Electrons, protons and neutrons

Is the atom the end?

Dalton said that atoms cannot be divided. For about a hundred years, scientists believed it. Then experiments showed that something much smaller than an atom could be pulled out of it. The atom has parts.

The electron

In 1897, the English scientist J. J. Thomson studied the glow in a glass tube with very little air, when a high voltage is applied. Something travelled from the negative plate to the positive plate. These rays were bent toward a positive plate, so they were negatively charged. Thomson showed that the rays are made of tiny particles that come from the atoms of the metal. These are the electrons.

• The charge of an electron is $-1.6 \times 10^{-19}\ \text{C}$. We call this charge $-1$.
• The mass of an electron is $9.1 \times 10^{-31}\ \text{kg}$. This is only $\dfrac{1}{1836}\ \text{u}$.

The proton

An atom is neutral, so it must also have a positive charge. In 1886, Goldstein found rays that travel the other way in a tube. They were positively charged particles. The lightest of them, from hydrogen, is the proton.

• The charge of a proton is $+1.6 \times 10^{-19}\ \text{C}$. We call this charge $+1$.
• The mass of a proton is $1.67 \times 10^{-27}\ \text{kg}$, which is about $1\ \text{u}$.

The neutron

The atom of helium has two protons, so we expect a mass of $2\ \text{u}$. But the mass is $4\ \text{u}$. Something else, with mass and no charge, must be in the atom. In 1932, James Chadwick found it. It is the neutron.

• The charge of a neutron is $0$.
• The mass of a neutron is about $1\ \text{u}$, almost the same as a proton.

You should find that the electrons of an atom make up far less than a tenth of one per cent of its mass.

A neutral atom

In a neutral atom, the number of electrons is equal to the number of protons. The positive and the negative charges cancel, so the atom has no charge. The atom of hydrogen has one proton and one electron, and in its common form it has no neutron.

Practise with new numbers

Try it in Python

This program lists the three particles with their mass in u and their charge.

Remember these

• Electron: charge $-1$, mass $\frac{1}{1836}\ \text{u}$. Found by Thomson in 1897.
• Proton: charge $+1$, mass $1\ \text{u}$. Found from Goldstein's rays.
• Neutron: no charge, mass $1\ \text{u}$. Found by Chadwick in 1932.
• A neutral atom has equal numbers of electrons and protons.

Check yourself

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

1. Which particle has a mass of about $1\ \text{u}$ and a charge of $+1$?

2. A neutral atom has $11$ protons. How many electrons does it have?

3. Why do the electrons hardly add to the mass of an atom?

4. Who discovered the neutron, and why was it so hard to find?