The number of protons in an atom is its:

**The reasoning** The **atomic number** is literally *defined* as the number of protons in an atom's nucleus. It's the atom's unique identity card. For example, hydrogen always has 1 proton (atomic number = 1), carbon always has 6 protons (atomic number = 6), and oxygen always has 8 protons (atomic number = 8). If you change the number of protons, you change the element entirely! **Why the wrong options tempt you** **A) Mass number** — This is the *total* of protons + neutrons. Students confuse it because both involve counting particles, but mass number includes neutrons too. **C) Atomic mass** — This is the average mass of all isotopes of an element (measured in atomic mass units). It sounds similar to "mass number" but includes decimals and considers isotope abundance. **D) Valency** — This tells you how many bonds an atom can form (based on electrons in the outer shell), not the protons in the nucleus. **Quick takeaway** Remember: **Atomic number = Protons = Element's ID**. It never changes for a given element, unlike mass number (which varies with isotopes) or valency (which depends on electron arrangement).