Question

How many neutrons in `""^35Cl`, `""^36Cl`, and `""^37Cl`, respectively? How many electrons in each isotope?

Answer 1

Isotopes has same atomic number or Proton but different Number of Neutrons
`Cl-35` `"Proton"=17`, `Neutron=35-17=18`
`Cl-36` `"Proton"=17`, `Neutron=36-17=19`
`Cl-37` `"Proton"=17`, `Neutron=37-17=20`

Answer 2

Well, for the chlorine atom `Z=17`..........

Explanation:

And what does this mean? It means that the nucleus of EVERY chlorine atom contains 17 positively charged, massive particles, i.e. protons. And if there are 17 positive charges, there must also be 17 electrons, i.e. 17 extra-nuclear particles of negligible mass that are conceived to whizz about the nuclear core.....

And thus the chlorine isotopes ONLY differ in the number of neutrons, massive particles of NEUTRAL charge, that are contained in the nuclear core.

For `""^35Cl`, there are 18 neutrons; for `""^36Cl`, there are 19 neutrons; and for `""^37Cl`, there are 20 neutrons.

How did I know that `Z=17` for chlorine? Did I know it from memory? How will you know in your next chemistry exam?