Explain why atoms have different isotopes?
Atoms have DIFFERENT ISOTOPES due to DIFFERENT NUMBERS OF NEUTRONS.
A natural chemical element may be monoisotopic or has isotopes. Isotopes are atoms but they differ from other isotopes by the number of neutrons. This involve a different atomic mass and different physical properties or sometimes (for light isotopes) different chemical properties. Also, all chemical elements have radioactive, artificial isotopes.
Dear questioner, As an answer to your question I should mention there are atoms which have the same number of protons but are considered different. These atoms are called Isotopes. Isotopes of an element have the same number of protons but the number of Neutrons are not the same. Isotopes have the same chemical virtues but in the physical virtues related to mass they are a bit different.