NCERT Solutions for Class 9th Science Chapter 8 .2.2 A. Rutherford's model of an atom — Pause and Ponder

The particles would still be deflected, but they would be attracted to the nucleus instead of repelled — so the deflections would bend the other way, and nothing would ever bounce straight back off a nucleus.

Two extra problems make the experiment much worse with negative particles:

• An electron has the same mass as the atom's own electrons, so it is scattered badly by them. The α-particle, being about 7300 times heavier than an electron, ploughs past them and only the nucleus can turn it. That is what makes the α-particle such a clean probe.
• An attractive force pulls the particle towards the target rather than pushing it away, so it can be captured instead of scattered.

Because in Thomson's atom there is nothing hard enough to bounce off.

Work through what each model predicts.

• Thomson's model: the positive charge is spread evenly through the whole atom, and the electrons are far too light to matter. An α-particle passing through meets a weak, thinly spread positive charge on every side, and the pushes from different directions largely cancel. The most it can suffer is a small nudge — a deflection of a fraction of a degree. A reversal is impossible.
• What was observed: a few α-particles came almost straight back, deflected through more than 90°.

To reverse a fast, heavy, positively charged particle you need a very large repulsive force, and to get a very large force you need a very large positive charge concentrated in a very small volume, so the α-particle can get extremely close to all of it at once. Rutherford's own remark was that it was as incredible as firing a shell at tissue paper and having it come back at you.

This asks for your own question, so there is no single correct answer. A good question here should point at something the model in front of you does not settle. Here is a model answer you can adapt.

Sample answer: “Sir, your model says the electrons revolve around the nucleus like planets around the Sun. But a revolving electron is accelerating, and an accelerating charge should radiate energy and spiral into the nucleus within a fraction of a second. Since atoms clearly do not collapse, what do you think is holding the electron up?”

This is a strong question because

• it accepts what the evidence has established (the nucleus is real);
• it attacks the part of the model that goes beyond the evidence (the planetary orbits);
• it is exactly the gap that Niels Bohr filled two years later, in 1913, with the idea of stationary states.

Other good questions you could ask:

• “How did you calculate the diameter of the nucleus from the fraction of particles that were deflected?”
• “If the nucleus contains several protons, all positive, why does their mutual repulsion not blow it apart?” (Rutherford could not have fully answered this — the neutron and the nuclear force came later.)
• “Would the experiment have given the same result with a foil of a light metal such as aluminium instead of gold?”