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NCERT Solutions for Class 9th Science Chapter 8 .2.1 Thomson's model of an atom — Pause and Ponder

Book page 1438 Updated on2026-09-08

Q1.
Suppose you made up your own ‘atom’, as Thomson described, using clay for the positive charge and small beads for the electrons spread through it. What will happen if: (i) the positive charge on the clay is lesser than the total negative charge of the beads? (ii) by mistake, the clay itself carries a bit of negative charge? Would your model still represent a neutral atom?
Answer

(i) The model would carry a net negative charge — it would represent a negative ion (an anion), not a neutral atom.

Net charge = (positive charge on clay) + (total negative charge on beads)
If positive < negative in size, the sum is negative
e.g. clay +8, beads 10 × (–1) = –10 → net = –2

A real example is the oxide ion O2–: 8 protons, 10 electrons, net charge –2.

(ii) No — it would not represent a neutral atom, and worse, it would no longer represent an atom at all.

  • Arithmetically the charges could never cancel: negative clay + negative beads can only add up to something negative.
  • Conceptually the whole point of Thomson's model is that the atom's positive charge lives in the sphere and its negative charge lives in the embedded particles. If the sphere itself is negative, there is nothing left to balance the electrons.
Why it happens: neutrality is an exact bookkeeping condition — number of protons = number of electrons, so total positive charge = total negative charge in size and they cancel. Any model of a neutral atom must satisfy it.
Q2.
Could an orange or a lemon, which also contain seeds inside soft pulp, be a good comparison? In what ways does it match Thomson’s idea and where does it fall short?
Answer

Yes, it works as far as the watermelon does — and it fails in exactly the same places.

Where it matches

  • Seeds (electrons) are embedded inside a bulk material (the positively charged pulp), not stuck on the outside.
  • The seeds are spread through the fruit rather than gathered at one point — Thomson's electrons are distributed throughout the sphere.
  • The fruit is roughly spherical, like the model's sphere of positive charge.

Where it falls short

  • An orange is divided into segments with membranes, and its seeds sit in a few segments — Thomson's positive charge is perfectly uniform, with no compartments.
  • The seeds are fixed; electrons are in motion.
  • An orange has a thick peel — a distinct outer boundary; the atom has no skin.
  • Scale is completely wrong. A seed is a good fraction of the size of the fruit. An electron is vanishingly small compared with the atom, and its mass is negligible compared with the atom's mass.
  • Most importantly, the model itself is wrong: there is no spread-out positive pulp in a real atom. All the positive charge is squeezed into a nucleus 105 times smaller than the atom.
Tip: every analogy in science has a boundary. State what the analogy is supposed to show (here: negative particles embedded in positive matter) and stop using it beyond that.
Q3.
Why did Thomson conclude that electrons are present in all atoms?
Answer

Because the cathode rays came out identical no matter what the tube was made of or filled with.

Thomson repeated the discharge-tube experiment while changing two things:

  • the metal of the cathode, and
  • the gas filling the tube at low pressure.

In every case the rays behaved the same way in electric and magnetic fields — same sign of charge, same charge-to-mass ratio. The particles were always negatively charged and always far lighter than an atom.

Why it happens: this is a controlled experiment. If the particle depended on the source, changing the source would change the particle. It did not. The only explanation left is that the particle is not characteristic of any one element but is a component of every atom. So Thomson concluded that electrons are a fundamental part of all matter — and that atoms, therefore, are divisible.
Did you know? The electron was the first subatomic particle ever identified. Thomson received the Nobel Prize in Physics in 1906 for his study of the electrical conductivity of gases, the work that led to it.
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