Relative Masses of Protons, Neutrons and Electrons

One of the things that you need to know about protons, neutrons and electrons is their relative masses. We will start by looking at their actual masses in grams, and then use these to work out their relative masses. You do not need to learn the actual masses, but starting with them will help you to understand why relative mass is so useful.

The actual masses are very small numbers

Protons and neutrons both have the same amount of mass. A single proton or neutron has a mass of roughly 1.67 x 10^-24g.

This number is written in standard form, which is a way of writing very small or very big numbers so that they take up less space. You need to be familiar with standard form for GCSE chemistry, so if you haven’t already learnt about it, make sure you do at some point.

When the mass of a proton or neutron is written out in full, rather than in standard form, it is 0.00000000000000000000000167g.

The mass of an electron is roughly 9.1094 x 10^-28g. Written out in full this is 0.00000000000000000000000000091094g.

To summarise, the actual masses of the three particles are as follows:

As you can see, these are not easy numbers to work with because they are extremely small. This is where relative mass comes in. In chemistry, most of the time we are more interested in how the masses of the different particles compare to each other, than what the masses actually are. Therefore, we can use relative mass and avoid having to work with these difficult numbers.

Protons and neutrons have a relative mass of 1

Since the masses of the proton and the neutron are the same, we give the proton and the neutron both a relative mass of 1. Remember that there are no units for relative mass, so it is not 1 of anything. The proton’s relative mass is just 1 and the neutron’s relative mass is also just 1.

Electrons have a relative mass of 0.0005

The actual mass of the electron is about 2000 times smaller than the actual mass of the proton or neutron. Therefore the electron’s relative mass should be about 2000 times smaller than the relative mass of the proton or neutron. Therefore the electron’s relative mass can be found by dividing 1 by 2000, which gives 0.0005.

The electron’s mass is negligible

The mass of the electron is so much smaller than that of the proton and the neutron that in many situations it can just be ignored, since it has very little effect on anything.

For example, later in the course we will be interested in the masses of atoms (which are made of protons, neutrons and electrons). Since electrons have so much less mass than protons and neutrons, almost all of the mass of an atom comes from the protons and neutrons. The amount of mass that an atom gets from its electrons is so tiny that for most purposes we can just ignore it.

In other words, the electron’s mass is negligible (which means so small that it can be ignored). Since the electron’s mass is negligible compared to the mass of the proton and the neutron, its relative mass is often written as 0. This is not quite as precise as using 0.0005, but for many purposes it makes no difference and is more convenient.

The relative masses of protons, neutrons and electrons are shown below:

The relative masses of protons, neutrons and electrons

Flashcards

Flashcards help you memorise information quickly. Copy each question onto its own flashcard and then write the answer on the other side. Testing yourself on these regularly will enable you to learn much more quickly than just reading and making notes.