The mass of an atom comes from
the protons and neutrons that are found in the nucleus. The number of protons
is the atomic number of an element. To find the number of neutrons, subtract
the atomic number from the mass of an atom. So, if an atom is missing a neutron
or has an extra neutron, the atom is called an isotope. An atom is still the
same element if it is missing an electron. That is also true for isotopes. They
are the same element but their number of neutron differs.
