Nitrogen is the seventh element on the periodic table. It has an atomic number of 7, so it also has seven protons. Because nitrogen has an atomic weight of 14.007 amu, its mass number is 14. Subtracting the number of protons (7) from the element's mass number (14) makes it possible to determine the number of neutrons in the atom.
Many nitrogen atoms have seven neutrons (14-7 = 7). Some atoms of nitrogen, however, have eight electrons. They are atoms of N-15, an isotope of nitrogen.
In a neutral atom, the number of protons must equal the number of electrons. This means a neutral atom of nitrogen must have seven electrons to match its seven protons.