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If an atom were as big as a cricket ground, its nucleus would be just a grain of black pepper at the centre — yet that speck holds almost all the mass!
- Most of the atom is empty space (most α-particles passed through).
- The nucleus is tiny, dense and carries all the positive charge .
- The nucleus holds most of the mass of the atom.
- Electrons revolve around the nucleus like planets around the Sun ( planetary model ).
How many atoms make a sheet of paper 0.1 mm thick? If one atom is about \(10^{-10}\) m and the sheet is \(10^{-4}\) m thick, then number of atoms \(\approx \dfrac{10^{-4}}{10^{-10}} = 10^{6}\) — about one million atoms stacked together!
Predict: what would you expect if the gold foil were made thicker ? With a thicker foil there are more layers of nuclei, so the chance of an α-particle hitting a nucleus rises — more particles would be deflected and bounce back , and fewer would pass straight through.
NCERT Question 4 — What conclusion did Rutherford draw
- 4. If α-particles were replaced with negative particles, they would be attracted to the positive nucleus instead of being repelled, so the pattern of deflection would change.
- 5. A few particles bouncing back can only happen if they hit something small, hard and heavy . A spread-out plum-pudding charge could never do this — so the model is ruled out.
- 6. (Open) e.g. “How did you imagine the nucleus before you could ever see it?”
- Nucleus — the extremely small, dense central region of the atom that carries all the positive charge and most of the mass.
- Planetary model — Rutherford model in which electrons revolve around the nucleus like planets around the Sun.
