If you are doing your chemistry or physics homework there should be one in front of you.
Each element is assigned an atomic number, #Z#, that specifies the number of #"protons"#, massive positively charged particles, that are in that element's nucleus. For #Cl#, #Z=17# (you don't have to remember this, because you SHOULD be assigned a Table for all your chemistry and physics tests!).
If there are 17 positive charges, there MUST be 17 negative charges associated with the NEUTRAL element (how many with #Cl^-#?), and these are the #"electrons"#, small particles of unit negative charge, that are conceived to whizz about the nuclear core.
And now the last atomic particle, the #"neutron"#, a massive particle of NEUTRAL charge, that resides in the nucleus. The neutron and the proton, at impossibly short internuclear distances, engage in the strong nuclear force, which at short range, is strong enuff to overcome the electromagnetic repulsion that would otherwise repel the positively charged protons. Different numbers of neutrons may reside in the nucleus, and this gives rise to the existence of #"isotopes"#, viz:
For the simplest atom, hydrogen, mostly we have #""^1H#, the #"protium isotope"#. A small number of hydrogen nuclei contain 1 neutron (in addition to the defining proton) to give #""^2H#, the #"deuterium isotope"#. And an even smaller number of hydrogen nuclei contain 2 neutrons to give #""^3H#, the #"tritium isotope"#.
Most elements exhibit a number of isotopes, and the quoted atomic mass is the weighted average of the isotopic masses (and hence we tend to see non-integral atomic masses. But remember that #Z# defines the element, and thus defines the number of electrons as well in the neutral element.
So how many electrons in #""^37Cl#? How many protons? And how many neutrons? What about in #""^35Cl#?
