"Versions of the same element."

Neutrons and protons make up the atom's nucleus. The number of protons in the nucleus of an atom is important in identifying the type of atom. All atoms of hydrogen, for example, have 1 proton in the nucleus. All atoms of neon have 10 protons in their nucleus. The number of neutrons in the nucleus of atoms, however, is not that critical and small variations among atoms of the same element do exist.

Preferred combinations of neutrons and protons do exist, where the forces holding protons and neutron together seem to balance best.

Hydrogen has three isotopes. Isotopes are written in the manner shown on the right with the mass number on top and the atomic number at the bottom . Tritium, an isotope of hydrogen, is shown on the right. Click on the blue writing to see the image.
-Hydrogen with one solitary proton in the nucleus.
-Deuterium with one proton and one neutron.
-Tritium with one proton and two neutrons
The trend is that light elements tend to have about as many neutrons as protons while heavy elements need more neutrons than protons in order to have a tightly held, stable nucleus.
For example:
Carbon atoms all have 6 protons but some may have 5 neutrons or 6 neutrons. Too many neutrons however, makes the nucleus very unstable and it is unlikely that an atom will have too many neutrons in its nucleus.
The number of protons and the number of neutrons added together is known as the mass number of the atom.
A carbon atom with 6 protons and 6 neutrons is said to have an atomic mass of 12. A carbon atom with 6 protons and 7 neutron is said to have an atomic mass of 13.
Atoms of the same element that have different number of neutrons in their nucleus are called isotopes.
Two carbon atoms, one with 6 neutrons and the other with 7 neutrons are said to be isotopes. Isotopes of the same element have identical chemical properties. Chemical properties of an atom are determined by the arrangement of electrons around the nucleus.