For the number of protons, we look at the atomic number of the element given. Take note that atoms must have equal numbers of protons and electrons. For the number of neutron, we must know the mass number. To find the mass number, all we need to do is round the atomic weight to the nearest whole number (if the atomic weight is given).

After knowing what the mass number is, we use the formula below to calculate the number of neutrons.

Mass Number = (Number of Protons) + (Number of Neutrons).

AtomHow do you determine the number of electrons, protons, neutrons in an atom?

I concur to previous posts.  Each element on the periodic chart has its own square.  At the top of the square is a number called the atomic number, which indicates the number of protons in the atom.  The number of protons is also equal to the number of electrons.  At the bottom of the square is a number called the mass number, which is the weighted average of all known isotopes of the atom.  Subtract the atomic number from this, you get the number of neutrons.