What is an isotope?

Dave - What defines the element is the number of protons it's got and also the number of electrons it has got outside so it defines the chemistry of that element. You can then change the number of neutrons inside that atom and that will change its nuclear properties but won't change its chemical properties and these things with different nuclear properties are called isotopes of an element. Ben - So it's still the same element but different numbers of neutrons so that the 238 is the number of neutrons. Dave - The 238 is the total mass, the total number of protons and neutrons and so, 239 will have an extra neutron.