An atom of strontium has at least four different isotopes. What is different between an isotope of 88Sr and an isotope of 84Sr?

88 and 84 are mass numbers the mass number of an element is the sum of the number of protons and the number of neutrons (in which they are each equal to 1 unit (amu)) the number of protons is always the same for a given element (e. g. any C atom has 6 protons), but the number of neutrons varies. Two atoms of the same element (number of protons) with a different number of neutrons are called "isotopes". So the two strontium atoms differ by 4 neutrons. 84Sr has four neutrons less than 88Sr