The atomic number of an element refers to the number of protons that it has in its nucleus. Carbon for instance has an atomic number of 6 because there are 6 protons in its nucleus.
This atomic number will also tell you the number of electrons the element (in its neutral state) has because protons and electrons are the same number. The reason the mass number is 18 in this scenario is that the element has 10 neutrons in its nucleus.
