Isotopes

Many elements are not absolutely unique. An element is defined purely by the number of protons in the nucleus, so any to nuclei with the same number of protons are the same element. Sometimes nuclei may have different numbers of neutrons, but the same number of protons. Because the nuclei have the same number of protons, they are the same element, but because they have different numbers of neutrons, the nuclei are not exactly the same. The one with more neutrons will be heavier.
Nuclei with the same number of protons by a different number of neutrons are called isotopes. All isotopes of the same element have identical chemical properties, because chemistry takes place between electrons in shell. Isotopes of the same element have the same number of protons and the same number and arrangement of electrons.
Isotopes have several uses.Some isotopes are radioactive e.g. Carbon 14. Animal take in Carbon 14 produced naturally by collisions of cosmic rays (from outer space) with Carbon 12 atoms. Animal take in Carbon 12 and Carbon 14 as part of their life cycle. When they die, this stops. Carbon 14 decays, and the proportion of Carbon 12 to Carbon 14 can give an estimate of the time of death of animals up to that died several thousand or tens of thousands of years ago.