Number of protons in an atom is its:

## The reasoning Think of an atom's identity card. What makes carbon *carbon* and not oxygen? It's the **number of protons** in its nucleus. This unique number is called the **atomic number (Z)**. For example: - Hydrogen has 1 proton → atomic number = 1 - Carbon has 6 protons → atomic number = 6 - Oxygen has 8 protons → atomic number = 8 The atomic number appears as the smaller number at the bottom left of an element's symbol on the periodic table. It's literally *defined* as the number of protons. ## Why the wrong options tempt you **A) Mass number** – This is protons *plus* neutrons (the total particles in the nucleus). Close, but includes extras! **C) Atomic mass** – This is the average mass of all isotopes of an element, measured in atomic mass units. It's about *weight*, not just proton count. **D) Valency** – This tells you how many bonds an atom can form. It's about electrons in the outer shell, not protons in the nucleus. ## Quick takeaway **Atomic number = protons only.** It's the element's fingerprint—change the proton count, and you get a completely different element!