Question

A beam of alpha-particles is fired at a piece of gold foil as shown in the image. After striking the gold foil in which direction do most of the alpha-particles travel?

• A. $P$
• B. $Q$
• C. $R$
• D. $S$

Answer 1

Answer: (A)

$P$

Alpha particles are positively charged particles made up of two protons, two neutrons, and zero electrons. Due to the fact that protons have a +1 charge and neutrons hold no charge. This would give the particle a +2 charge overall.
Originally, Rutherford thought that particles would fly straight through the foil. However, he found that the particles would be shifted or deflected when passing through the foil. This is due to the fact that like charges repel each other.
As the positively charged alpha particle would fly through the foil, it would come in proximity with a positively charged nucleus of an atom.
Thus we can conclude that the particle travels in $P$ direction.

Ernest Rutherford performed an experiment to know the arrangement of electrons within an atom. In this experiment, fast-moving alpha particles were made to fall on a thin gold foil(100 nm thickness). The $\alpha$-particles are doubly charged helium ions. Since they have a mass of 4 u, the fast-moving $\alpha$-particles have a considerable amount of energy. It was expected that $\alpha$-particles would be deflected by subatomic particles in the gold atom. Since $\alpha$-particles were much heavier than protons, he did not expect to see large deflections.

But, the $\alpha$-particle scattering experiment gave totally unexpected results.
The following observations were made:
(i) Most of the fast-moving -particles passed straight through the gold foil.
(ii) Some of the -particles were deflected by the foil by small angles.
(iii) Surprisingly one out of every 12000 particles appeared to rebound.

Following are the observations which conclude the presence of the nucleus at the center of the atom:

1) Some of the alpha rays are deflected back with some angle. (presence of nucleus)

2) Very few alpha particles are bounced back with zero-degree angles. (Alpha rays hit the center of the nucleus)

Hence option A,C,DA,C,D are correct.