8.5 Atomic Number

Last updated at July 3, 2026 by

💡 Your ID number

Every element has a unique ID — the number of protons in its nucleus. Change that number and you change the element itself.

What is atomic number (Z)?
Atomic Number Atomic identity of the element Number count of protons Number of protons in the nucleus; it fixes the identity of an element (symbol Z).
  • Number of protons in the nucleus.
  • It uniquely identifies the element and its chemical behaviour.
  • In a neutral atom, protons = electrons, so Z also equals the number of electrons.

NCERT Question 7 — Assertion (A): The discovery of

Assertion (A): The discovery of subatomic particles helped in understanding the atomic structure. Reason (R): The number of electrons is equal to the number of protons in an atom. Choose the correct option: (i) Both A and R are true, and R is the correct explanation of A. (ii) Both A and R are true, but R is not the correct explanation of A. (iii) A is true, but R is false. (iv) A is false, but R is true.
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Important Definitions
  • Atomic number (Z) — the number of protons in the nucleus of an atom; it fixes the identity of the element.
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