Isotopes of an element have the same:

**The reasoning** Isotopes are *versions* of the same element that differ only in their neutron count. What makes an element what it is? Its **atomic number** — the number of protons in the nucleus. Carbon is carbon because it has 6 protons. Whether it's Carbon-12, Carbon-13, or Carbon-14, they ALL have 6 protons. Change the proton number, and you've got a completely different element (like Nitrogen with 7 protons). - Same protons? Same element, just different isotopes. - Different neutrons? That's what creates the different isotopes. **Why the wrong options tempt you** **A & D)** These are literally what *change* between isotopes! Carbon-12 has 6 neutrons; Carbon-14 has 8 neutrons. Different neutron counts mean different atomic masses. **B)** Mass number = protons + neutrons. Since neutrons differ, mass numbers differ too (12 vs. 14). **Quick takeaway** Isotopes = same house (protons), different furniture (neutrons). The proton count defines the element; neutrons just add weight. **Protons = identity; neutrons = variety.**