Chemical nomenclature
Every element is represented by a chemical symbol. For example the element sodium has the chemical symbol Na.
The atomic number is written at the bottom left hand side of the symbol. It tells you how many protons the element has - in this case 11.
The mass number is written at the top left hand side of the symbol. It tells you the total number of protons and neutrons - in this case 23.
You can work out the number of neutrons by subtracting the atomic number from the mass number. Here you can work out that an atom of sodium contains 12 neutrons.
Isotopes
Isotopes are atoms of the same element with different mass numbers. They have the same number of protons (otherwise they would be different elements) but have different numbers of neutrons.
These two carbon atoms are isotopes. Each has the same atomic number and has six protons.
They have different mass numbers because they have different numbers of neutrons. Carbon-12 has six neutrons, while carbon-14 has eight neutrons.