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đź’ˇ 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)?
- 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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âś… Test Yourself
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What does the atomic number (Z) count?
Show Answer
The number of protons in the nucleus of an atom. -
In a neutral atom, what equals the number of protons?
Show Answer
The number of electrons. -
What is the atomic number of helium?
Show Answer
2 — it has 2 protons and 2 electrons. -
What decides the identity of an element?
Show Answer
Its atomic number.
Important Definitions
- Atomic number (Z) — the number of protons in the nucleus of an atom; it fixes the identity of the element.
