Finding the electron, the proton and the neutron

A glowing tube

A neon sign glows when electricity passes through gas at a very low pressure. The first experiments on the inside of the atom used the same idea.

Scientists pumped nearly all the air out of a glass tube. At each end there was a metal plate, called an electrode. They joined the plates to a source of very high voltage, about $10\,000\ \text{V}$. The tube glowed, and a stream of something travelled from the negative plate (the cathode) to the positive plate (the anode). These were the cathode rays.

What the cathode rays are

Many experiments showed the same things:

• The rays travel in straight lines. A metal cross in the way makes a sharp shadow.
• The rays turn a small paddle wheel. So they carry momentum, and they are made of particles.
• The rays bend in an electric field toward the positive plate. So the particles carry negative charge.
• The rays behave in the same way whatever gas is in the tube and whatever metal the cathode is made of.

The last point is the important one. If the same particle comes out of every gas and every metal, then that particle is a part of every atom. J. J. Thomson named it the electron.

Thomson measures the charge to mass ratio

Thomson sent the rays through an electric field and a magnetic field at the same time. He adjusted the two fields until the rays went straight. This gave the speed of the particles. Then the bend in the electric field alone gave the ratio of charge to mass:

This number is large. For comparison, the ion of hydrogen in electrolysis has a charge to mass ratio of only $9.58 \times 10^{7}\ \text{C kg}^{-1}$. The electron has about $1840$ times more charge for each kilogram. Either the electron has a much larger charge, or it is much lighter. Which one is true?

Millikan measures the charge

In 1909, Robert Millikan sprayed tiny drops of oil between two horizontal plates. Some drops picked up an electric charge. He adjusted the voltage until a drop stayed still in the air, so that the force of the electric field balanced its weight. From this he could find the charge on each drop.

He found that the charge on every drop was a whole number multiple of one value:

So charge comes in small equal pieces, and the electron has one piece. Now the mass follows:

The electron is about $1840$ times lighter than a hydrogen atom. Its charge is the same size as the charge of a proton.

You should find that the voltage jumps in steps as the charge changes, never smoothly: charge comes in whole pieces.

The proton

An atom is neutral. If it has negative electrons, it must have some positive charge. In 1886, Goldstein used a tube with holes in the cathode. He found rays that moved the other way, in the direction of the cathode, and passed through the holes. These were called canal rays. They are positive ions of the gas in the tube.

The charge to mass ratio of these rays depends on the gas, and it is largest for hydrogen. The positive particle of hydrogen is the proton.

The neutron

Helium has two protons, yet an atom of helium is four times as heavy as an atom of hydrogen. Something with mass, and with no charge, had to be inside the atom.

In 1932, James Chadwick fired alpha particles at beryllium. A very penetrating radiation came out. It was not bent by electric or magnetic fields, so it has no charge. It knocked protons out of paraffin wax, and from the way they moved, Chadwick worked out its mass. It is about the same as the mass of a proton. This was the neutron.

The neutron was found late because it has no charge, and our instruments of that time found particles by their charge.

The three particles

Practise with new numbers

Try it in Python

This program finds the mass of the electron from the two measurements.

Remember these

• Cathode rays are streams of electrons. They bend toward the positive plate, and they are the same for every gas and metal.
• Thomson found $e/m_e = 1.759 \times 10^{11}\ \text{C kg}^{-1}$. Millikan found $e = 1.602 \times 10^{-19}\ \text{C}$.
• The mass of the electron is $9.109 \times 10^{-31}\ \text{kg}$, about $\dfrac{1}{1840}$ of the mass of a proton.
• The proton was found in canal rays. The neutron was found by Chadwick in $1932$.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. Thomson found the same charge to mass ratio with hydrogen, air and neon in his tube. What did he conclude?

2. An oil drop holds $6.408 \times 10^{-19}\ \text{C}$. How many extra electrons does it carry?

3. Canal rays bend the opposite way to cathode rays in an electric field. What does that tell you?

4. What did Chadwick fire at beryllium to find the neutron?