When reacting with a chlorine atom, a sodium atom reacts by losing an electron to form the na+ ion, which is

A positive charge.

Positive charges happen when an atom loses an electron. In a normal state, atoms have equal amounts of protons (+) and electrons (-). So when they lose an electron they will have more protons (+) which will make it positive. Sodium has 11 protons + and 11 electrons -, take away an electron and it will go from a neutral charge to positive.