Atoms of different elements usually have different mass numbers, but they can be the same. For example, the mass number of argon atoms and calcium atoms can both be 40.
The symbol for an atom can be written to show its mass number at the top, and its atomic number at the bottom.
To calculate the numbers of subatomic particles in an atom:
The atomic number of a chlorine atom is 17 and its mass number is 35. Calculate the number of protons, neutrons and electrons it contains.