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NCERT · Class 9 · Science

Chapter 4: Structure of the Atom

Practice each question, then open the answer or worked solution.

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Exercise-4.2.4

Textbook page 49 · 2 questions
Q1Written answer

Name the three subatomic particles of an atom.

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Worked solution

An atom consists of three subatomic particles:

  • Protons  – Positively charged
  • Electrons – Negatively charged
  • Neutrons – Neutral in nature (no charge)
Q2Written answer

Helium atom has an atomic mass of 4 u and two protons in its nucleus. How many neutrons does it have?

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Worked solution

Given: Atomic mass of helium atom = 4u, 2 protons in helium nucleus

Atomic mass = number of protons + number of neutrons

4 = 2 + number of neutrons

Number of neutrons = 4 – 2 = 2

Hence, Helium has 2 neutrons.