Isotopes have the same:

**The reasoning** Isotopes are atoms of the **same element** but with different numbers of neutrons. The key word here is "same element." What defines an element? Its **atomic number** — which is the number of protons. For example, Carbon-12 and Carbon-14 are both carbon because they both have **6 protons**. But Carbon-12 has 6 neutrons while Carbon-14 has 8 neutrons. Same element, different masses. The principle: **The number of protons determines the element's identity.** Change the protons, you get a different element entirely! **Why the wrong options tempt you** - **A) Neutrons** — This is the opposite! Isotopes differ specifically because they have different numbers of neutrons. - **C) Mass number** — Mass number = protons + neutrons. Since neutrons differ, mass numbers must differ too (12 vs 14 in our carbon example). - **D) Atomic mass** — Same trap as C. Different neutrons = different total mass. **Quick takeaway** Isotopes are like siblings from the same family (same protons/element) but with different weights (different neutrons) — the family name (element) never changes!