The two isotopes of chlorine
Chlorine has two isotopes.
3517Cl 3717Cl
Both have 17 protons, as they must. Work the neutrons out with A − Z and you get 18 in the first and 20 in the second.
They are not present in equal amounts
In a sample of chlorine gas, the two isotopes are not found in equal quantities. In every hundred parts of the gas there are:
| Isotope | Parts in every hundred |
|---|---|
| 3517Cl | 75 |
| 3717Cl | 25 |
Relative abundance
This is called the relative abundance percentage of each isotope. Three out of every four chlorine atoms you meet are the lighter one.
What this shows
The two behave identically in every chemical reaction — they have the same 17 electrons. Nothing you can do with chlorine in a test tube will separate them.