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Protons, neutrons, electrons and isotopes: Class 9 practice

Atomic number Z = protons (= electrons in a neutral atom). Read the method, follow the three worked examples, then try the six questions.

What you should know first

  • Atomic structure

The idea behind protons, neutrons, electrons and isotopes

Atomic number Z = protons (= electrons in a neutral atom). Mass number A = protons + neutrons, so neutrons = A − Z. Isotopes have the same Z but different A. Average atomic mass weights each isotope by its abundance.

Step-by-step method

  1. Read Z and A.
  2. Neutrons = A − Z; electrons = Z (adjust for charge).
  3. Average mass = Σ(mass × % abundance)/100.

Worked example 1 (Basic)

Find protons, neutrons and electrons in C with Z = 6 and A = 14.

  1. Protons = 6
  2. Neutrons = 14 − 6 = 8
  3. Electrons = 6 = 6
  4. Answer: p 6, n 8, e 6

Worked example 2 (Standard)

Find protons, neutrons and electrons in C− with Z = 6 and A = 14.

  1. Protons = 6
  2. Neutrons = 14 − 6 = 8
  3. Electrons = 6 + 1 = 7
  4. Answer: p 6, n 8, e 7

Worked example 3 (Challenge)

An element has isotopes of mass 11 (75%) and 13 (25%). Find its average atomic mass.

  1. (11 × 75 + 13 × 25)/100
  2. = 11.5 u
  3. Answer: 11.5 u

Common mistakes and misconceptions

  • Adding electrons to the mass number
  • Forgetting ion charge when counting electrons

Practice questions

  1. (Basic) Find protons, neutrons and electrons in K with Z = 19 and A = 40.
  2. (Basic) Find protons, neutrons and electrons in Cl with Z = 17 and A = 35.
  3. (Standard) Find protons, neutrons and electrons in Mg+ with Z = 12 and A = 24.
  4. (Standard) Find protons, neutrons and electrons in O− with Z = 8 and A = 18.
  5. (Challenge) An element has isotopes of mass 12 (75%) and 14 (25%). Find its average atomic mass.
  6. (Challenge) An element has isotopes of mass 28 (80%) and 30 (20%). Find its average atomic mass.

Answer key with full working

  • Q1: Protons = 19 → Neutrons = 40 − 19 = 21 → Electrons = 19 = 19 → Answer: p 19, n 21, e 19
  • Q2: Protons = 17 → Neutrons = 35 − 17 = 18 → Electrons = 17 = 17 → Answer: p 17, n 18, e 17
  • Q3: Protons = 12 → Neutrons = 24 − 12 = 12 → Electrons = 12 − 1 = 11 → Answer: p 12, n 12, e 11
  • Q4: Protons = 8 → Neutrons = 18 − 8 = 10 → Electrons = 8 + 1 = 9 → Answer: p 8, n 10, e 9
  • Q5: (12 × 75 + 14 × 25)/100 → = 12.5 u → Answer: 12.5 u
  • Q6: (28 × 80 + 30 × 20)/100 → = 28.4 u → Answer: 28.4 u

Syllabus note

This skill is listed under "Structure of the Atom" for Class 9 in the NCERT/CBSE books. ICSE, IGCSE, IB and state boards may teach it in a different chapter or year.

Answers on this page are generated and automatically re-checked by a second, independent calculation. They have not been reviewed by a teacher — if you spot a problem, tell us on WhatsApp.

Related practice and notes

Frequently asked questions

Which chapter is protons, neutrons, electrons and isotopes in?

"Structure of the Atom", Class 9 Chemistry (NCERT/CBSE). Other boards may place it differently.

How were the answers checked?

Answers on this page are generated and automatically re-checked by a second, independent calculation. They have not been reviewed by a teacher — if you spot a problem, tell us on WhatsApp.

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