In 1911 New Zealander Ernest Rutherford performed an important experiment.
He took a piece of polonium metal (Po), which emits alpha particles (α), and placed it in a lead box with a small hole.
Alpha particles passed through other lead plates through holes in their center. Then they went through a very thin blade (10-4mm) of gold (Au).
Rutherford adapted a zinc sulfide (fluorescent) movable bulkhead to record the path the particles traveled.
The physicist observed that most alpha particles crossed the gold blade and only a few deviated, or even receded.
From these results, he concluded that the atom was not a positive sphere with electrons plunged into this sphere. Concluded that:
- the atom is a huge void;
- the atom has a very small nucleus;
- the atom has positive nucleus (+), as alpha particles deviate sometimes;
- The electrons are around the nucleus (in the electrosphere) to balance the positive charges.
Rutherford's atomic model then suggested an atom with circular orbits of electrons around the nucleus. He compared the atom to the solar system, where the electrons would be the planets and the nucleus would be the sun.
Today, the atom is known to be 10,000 to 100,000 times larger than its nucleus. On a macroscopic scale, one can compare an atom with a football stadium.
If the atom were the Maracana stadium, its nucleus would be an ant in the center of the field. So the atom is huge relative to its nucleus.
However, the Rutherford atom has some flaws. If the atomic nucleus is made up of positive particles, why do these particles not repel each other and the nucleus does not collapse?
If the particles are of opposite charges, why don't they attract? The electrons would gradually lose energy by spiraling toward the nucleus and, as it did so, would emit energy in the form of light.
But how do electrons move around the nucleus without atoms collapsing?
These questions were answered in 1932 by James Chadwick. He observed that the radioactive beryllium (Be) nucleus emitted particles with no electric charge and a mass equal to that of the protons (+). He called this particle neutrons.
Then came the third subatomic particle. Now we know that in the nucleus of the atom there are protons and neutrons and in the electrosphere there are electrons.
Then this relationship was established:
In the table above, it can be seen that the electron is 1,836 times smaller than the mass of a proton.