The combining capacity of an atom is called its valency.
Now, sodium has only one electron in its outermost shell which it can lose very easily to gain an octet configuration. Hence, the valency of sodium is 1.
Chlorine has seven electrons in its outermost shell and it could complete its octet by gaining one electron. Hence, the valency of Chlorine is 1.
Argon has fully filled outermost hell with no vacancy available for an extra electron.
Hence, it’s valency is 0.