The Nucleus of an Atom

The atomic number is the number of protons found in the nucleus of an atom of that element. Therefore, it is identical to the charge number of the nucleus. It is conventionally represented by the symbol $Z$.

The atomic number uniquely identifies a chemical element. In an uncharged atom, the atomic number is also equal to the number of electrons.

The atomic number, $Z$, should not be confused with the mass number, $A$.

Mass number is the number of nucleons, i.e. the total number of protons and neutrons in the nucleus of an atom. The number of neutrons, $N$, is known as the neutron number of the atom; thus, $A = Z + N$ (these quantities are always whole numbers).

Isotopes are atoms with the same atomic number $Z$ but different neutron numbers $N$, and hence different atomic masses.

Stable and unstable atoms

There are forces within the atom that account for the behavior of the protons, neutrons, and electrons. Without these forces, an atom could not stay together.

Recall that protons have a positive charge, electrons a negative charge, and neutrons are neutral. According to the laws of physics, like charges repel each other and unlike charges, attract each other. A force called the strong force opposes and overcomes the force of repulsion between the protons and holds the nucleus together.

Unstable atoms will lose neutrons and protons as they attempt to become stable.

1. Binding energy is the net energy that is the result of the balance with the strong force and the repulsive force, and this is the amount of energy that holds the nucleus together.
2. A stable atom is an atom that has enough binding energy to hold the nucleus together permanently.
3. An unstable atom does not have enough binding energy to hold the nucleus together permanently and is called a radioactive atom.