If there are 9 protons, there must be 10 neutrons for the total to add up to 19. The atomic number is tied to the position of the element in the Periodic Table and therefore the number of protons defines what sort of element you are talking about. The number of neutrons in an atom can vary within small limits. They all have the same number of protons, but the number of neutrons varies. The fact that they have varying numbers of neutrons makes no difference whatsoever to the chemical reactions of the carbon.
Isotopes are atoms which have the same atomic number but different mass numbers.
They have the same number of protons but different numbers of neutrons. The electrons Working out the number of electrons Atoms are electrically neutral, and the positiveness of the protons is balanced by the negativeness of the electrons. The arrangement of the electrons The electrons are found at considerable distances from the nucleus in a series of levels called energy levels. Each energy level can only hold a certain number of electrons. At GCSE you stop there because the pattern gets more complicated after that.