yes tarin is right...

 

because of heterolytic cleavage, R⁺ and X^- are formed. Na being higly electropositive, easily donates its valence electron to R⁺ and itself becomes Na⁺ and since R⁺ gains one electron, it becomes R^. That is, a radical is formed and not an anion.

R⁺ + Na -------------> R^. + Na⁺

After that,

Na⁺ + X^- --------> NaX

Then, sodium again donates an electron to R radical and the radical gets converted to an anion.

R^. + Na -------> R^- + Na⁺ which is nothing but RNa

 

Finally, R^-Na⁺ + RX -----> R-R + Na⁺X^-