The answer is option B. The number of protons
If two atoms are isotopes of the same element then the number of protons in those two atoms must be same but the number of neutrons must not be same. If number of neutrons of an atom vary but number of protons in them are same then it simply means that they both are isotopes of a particular element. As we know that every atom has a nucleus which has protons and neutrons and due to these protons and neutrons an atom has weight. Apart from that, an atom has electrons too but they are outside the nucleus. Electrons are extremely light in weight so they do not contribute towards weight but they have a great role in chemical reactions. During chemical reactions, it is electron which directly participates, while proton and neutrons do not take part or move during chemical reactions. Also, in a neutral atom number of protons is equal to number of electrons but when an atom becomes negatively or positively charged, it can gain or loose electrons so electrons have tendency to vary. That is why we cannot consider electrons to be always same for isotopes of an element. It is clear that for an isotope, the number of electrons and neutrons will vary but the number of protons will always be fixed.
