A Level Physics 9702: Evidence from Alpha-Particle Scattering (Structured)

In an experiment based on the original alpha-particle scattering investigation, a narrow beam of $\alpha$ -particles is directed at a thin sheet of gold foil in an evacuated chamber. A detector that can be rotated around the foil counts the number of $\alpha$ -particles scattered through different angles.

Three key observations are made:

Observation 1: the great majority of $\alpha$ -particles pass straight through the foil with little or no deflection.
Observation 2: a small fraction of the $\alpha$ -particles are deflected through large angles, some greater than $90°$ .
Observation 3: a very small fraction of the $\alpha$ -particles are deflected back through angles close to $180°$ , almost directly back towards the source.

(a) State the nuclear model of the atom that these observations led scientists to propose. [2]

(b) Explain what Observation 1 shows about the structure of a gold atom. [2]

(c) Explain what Observations 2 and 3 show about the nucleus of a gold atom, referring to both its size and its charge. [3]

Part (a): The nuclear model of the atom

These observations led to the nuclear model: an atom consists of a tiny, dense, positively charged nucleus at its centre, containing almost all of the atom’s mass, surrounded by a much larger region of mostly empty space through which orbiting electrons move.

Part (b): What Observation 1 shows

The great majority of $\alpha$ -particles travel straight through the gold foil with little or no deflection. For this to happen, an $\alpha$ -particle must be passing through the atom without coming close to anything that exerts a significant force on it.

This shows that most of the volume of a gold atom is empty space. There is nothing there dense enough or charged enough to deflect a fast-moving $\alpha$ -particle.

Part (c): What Observations 2 and 3 show about the nucleus

A small number of $\alpha$ -particles are deflected through large angles, and a very small number are deflected back through angles close to $180°$ .

Since $\alpha$ -particles are positively charged, only a strong electrostatic repulsion from another concentration of positive charge could push (or turn back) an $\alpha$ -particle so strongly. This shows that the positive charge of the atom is concentrated, rather than spread out as in the earlier “plum pudding” model, i.e. the nucleus is positively charged.
Because large-angle and backward deflections are so rare, only a very small fraction of the incoming $\alpha$ -particles can be passing close enough to this concentration of charge to be affected by it. This shows that the region containing this charge (and, since momentum must be reversed, most of the atom’s mass) occupies only a tiny fraction of the total volume of the atom. The nucleus is very small compared with the atom as a whole.

Final answers

(a) A tiny, dense, positively charged nucleus (containing almost all the atom’s mass) surrounded by mostly empty space with orbiting electrons.
(b) Observation 1 shows that most of an atom’s volume is empty space.
(c) Observations 2 and 3 show that the nucleus is positively charged (from the repulsion needed to deflect positive $\alpha$ -particles) and very small compared with the atom as a whole (from how rare such deflections are).