Since Nitrogen is in group 15, there will be 5 electrons in in its valance shell. 2 in the first, 5 in the second, which will equal to its atomic number of 7.
For a full 2nd shell, you would need to have 8 electrons total, so to fill nitrogens valence shell, 3 would have to be added. By adding 3 electrons the charge would now be negative because there are now more electrons than protons. (7 protons, 7 electrons+3 electrons=10 electrons) 7 (-10) = -3. Which would make the overall charge become negative 3.