Step-by-step explanation:
Atomic number of sodium is 11 and its electronic distribution is 2, 8, 1. So, it means there is only one valence electron present in a sodium atom. This valence electron is represented by a dot in the Lewis structure.
Atomic number of chlorine is 17 and its electronic distribution is 2, 8, 7. So, it means there are seven valence electrons present in a chlorine atom. These valence electrons are represented by seven dots in the Lewis structure.
Also, in order to attain stability sodium will lose one valence electron and this valence electron will be gained by chlorine atom.
Therefore, an arrow pointed from dot near Na to Cl shows that an electron is being donated from Na to the Cl atom.