Mass of an electron

The mass of an electron is 9.109 × 10⁻³¹ kg (9.109 383 7139 × 10⁻³¹ kg in the 2022 CODATA values). That is about 1/1836 of the mass of a proton. In atomic mass units it is 0.000 549 u, and its rest energy is 0.511 MeV. For most calculations, 9.11 × 10⁻³¹ kg is accurate enough.

The mass of an electron in different units

The value isn't exact: it is measured, and the last digits carry a tiny uncertainty. The values above are the 2022 CODATA recommended values published by NIST.

Electron, proton and neutron compared

A proton has 1,836 times the mass of an electron, and a neutron about 1,839 times. At GCSE you usually just need to know that the electron's relative mass is very small compared with the proton and neutron. That is why, as AQA's GCSE Chemistry specification says, nearly all of an atom's mass is in its nucleus. At A-level, AQA's Physics specification asks for the charge and mass of each particle in both SI units and relative units.

Check you can tell the particles apart by their mass and charge, then press Mark to check your answer in the course.

How small is the electron's share of an atom's mass?

In a hydrogen atom, one proton and one electron, the electron is about 0.05% of the mass. In a carbon-12 atom, the six electrons have a total mass of 6 × 9.109 × 10⁻³¹ = 5.47 × 10⁻³⁰ kg. The whole atom is 12 u, or 1.99 × 10⁻²⁶ kg, so the electrons make up only about 0.03% of it.

That is why the mass number of an atom counts only protons and neutrons: the electrons barely change the total. The Particles course shows the three particles side by side and goes on to quarks and leptons, the family the electron belongs to.

Specific charge of the electron

The specific charge of a particle is its charge divided by its mass, and AQA's A-level Physics specification includes it for the proton and the electron. The electron's charge is −1.60 × 10⁻¹⁹ C, so its specific charge has a size of:

That is 1.76 × 10¹¹ C kg⁻¹. Because the electron is so light, its specific charge is about 1,836 times bigger than the proton's, even though their charges are the same size. It also works the other way: measure e/m in an experiment, measure e separately, and you can work out the electron's mass.

The electron's rest energy

Mass and energy are linked by E = mc², so particle physicists often give masses as energies. The electron's rest energy is 0.511 MeV. The positron, the electron's antiparticle, has exactly the same mass and rest energy but the opposite charge. When an electron and a positron annihilate, their rest energies turn into two photons, so at least 2 × 0.511 = 1.022 MeV is released. AQA's A-level specification asks you to compare particle and antiparticle masses and rest energies in MeV, and to know the energies in annihilation and pair production.

Worked examples

Example 1: how many electrons make a gram?

1 g is 1 × 10⁻³ kg. Divide by the mass of one electron:

It would take about 1.1 × 10²⁷ electrons to make a single gram.

Example 2: the mass of a mole of electrons

A mole is 6.022 × 10²³ particles. So a mole of electrons has a mass of 6.022 × 10²³ × 9.109 × 10⁻³¹ = 5.49 × 10⁻⁷ kg, about 0.55 mg. A mole of protons is about 1 g.

Example 3: comparing with a proton

How many times heavier is a proton? 1.673 × 10⁻²⁷ ÷ 9.109 × 10⁻³¹ ≈ 1,836. If the electron were the mass of a grape, around 8 g, the proton would be about 15 kg.

Common mistakes

• Wrong power of ten. It is 10⁻³¹ kg, not 10⁻²⁷ kg, which is the proton's.
• Mixing kg and g. 9.11 × 10⁻³¹ kg is 9.11 × 10⁻²⁸ g.
• Saying electrons have no mass. Their mass is very small, not zero. A photon is the particle with zero mass.
• Adding electrons to the mass number. The mass number counts protons and neutrons only.

To practise atoms and particles in a short timed paper, try today's GCSE Physics Higher or A-level Physics: Year 1 paper. Each paper mixes topics.

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