No, it is not the most common isotope of nitrogen. The most common isotope would have a mass number of 14. If you look at the periodic table of elements, nitrogen is element number 7. That means it has 7 protons. To get the mass number, you need to add together the number of protons and neutrons in the atom. The number of neutrons can differ from atom to atom, so the mass number is the number of protons and neutrons on average. The listed atomic mass of nitrogen is 14.007. To figure out how many neutrons are normally in a nitrogen atom, you subtract the atomic number (7 in this case) from the rounded off atomic mass. 14 -7 = 7. Nitrogen typically has 7 neutrons. That means the mass number is 7 protons + 7 neutrons=14. The most common isotope of nitrogen is 14.
Atoms are neutral when they have equal numbers of protons and electrons. That means a neutral nitrogen atom has 7 protons and 7 electrons. Each added electron will increase the charge by a single negative. If nitrogen has a charge of -3, then it has three extra electrons. That gives it a total of 10 electrons.