Question

Give reasons for the following: 

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

(ii) Why are potassium and caesium used in photoelectric cells? 

(iii) Why Li₂CO₃ decomposes at lower temperature whereas Na₂CO₃ at higher temperature?

Answer 1

(i) Beryllium and magnesium atoms are smaller in size and their electron are strongly bound to the nucleus. They need large amount of energy levels which is not available in the Bunsen flame. So they do not impart colour to the flame.

(ii) Due to low ionization energies, alkali metals, specially potassium and caseium, eject electrons when exposed to light, therefore, photoelectric cells.

(iii) As we move down the alkali metal group, the electropositive character increases. This causes an increase in the stability of alkali carbonates. However, lithium carbonate is not so stable to heat. This is because lithium carbonate is covalent. Lithium a large carbonate ion, leading to the formation of more stable lithium oxide.

Li₂CO₃ \(\overset{\Delta}{\rightarrow}\) Li₂O + CO₂

Therefore, lithium carbonate decomposes at a low temperature while a stable sodium carbonate decomposes at a high temperature.