why on using Hg electrodes Na is deposited at cathode on electrolysis of aq NaCl?

The reaction at anode :
2Cl–→ Cl2+ 2e–

The chlorine gas that results vents at the top of the outside cells where it is collected as a byproduct of the process.
The reaction at the mercury cathode :
2Na++ 2e–→ 2Na (amalgam)
The sodium metal formed by this reaction dissolves in the mercury to form an amalgam.
The anode reaction in the center cell takes place at the interface between the mercury and the sodium hydroxide solution.
2Na (amalgam) → 2Na++ 2e–
Overall Cell Reaction
2H2O + 2e–→ 2OH–+ H2