How are electrons and neutrons determined? (on the periodic table?)?

The number of electrons is the same as the number of protons (the atomic number on the periodic table). You can't tell the number of neutrons by looking at the periodic table alone. You need to have the atomic mass and subtract the number of protons from that to get the number of neutrons. If the number of neutrons is not the same as the number of protons, it is an isotope.