An atom's atomic number is a subscript to the left of the symbol for that element. For chlorine, Cl, the atomic number is 17.
The atomic number is the number of protons an atom has in its nucleus. These are positively charged subatomic particles. In this case, we learn that chlorine contains 17 protons. From that, we also know that 17 electrons are present which are negatively charged subatomic particles. Electrons may be found in a space called an orbital. There are two electrons in the first orbital, eight in the next, and there will be seven electrons in the third orbital for chlorine.
Because the number of protons and electrons are the same, they will balance each other out which will result in an atom with a neutral electrical charge. Therefore, a neutral atom of chlorine contains 17 protons and 17 electrons.
If we look up chlorine's atomic mass number on the periodic table, it is 35 which indicates the sum of protons and neutrons in the nucleus. If we subtract 17 (number of protons) from 35, we find there are 18 neutrons in a neutral chlorine atom's nucleus. Neutrons are neutrally charged subatomic particles and do not add to the charge on an atom.