Chemistry · Structure Of Atom · NEET
| Positive charge location | Thomson: spread evenly through whole atom | Rutherford: packed in one tiny nucleus |
| Alpha result predicted | Thomson: all pass with tiny bends | Rutherford: some bounce straight back |
| Atom structure | Thomson: solid sphere of charge | Rutherford: mostly empty space |
It proved three things. (1) The atom is mostly empty space, because most alpha particles went straight through. (2) All the positive charge and almost all the mass sit in one tiny center, the nucleus, because a few particles bounced back. (3) This nucleus is very small compared to the whole atom. For NEET, remember: the experiment gave us the nuclear model of the atom.
Because the atom is mostly empty space. The tiny nucleus takes up almost no room. Most alpha particles never come near a nucleus, so nothing pushes them off their path and they go straight through. NCERT states the atom radius is about 10^-10 m but the nucleus radius is only about 10^-15 m, so the nucleus is roughly 100,000 times smaller.
An alpha particle is positive. The nucleus is also positive and very heavy. When a rare alpha particle heads almost straight at a nucleus, the two positive charges push each other apart (repulsion, called Coulomb force). Because the nucleus is so heavy and packed, it can push the light alpha particle straight back. Rutherford said this was as surprising as a bullet bouncing off tissue paper.
These particles passed close to a nucleus, but not straight at it. The positive nucleus pushed them, so their path bent a little. The closer they passed, the bigger the bend. Only the ones aimed almost dead-center bounced right back.
Gold can be hammered into an extremely thin sheet (only a few atoms thick). A thin foil means each alpha particle mostly meets just one layer of atoms, so the results are easy to read. Gold is also a heavy atom, so its nucleus is heavy enough to push alpha particles back clearly.
An alpha particle is a helium nucleus. It has 2 protons and 2 neutrons, so it carries a charge of +2 and has a mass of about 4 units. It is positive and heavy, which is why it is used to probe the atom. Do not confuse it with a beta particle (an electron) or a gamma ray (energy).
Thomson's model (plum pudding) said positive charge was spread evenly through the whole atom, with electrons stuck inside like seeds. If that were true, alpha particles would all pass through with only tiny bends. But some bounced back, so charge cannot be spread out. Rutherford concluded the positive charge is squeezed into one tiny nucleus.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Hans Geiger and Ernest Marsden, his students, carried out the actual bombardment of the gold foil with alpha particles.
A circular fluorescent screen coated with zinc sulphide glowed with a tiny flash wherever an alpha particle hit it. By counting flashes at different angles, they measured how the particles scattered.
No. It correctly places a tiny nucleus at the center, but it cannot explain why electrons do not spiral into the nucleus. That failure is called a drawback of the Rutherford model and led to Bohr's model.
Their nuclei are lighter and have less positive charge, so they push alpha particles less. The deflection would be much smaller and far fewer particles would bounce straight back.
Yes. NEET asks the observations, the conclusions (empty atom, tiny dense positive nucleus), and the difference from Thomson's model. It is a common source of assertion-reason and statement-based questions.
