Animated Solution for Chemistry - Atomic Structure: If the Thomson model of the atom was correct, then the result of Rutherford's gold foil experiment would have been

Imagine you are holding a watermelon. In the late 19th century, J.J. Thomson proposed that an atom looked exactly like that. He envisioned the atom as a continuous, diffuse cloud of positive charge—the red, fleshy part of the watermelon—with tiny, negatively charged electrons embedded within it like black seeds. This was the famous Plum Pudding Model.

Now, let's conduct a thought experiment. What if we were to fire a barrage of tiny, high-speed bullets at this atomic watermelon? This is exactly what Ernest Rutherford set out to do using $\alpha$-particles.

Before we look at the collision, we must understand our projectile. An $\alpha$-particle is essentially a Helium nucleus, denoted as ${}_2^4{\text{He}}^{2+}$. It is incredibly heavy compared to an electron—about four times the mass of a proton—and it carries a positive charge of $+2e$.

Because these particles are so massive and are fired at very high velocities, they possess a tremendous amount of kinetic energy and momentum. They are the heavy artillery of the subatomic world.

So, what happens when these heavy, fast-moving $\alpha$-bullets hit the diffuse positive cloud of the Thomson atom?

Since the positive charge in Thomson's model is spread out evenly over the entire volume of the atom, there is no dense, massive center to stop the $\alpha$-particles or bounce them back. The $\alpha$-particles will simply plow right through the atom, much like a cannonball passing through a cloud of smoke.

However, there is a subtle catch. The $\alpha$-particles are positively charged, and the atom's diffuse cloud is also positively charged. As the $\alpha$-particles pass through, they will experience a gentle, continuous electrostatic repulsion from the positive sphere.

This weak repulsive force is nowhere near strong enough to reverse the path of the massive $\alpha$-particles. But it will do two things:

1. Small Deflections: The continuous lateral push will nudge the $\alpha$-particles slightly off their straight-line trajectories, causing them to emerge at small angles of deflection. 2. Reduced Speed: Pushing through this repulsive positive field requires the $\alpha$-particles to do work against the electrostatic force. This drains some of their kinetic energy, meaning they will emerge on the other side with a reduced speed.

Therefore, if Thomson's model were the absolute truth, Rutherford would have observed all the $\alpha$-particles passing through the gold foil, but with slight deflections and a noticeable drop in their speed. This perfectly aligns with option (d).

Of course, we know history played out very differently. When Rutherford actually performed the experiment, he was astounded to find that while most particles went straight through, a tiny fraction bounced right back at large angles!

As Rutherford himself famously said, it was as if you fired a 15-inch shell at a piece of tissue paper and it came back and hit you. This single observation shattered the Thomson model forever and proved the existence of a tiny, incredibly dense, positively charged nucleus at the center of the atom.