Note - if an atom is neutral, the number of protons equals the number of electrons.
Protons have a positive charge while electrons have a negative charge. In order to form a neutral (no charge) atom there must be the same number of protons as electrons.
Therefore there are 17 protons since there are 17 electrons. Both cancel each other out since 17+ -17= 0
In an atom the number of protones are equal to the number of electrons. therefore that atom will be neutral in charge.
So, a neutrally charged clorine atom is having 17 electrons & 17 protons.