Atomic Number - The number of protons in an atom; also the number of electrons in an atom thus the charge is zero.
Mass Number - The number of protons in the nucleus plus the number of nuetrons in the nucleus; the total number of particles in the nucleus.
Element - Atoms with the same atomic numbers; 92 naturally occuring elements.
Isotopes - Atoms with the same atomic number but different mass numbers (same element but with different number of neutrons).