Discovery of Subatomic Particles

Introduction: Is the Atom Really Indivisible?

For more than two thousand years, philosophers and chemists believed that the atom was the smallest, simplest unit of matter — a tiny solid sphere that could not be cut into anything smaller. The Greek word "atomos" itself means "uncuttable". John Dalton, in 1808, repeated the same idea in his atomic theory and used it as the foundation of modern chemistry.

But by the end of the nineteenth century, experiments with electricity and gases at low pressure began producing strange glowing rays that seemed to come out of atoms. If atoms could throw out smaller pieces of themselves, then they could not really be indivisible. This part of the chapter follows the journey of three remarkable discoveries — the electron, the proton, and the neutron — that opened up the inside of the atom.

8.1 Are Atoms Divisible?

Dalton's theory pictured the atom as a tiny solid ball with no internal parts. The theory worked beautifully for explaining the laws of chemical combination, but it gave no explanation for why matter behaves the way it does when electricity passes through it. By the late 1800s scientists were studying the conduction of electricity through gases at very low pressure inside sealed glass tubes. The bright glow they saw inside these tubes — and the way the glow shifted when magnets or electric fields were placed nearby — could only be explained if atoms themselves contained charged particles. The first of these particles to be identified was the electron.

8.2 Discovery of the Electron — J.J. Thomson, 1897

J.J. Thomson worked with a discharge tube (also called a cathode-ray tube). It was a glass tube fitted with two metal electrodes and connected to a high-voltage battery. When most of the air inside was pumped out and a few thousand volts were applied across the electrodes, a stream of glowing rays appeared to travel from the cathode (negative electrode) towards the anode (positive electrode). These were called cathode rays.

What Thomson observed

• The rays travelled in straight lines from the cathode to the anode.
• They cast sharp shadows of any solid object placed in their path, showing they consisted of particles.
• When a small paddle wheel was placed in the tube, the wheel rotated — proving that the rays carried both mass and momentum.
• When a magnetic field or an electric field was applied, the rays were deflected. Crucially, they always bent towards the positive plate, which meant the particles in the rays carried a negative charge.
• The same kind of rays appeared no matter which gas was inside the tube or which metal was used as the cathode. The particles must therefore be common to all matter.

Thomson named these particles electrons and concluded that the electron is a fundamental constituent of every atom. He measured the ratio of charge to mass (\(e/m\)) of the electron and found that the electron is extraordinarily light — far lighter than even the lightest atom (hydrogen).

8.3 Discovery of the Proton — Goldstein, 1886

Even before Thomson had finished his work on the electron, the German physicist Eugen Goldstein had been experimenting with discharge tubes that had a perforated cathode. When the tube was switched on, he saw not only the usual cathode rays going in one direction, but also a faint stream of rays going in the opposite direction — passing through the holes in the cathode. He called these anode rays or canal rays.

Properties of anode rays

• They were deflected towards the negative plate of an electric field, showing they were positively charged.
• The mass of the particles in these rays depended on the gas filled in the tube. The lightest particles appeared when the gas was hydrogen.
• The hydrogen-derived positive particle was named the proton. It became clear that this was a fundamental positively charged particle present in atoms.

The proton has a charge that is exactly equal in magnitude to that of the electron but opposite in sign. Its mass, however, is roughly 1836 times that of an electron — practically the same as the mass of a hydrogen atom (1 unified atomic mass unit, or 1 u).

8.4 Discovery of the Neutron — Chadwick, 1932

By 1920, physicists realised something was wrong with the masses of atoms. A helium atom, for instance, has only 2 protons but its mass is roughly 4 u, not 2 u. Where was the extra mass coming from? Ernest Rutherford predicted that there must be a neutral particle inside the nucleus to account for it.

The hunt ended in 1932 when James Chadwick bombarded a thin sheet of beryllium with α-particles (helium nuclei) and detected an unknown radiation. Unlike electrons or protons, this radiation was not deflected by electric or magnetic fields, which meant it carried no charge. By measuring how it bounced off other atoms, Chadwick worked out that its mass was roughly the same as that of the proton. He named the new particle the neutron.

8.5 Comparing the Three Subatomic Particles

8.6 Activity — The Magnet and the Cathode Ray

Quick Recap

• The atom is divisible — it contains electrons, protons and neutrons.
• J.J. Thomson (1897) discovered the electron through cathode-ray experiments. Charge \(= -1\), almost no mass.
• E. Goldstein (1886) observed canal rays; the proton was identified as the lightest positive particle. Charge \(= +1\), mass \(\approx 1\) u.
• J. Chadwick (1932) discovered the neutron. Charge \(= 0\), mass \(\approx 1\) u.
• Protons and neutrons sit inside the nucleus; electrons move outside it.