# 1.2 Introducing the neutron

So far you know that inside the nucleus are protons.

But there are further particles that can reside there; these have the same mass as protons but carry no charge. They are called neutrons.

Why is the word ‘can’ used above? Which atom does not contain any neutrons in its nucleus?

### Answer

Hydrogen. You’ll recall this has an atomic number of one. So its nucleus can only contain one proton.

The relative charges and relative masses of electrons, protons and neutrons are shown in Table 1. Notice how tiny electrons are compared to protons and neutrons.

## Table 1 A comparison of the relative charge, and relative mass of electrons, protons and neutrons

Subatomic particle | Relative charge | Relative mass
--- | --- | ---
Electron | -1 | 1/1836
Proton | +1 | 1
Neutron | 0 | 1

So in addition to an atomic number, atoms are also defined by their mass, or more precisely what is know as their mass number, where.

Mass number = number of protons + number of neutrons.

And elements can be represented as follows:

Where A is the mass number and Z the atomic number.

Looking again at hydrogen (Z=1); adding in the mass number(A), its chemical symbol now becomes indicating its atoms contain one proton and one electron.

OpenLearn - Discovering chemistry
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