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Class 9 Science: Journey Inside the Atom, 8 practice questions with answers

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These are 8 practice questions on Chapter 8, Journey Inside the Atom, of the new NCERT Class 9 Science textbook, Exploration (First Edition, April 2026). Each has a short worked answer and the textbook page it comes from. They cover Thomson's electron, the gold foil experiment, Chadwick's neutron, atomic number and mass number, isobars, and two numericals on average atomic mass. Try each one on paper before you open the answer. For how scientists use models, laws and units, see the Class 9 Science Chapter 1 questions.

These questions are our own, drafted with AI help. Every number was re-computed by a test script, but no practising teacher has reviewed them yet. If you find a mistake, please tell us.

What the chapter says you need

Facts from the NCERT textbook (Exploration: Science, Textbook for Grade 9, Chapter 8, printed pages 140 to 161):

Who found what

Question 1. Which subatomic particle did J. J. Thomson discover? (The nucleus; the electron; or the neutron.)

Answer: the electron. Thomson's best-known discovery was the electron, the first subatomic particle found. (Page 142.)

Question 2. Which scientist discovered the neutron in 1932? (James Chadwick; J. J. Thomson; or Niels Bohr.)

Answer: James Chadwick. James Chadwick discovered the neutron, a neutral particle about as heavy as a proton. Thomson discovered the electron, and Bohr proposed energy levels. (Page 147.)

Question 3. In the gold foil experiment, most alpha particles passed straight through the foil. What did Rutherford conclude from this? (The positive charge is spread evenly through the atom; electrons carry most of the mass of the atom; or most of an atom is empty space.)

Answer: most of an atom is empty space. Because most of the atom is empty, most alpha particles meet nothing and go straight. The few large deflections showed a tiny, dense, positive nucleus. (Pages 143 and 144.)

Atomic number and mass number

Question 4. An atom has 17 protons and 20 neutrons in its nucleus. What is its mass number?

Answer: 37. Mass number = protons + neutrons = 17 + 20 = 37. Electrons have negligible mass, so they are not counted. (Pages 147 and 150.)

Question 5. An atom of aluminium has mass number 27 and atomic number 13. How many neutrons are in its nucleus?

Answer: 14. Mass number = protons + neutrons, and the atomic number is the number of protons. Neutrons = 27 − 13 = 14. (Pages 149 and 150.)

Question 6. Calcium (atomic number 20) and argon (atomic number 18) both have atoms with mass number 40. What are such atoms called? (Isobars; isotopes; or nucleons.)

Answer: isobars. Isobars belong to different elements but have the same mass number (and different atomic numbers). Isotopes share the same atomic number instead. (Page 156. The textbook introduces isobars with calcium, potassium and argon, so this question uses two of its three examples.)

Isotopes and average atomic mass

Question 7. An element X occurs in nature as two isotopes: one of mass 10 u (20%) and one of mass 11 u (80%). What is its weighted average atomic mass, in u?

Answer: 10.8 u. Multiply each isotope's mass by its share and add: 10 × 20/100 + 11 × 80/100 = 2 + 8.8 = 10.8 u. (The method is that of the chlorine example, page 155, which uses different masses and shares.)

Question 8. Copper occurs in nature as two isotopes: copper-63 (about 69%) and copper-65 (about 31%). (a) What is the average atomic mass of copper, in u, to one decimal place? (b) Explain why the answer is not simply (63 + 65) ÷ 2 = 64.

Answer: (a) 63 × 0.69 = 43.47 and 65 × 0.31 = 20.15; the sum is 63.62, which is about 63.6 u. (b) Copper-63 is far more common, so the average leans towards 63; the simple average of 64 wrongly treats both isotopes as equally common. (The textbook makes the same point for chlorine, pages 155 and 156.)

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Sources

All opened and read on 21 October 2026. Page numbers are the printed page numbers of the textbook; printed page = PDF page + 139.

The facts and definitions are from the textbook pages above. The questions, worked answers and checks are our own, and every number was re-computed by script. This guide is general information; follow your teacher and the textbook.