NCERT Solutions for Class 9th Science Chapter 8 Chapter opener — Think It Over
Book page 140 Updated on2026-09-08
Q1.
Are atoms the smallest indivisible particles?
Answer
No. An atom can be broken into smaller particles, so it is not indivisible.
The idea that it was indivisible came from philosophy, not experiment — Acharya Kanada's parmanu and the Greek atomos (which literally means “indivisible”). Dalton kept the same assumption in 1808. Experiments then broke it:
Radioactivity — certain elements were found to emit invisible particles and energy on their own. Something was coming out of the atom.
Cathode rays (Thomson, 1897) — a stream of negatively charged particles, much lighter than the atom, came out of the cathode whatever metal the cathode was made of and whatever gas filled the tube. These were electrons, present in every atom.
Today we know an atom contains three subatomic particles — electrons, protons and neutrons — and that protons and neutrons themselves have inner structure.
Why it happens: the electron's independence of the cathode material is the key clue. If a particle identical in every way comes out of copper, aluminium or iron, it cannot belong to any one element — it must be a building block common to all of them.
Q2.
Why do electrons not fall into the nucleus even though they are attracted to protons in it?
Answer
Because an electron in a stationary state does not lose energy — that is Bohr's postulate, and it is exactly the point at which his model beat Rutherford's.
Follow the reasoning in two steps.
What classical physics predicts. An electron moving in a circle is continuously changing direction, so it is accelerating (Chapter 4). An accelerating charge should radiate energy. Losing energy, it would spiral inward and crash into the nucleus in a tiny fraction of a second. Every atom would collapse — and matter would not exist.
What actually happens. Atoms are stable; a piece of iron stays iron. So the classical prediction is wrong for an atom. Bohr proposed that electrons are allowed only in certain fixed shells (K, L, M, N …), and that while an electron stays in its shell its energy stays constant, even though it is moving. Energy is exchanged only when the electron jumps from one shell to another, and then only in a fixed amount equal to the energy difference between the two levels.
So the electrostatic attraction of the protons does hold the electron in — it is what keeps the electron bound to the atom at all — but the electron settles into an allowed energy level instead of falling all the way in.
Did you know? Bohr had no derivation for stationary states; he simply postulated them because they matched experiment. That is a legitimate scientific move — a postulate stands until a deeper theory (here, quantum mechanics) explains it.
Q3.
Why did scientists keep modifying atomic models?
Answer
Because each new experiment produced a result the old model could not explain, and a model that cannot explain the evidence has to be changed.
Model
What it explained
What broke it
Dalton (1808)
Atoms as indivisible building blocks of matter
Radioactivity and cathode rays — atoms emit smaller particles
Thomson
How positive and negative charge balance to make a neutral atom
Gold foil experiment — a few α-particles bounced straight back
Rutherford (1911)
Empty space, a dense central nucleus, orbiting electrons
Could not explain why atoms are stable
Bohr (1913)
Stability, through fixed energy levels K, L, M, N
Later experiments needed electron clouds, not sharp orbits
Quantum mechanical
Electrons as clouds — regions of probability
Still being refined
Why it happens: this is how science works, not a sign of failure. A model is a tool for prediction. When a prediction fails, the model is replaced by one that keeps everything the old model got right and explains the new result too. Notice that each new model kept the useful part of the last one — Bohr kept Rutherford's nucleus.