From the periodic table, we can find that:
Sodium has 11 electrons with a configuration of (2,8,1)
Chlorine has 17 electrons with a configuration of (2,8,7)
Based on this, we can see that sodium needs to lose one electron to become stable with valence shell having 8 electrons.
On the other hand, chlorine needs to gain one electron to become stable with valence shell having 8 electrons.
In sodium chloride, the sodium donates this one electron to chlorine forming an ionic bond so they both become stable.
