Lead Up to the Gold Foil Experiment
In the early 1800s, John Dalton presented his atomic theory. He proposed that all matter consists of atoms. He considered the atom to be the smallest indivisible building block of matter. So, an element consists of the same atoms. Compounds are combinations of various atoms. He was mostly right. But it turns out that atoms have smaller components!
The first subatomic particle got discovered in 1897. Do you remember what it was? It was the electron! JJ Thomson performed his famous cathode ray tube experiment. A cathode ray tube is a unique instrument. It consists of a glass tube with a metal electrode at each end. The tube gets evacuated and filled with a low-pressure gas. The most common gas is hydrogen. Why? Because it maintains the correct pressure and voltage. When the electrodes are connected, the current flows through the tube.
Today, we know the current occurs because the hydrogen gas gets ionized. At the time, it was unclear what particle flowed through the tube. Thomson tested the particles' properties by applying external electric and magnetic fields. He noticed that the fields pushed the particle beam. That indicated the particle had a charge! From his experiments, he deduced the mass-to-charge ratio. What did he discover? The mass was way smaller than any known atom! He even used different metals for the electrode. The results were the same! He concluded that he had discovered a negatively charged subatomic particle.
Modeling of the Atom
Before the electron's discovery, we assumed that the atom was indivisible. It was the foundational unit. So, it didn't have smaller units. We didn't need any atomic models. With Thomson's findings, we had to consider the atom's construction. We had two pieces of information. Atoms have a neutral charge. But they have a smaller negatively charged particle. He also knew that electrons were much smaller than atoms. To account for these, Thomson proposed his plum-pudding model. In it, the electrons get embedded within a positively charged sphere. He imagined the sphere as a thick jelly. The electrons distribute within the jelly to minimize their electrostatic repulsion. But the jelly isn't matter! Instead, it's just a charge.
Today, the plum-pudding model seems far-fetched. Maybe you're thinking, "That's what he came up with?!" But based on the available information, it makes sense. It even creates a fascinating geometry problem. What's the best configuration for the electrons? You have to distribute the electrons to minimize the repulsion. But, they need to reside on the surface of a sphere. It's actually very similar to molecular geometry problems. For example, four electrons would form a tetrahedron. That's the same as a central atom bonded to four external atoms. But it predates the VSEPR model.
The Gold Foil Experiment
In 1908, Ernest Rutherford wanted to prove the plum-pudding model. So, he set his graduate students to work. They fired helium nuclei (alpha particles) at thin sheets of gold foil. Based on Thomson's model, the helium nuclei should pass through the gold atoms. The deflection would be minimal. Why? The atom is a sphere of positive charge. It is not solid matter. So, they would pass through one another. Also, the electrons are so small that their effect should be negligible. What about the charges? Shouldn't they repel? Not for high-velocity alpha particles. Thomson's model predicted that the electrostatic repulsion was too weak to influence them.
So, what did they discover? Most of the alpha particles went through the gold foil. Initially, it seemed to confirm the plum pudding model. But soon, they realized a few were getting deflected backward. They quickly realized that the gold atoms must have a dense center of mass. Furthermore, the center of mass must have a positive charge. They called it the nucleus. What about the electrons? They orbit around the positive nucleus. There is empty space between the nucleus and the electrons. That's why some particles passed through the gold foil. But, the ones that collided with the nucleus got deflected. They had developed the nuclear model of the atom!
