Sodium chloride is ionic compound. It is formed by the transfer of electron from one atom to the atom of another element.
Both bonded atoms have very large electronegativity difference. The atom with large electronegativity value accept the electron from other with smaller value of electronegativity.
The electronegativity of chlorine is 3.16 and for sodium is 0.93. There is large difference is present. That's why electron from sodium is transfer to the chlorine. Sodium becomes positive and chlorine becomes negative ion. Both atoms are bonded together electrostatic attraction occur between anion and cations.
Sodium atom have one valance electron by losing this one valance electron sodium atom get the complete octet. Chlorine atom has seven valance electrons and needed to lose seven valance electrons or to get one electron and thus complete the octet. It is very easy for chlorine atom to get one electrons instead of losing all seven electron. Thus when it react with sodium it gain the valance electron of sodium and form ionic compound.
That's why only one atom of sodium combine with one atom chlorine.
