Class 9 Science · Original EZT practice
Journey Inside the Atom (Class 9 Science Chapter 8): summary and practice
The atom was once thought indivisible - then came the electron, a nucleus, energy levels, and the neutron. This chapter retraces that journey: Thomson's model, the gold foil experiment that overturned it, Bohr's model, the neutron's discovery, and how we write the symbols of elements. All questions are original EZT tutor practice following the chapter's own section order.
6 original worked examples, 6 fresh self-check questions and 4 mixed questions, with explained answers. These are not copied textbook exercises, an official chapter index or a full answer key. No affiliation with NCERT is claimed.
Official sources for topic and edition context, checked 22 October 2026:
1. Thomson's model and the gold foil experiment
Thomson pictured the atom as a positive sphere with electrons embedded in it - in 1911 Geiger and Marsden, working under Ernest Rutherford, fired alpha particles at thin gold foil and found most passed straight through while a few bounced back, evidence for a tiny, dense, positive nucleus.
2. Bohr's model
Bohr proposed that electrons move in fixed energy levels (shells) around the nucleus and do not radiate energy while in a permitted level - energy is absorbed or emitted only when an electron jumps between levels.
3. Completing the picture - the neutron
The neutral particle in the nucleus, the neutron, was discovered later by Chadwick - completing the proton-neutron nucleus and explaining the atom's remaining mass.
4. Symbols of elements
Elements are written with standard symbols, often from their English names and sometimes from Latin ones - sodium is Na (natrium) and iron is Fe (ferrum).
Choose, solve, check, then try a fresh question.
Before calculating, read the method prompt and name the condition that makes it useful. Write your own attempt. Open the steps only when you need them, and use the check to catch a wrong sign, unit or interpretation. Finish with the fresh question before reading its answer.
Read the gold foil result
Choose the method: Most particles passing through means mostly empty space.
EZT practice question
In the gold foil experiment, most alpha particles passed straight through. What did this show?
Show the worked solution
- Alpha particles are tiny positively charged particles fired at thin gold foil.
- If atoms were solid, most particles would have been blocked or deflected.
- Nearly all passed straight through - so most of the atom is empty space.
Answer: That most of the atom is empty space.
Check: This single observation destroyed the idea of the atom as a solid positive sphere.
Watch for this: Do not say the particles missed the foil - they passed through the atoms themselves.
Now try it yourself
What are alpha particles?
Check your answer
Tiny positively charged particles emitted by certain radioactive elements.
Their charge is what makes them deflect near a nucleus.
Explain the bounce-back
Choose the method: Strong deflection needs a small, dense, positive centre.
EZT practice question
A few alpha particles bounced back from the gold foil. What did Rutherford conclude?
Show the worked solution
- A positive alpha particle can only be turned back by a strong positive repulsion.
- So the atom's positive charge must be concentrated, not spread out.
- And since most of the mass resists the bounce too, that tiny region also carries most of the mass - the nucleus.
Answer: The atom's positive charge and most of its mass are concentrated in a very small region - the nucleus.
Check: Geiger and Marsden ran the experiment in 1911 under Rutherford - the conclusion gave us the nuclear model.
Watch for this: Do not credit the electrons - they are far too light to turn an alpha particle back.
Now try it yourself
Who performed the gold foil experiment under Rutherford, and in which year?
Check your answer
Geiger and Marsden, in 1911.
Their measurements led to the nuclear model of the atom.
State Bohr's postulates
Choose the method: Fixed levels, and no radiation inside a level.
EZT practice question
State two postulates of Bohr's model of the atom.
Show the worked solution
- Bohr kept the nucleus but fixed how electrons move.
- First: electrons revolve around the nucleus only in certain fixed energy levels or shells.
- Second: an electron in a permitted level does not radiate energy - energy is absorbed or emitted only when it jumps between levels.
Answer: (1) Electrons revolve around the nucleus only in certain fixed energy levels. (2) An electron in a permitted level does not radiate energy; energy is absorbed or emitted only when it jumps between levels.
Check: These two ideas explain why atoms are stable - the electron never spirals into the nucleus.
Watch for this: Do not say electrons can sit anywhere - Bohr's whole point is that only certain levels are allowed.
Now try it yourself
What problem of Rutherford's model did Bohr's fixed energy levels solve?
Check your answer
A revolving electron was expected to radiate energy and spiral into the nucleus; fixed levels explain why atoms are stable.
No radiation inside a level means no collapse.
Track the energy of a jump
Choose the method: Down a level emits; up a level absorbs.
EZT practice question
What happens when an electron jumps from a higher to a lower energy level?
Show the worked solution
- Energy levels differ in energy.
- Moving to a lower level leaves the electron with less energy.
- The difference is released - the electron emits energy.
Answer: It emits energy.
Check: The reverse jump, lower to higher, needs the atom to absorb exactly that much energy.
Watch for this: Do not say it emits energy while staying in one level - Bohr's postulate forbids that.
Now try it yourself
What must happen for an electron to jump to a higher energy level?
Check your answer
The atom must absorb energy.
The electron needs the energy difference between the two levels.
Decode unusual symbols
Choose the method: Some symbols come from Latin names.
EZT practice question
Write the symbols for sodium and iron, and say why they look unusual.
Show the worked solution
- Most symbols match the English name, like O for oxygen.
- Sodium and iron do not match - Na and Fe.
- They come from the Latin names natrium and ferrum.
Answer: Na and Fe - they come from the Latin names natrium and ferrum.
Check: The same pattern explains K for potassium (kalium) and Au for gold (aurum).
Watch for this: Do not invent symbols from the English name - So and Ir are not the symbols for sodium and iron.
Now try it yourself
Why is potassium written as K?
Check your answer
From its Latin name, kalium.
Like sodium and iron, the symbol keeps the older name.
Try a mixed set
Choose a method or explanation before opening each answer.
1. Thomson's model?
Show answer and reason
A positive sphere with electrons embedded in it.
Like seeds in a fruit - neutral overall.
2. Gold foil experiment?
Show answer and reason
Mostly empty atom with a tiny dense positive nucleus.
Most alpha particles passed through; a few bounced back.
3. Bohr's model in short?
Show answer and reason
Electrons move only in fixed energy levels.
They absorb or emit energy only when jumping levels.
4. Who discovered the neutron?
Show answer and reason
James Chadwick.
The neutral nuclear particle, adding mass without charge.
Try it yourself
Work these on paper before checking any notes. Answers are deliberately not shown here.
- A few alpha particles bounced back from the gold foil. What did Rutherford conclude?
- What happens when an electron jumps from a higher to a lower energy level?
- Write the symbols for sodium and iron, and say why they look unusual.
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Questions about this practice page
- What was Thomson's model of the atom?
- A positively charged sphere with electrons embedded in it, like seeds in a fruit - neutral overall.
- What did the gold foil experiment prove?
- That the atom is mostly empty space with a tiny, dense, positively charged nucleus; most alpha particles passed through and only a few deflected strongly.
- What is Bohr's model in short?
- Electrons move around the nucleus only in fixed energy levels, and they absorb or emit energy only when jumping between levels.
- Who discovered the neutron?
- James Chadwick - it is the neutral particle in the nucleus, adding mass without charge.
- Are these the NCERT exercise questions for Class 9 Science Chapter 8?
- No. EZT pages are original practice following the chapter's section order - use them to prepare for the textbook exercises, not as a substitute for your book.