Atomic number and mass number
Proton number and nucleon number
The proton number (Z) of an atom is the number of protons it contains. All the atoms of a particular element have the same proton number (number of protons). The atoms of different elements have different numbers of protons. All oxygen atoms have 8 protons and all sodium atoms have 11 protons.
The nucleon number (or mass number) of an atom is the total number of protons and neutrons it contains. The nucleon number of an atom is never smaller than the proton number. It can be the same, but is usually bigger.
Full chemical symbols
If you know the proton number and nucleon number you can calculate the number of each sub-atomic particle in an atom. The full chemical symbol for an element shows its nucleon number (A) at the top, and its proton number (Z) at the bottom.
This symbol tells you that the chlorine atom has 17 protons. It will also have 17 electrons, because the number of protons and electrons in an atom is the same.
The symbol also tells you that the total number of protons and neutrons in the chlorine atom is 35. Note that you can work out the number of neutrons by subtracting the atomic number from the mass number. In this example, it is 35 – 17 = 18 neutrons.