Chemistry helpppp????? pleaseeeee??

The atomic weight of an element is the sum of its protons and neutrons. In an element ( not an ion of the element ) the number of electrons equals the number of protons = atomic number, In the case cited above Cl^-1 is chloride anion and it has one extra electron than the element Cl so 37Cl^1 has 17 protons , 18 electrons and 20 neutrons answer B