Elements may have a varied number of neutrons in their nuclei. These are called isotopes , and many of them are unstable. Measurement of their instability is usually designated as the half-life of the isotope. The basic model of the nucleus of an atom is that it is made up of protons and neutrons bound together in a sphere. This force seems to require a certain number of neutrons within the nucleus for it to work.
Modern theories state that a gluon particle acts to hold the nucleus together.
The atomic number is the number of protons in the nucleus, and the atomic weight is a value slightly less than the sum of the number protons and neutrons in the nucleus. Thus Oxygen is number 8 and has an atomic weight of approximately 16, because it has 8 protons and 8 neutrons in its nucleus. Uranium is number 92 and has an atomic weight of approximately 238, because it has 92 protons and 145 neutrons in its nucleus. There are usually an equal number or slightly more neutrons than protons in the nucleus. With the heavier atoms, the ratio goes up to about 1.5 neutrons per proton.