How do you determine the number of electrons an atom will gain or loose

In general, metals will lose electrons to become a positive cation and nonmetals will gain electrons to become a negative anion. Hydrogen is an exception, as it will usually lose its electron. Metalloids and some metals can be can lose or gain electrons. This is not always true, as elements such as nitrogen can lose electrons to become positive. When an ionic compound forms, the more electronegative element will gain electrons and the less electronegative element will lose electrons. Hope this helps :)

nacl is added to the aqueous of what? ? in order to get a proper answer you have to specify the exact composition of the aqueous solution.