Atoms & structure
Isotope — আইসোটোপ
Quick answer
Isotopes are atoms of the same element with equal protons but different neutrons.
Definition
Isotopes are versions of one element that share the same atomic number — the same proton count — but carry different numbers of neutrons, so their mass numbers differ. They sit in the same place in the periodic table (the name means "same place") and behave almost identically in chemistry, because reactions follow electrons, not neutrons. Hydrogen has three: protium with no neutron, deuterium with one, and radioactive tritium with two. Carbon-12 and carbon-14, uranium-235 and uranium-238 are other famous pairs.
For example
Carbon-14, with 8 neutrons, slowly decays, so historians can date old wooden boats or bone tools by how much of it remains.