This is how the foundation of "quantum mechanics" was born

The word atom was invented by the ancient Greeks, meaning "indivisible". Some ancient Greek philosophers believed that everything was made up of indivisible atoms, but because ancient Greek physics was only in its infancy and there was no way to verify it, this view remained only in philosophy.

At the end of the 18th century, chemists discovered that the mass before and after a chemical reaction is constant, and the ratio of reactants participating in a chemical reaction is also constant. For example, if red powder is obtained when iron and oxygen react, then there must be 42 parts of oxygen for every 100 parts of iron. If it is black powder, then there must be 28 parts of oxygen for every 100 parts of iron. This shows that there is some kind of invisible basic unit during chemical reactions, and all reactions can only be carried out in multiples of this basic unit. Chemist John Dalton referred to the naming of the ancient Greeks and named this basic unit "atom", which was the birth of scientific atomic theory.

However, are atoms in the modern sense indivisible as the ancient Greeks believed? In 1897, physicist Thomson created a type of ray with an electric charge. Thomson believed that this ray was not an electromagnetic wave, but was composed of some tiny particles. He measured the mass of this particle and found that it was very light, one thousand eight hundredth of the smallest atom - hydrogen atom. So Thomson believed that atoms were divisible, and this charged particle was the component of atoms. Later generations named it electron.

Electrons carry a negative charge, but atoms are neutral. How can negatively charged electrons form neutral atoms? At that time, some physicists believed that the structure of atoms was like the solar system, with electrons orbiting around the periphery and positive charges in the center. This was the so-called planetary model of atoms. However, Thomson pointed out that unlike planets, electrons are charged, and circular motion releases energy, making atoms unstable. Therefore, he proposed another model: atoms are a uniform mass of positive charges, dotted with individual electrons, like raisins embedded in bread. The charges of the two cancel each other out, making the atom as a whole electrically neutral.

This model explained the stability of atoms and had an overwhelming advantage at the time, but a few years later it encountered a greater difficulty.

Ernest Rutherford, a student of Thomson, discovered that radioactive atoms can spontaneously emit rays. He divided these rays into three categories, the first of which was called alpha rays. He believed that alpha rays were also composed of tiny particles, and wanted to use the same method as his teacher to measure the weight of the particles. However, he found that these particles would fly around when they hit the air, making it impossible to measure accurately.

Thomson encountered this problem when he measured electrons, and his solution was to evacuate the air. Rutherford also measured the weight of particles by evacuating the air, but he found that the particles were very heavy, thousands of times heavier than electrons.

This is strange. Electrons are very light, so it is understandable that they are bounced off atoms in the air. Alpha rays are so heavy that it should be easy for them to pass through the atoms in the raisin bread model, so why are they bounced off? It seems that there is some serious problem with the raisin bread model.

Physicists in the early 20th century had no way to directly see the internal structure of atoms. However, there were indirect ways, and one of them was to see what happened when particles hit atoms. The atoms in the air are spread all over the space and can bounce everywhere, which makes the result a mess. But if the experimental conditions are changed to allow the rebound to occur only at a specified time and place, it is possible to solve this mystery.

Rutherford then conducted a series of experiments. He pressed the metal into a very thin layer, placed it in a vacuum environment, and then hit it with alpha rays. The results showed that most of the alpha rays passed directly through, a small amount was deflected, and in rare cases, a huge rebound of more than 90 degrees occurred.

How to understand this phenomenon? If the atom has a uniform charge, there will not be such a big difference in the results. Therefore, Rutherford believed that the positive charge in the atom is not spread out like the raisin bread model, but is highly concentrated in a small area inside the atom to form a nucleus. If the alpha ray is far away from the nucleus, it will pass directly through; if it hits the nucleus, it will be bounced off at a large angle. As for the electrons, they are distributed on the periphery. In this way, he proved with experiments that the planetary model is correct.

Although this evidence is strong enough to completely overturn the raisin bread model, it also forced physicists to face the original problem of the planetary model: electrons orbiting the nucleus should be unstable, so why are they actually stable? Quantum mechanics was born in the process of solving this problem. Without Rutherford, half of modern physics would have been born many years later.