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What is an Isotope?
Isotope is a form of a chemical element wich have same number of protons but different number of neutron.
from the picture we know that chlor that has 37 and chlor that has 35 mass number are isotope, because both of them have same number of protons, it is 17. And they have different number of neutron, chlor that has 37 mass number has 20 neutrons but chlor that has 35 mass number has 18 neutrons.