials. Therefore, they act as reducing agents. The reducing character of alkaline earth metals increases as we move down the group from Be to Ba because the ionisation enthalpies increase and electrode potentials become more and more negative with increasing atomic number from Be to Ba.
(i) Ionisation enthalpy (ii) Enthalpy of hydration (iii) Enthalpy of vaporisation The combined effect of these factors is approximately the same for Ca, Sr, and Ba. Hence, their electrode potentials are nearly constant.