The mass of one electron is 9.109 × 10⁻³¹ kg (more precisely 9.1093837 × 10⁻³¹ kg). The same value is 9.109 × 10⁻²⁸ g, 5.486 × 10⁻⁴ atomic mass units (u), or 0.511 MeV/c² in energy units. An electron is about 1,836 times lighter than a proton.
This is the electron’s rest mass, written me. It is one of the fundamental physical constants, and the values below are the internationally recommended CODATA figures published by NIST.
Mass of an electron in different units
| Unit | Mass of one electron | Where you will use it |
|---|---|---|
| Kilogram (kg) | 9.1093837 × 10⁻³¹ kg | SI calculations in physics |
| Gram (g) | 9.1093837 × 10⁻²⁸ g | Chemistry and CGS problems |
| Atomic mass unit (u or amu) | 5.4858 × 10⁻⁴ u | Comparing with atomic and nuclear masses |
| Energy equivalent (MeV/c²) | 0.51099895 MeV/c² | Particle and nuclear physics |
| Energy equivalent (joules) | 8.187 × 10⁻¹⁴ J | E = mc² calculations |
| Relative to a proton | 1 / 1836.15 | Quick comparisons |
For school and first-year engineering problems, 9.1 × 10⁻³¹ kg and 9.11 × 10⁻³¹ kg are both accepted roundings. Use more digits only when the question gives other data to that precision.
How the unit conversions work
Kilograms to grams: multiply by 1,000. So 9.109 × 10⁻³¹ kg × 10³ = 9.109 × 10⁻²⁸ g.
Kilograms to atomic mass units: 1 u = 1.66054 × 10⁻²⁷ kg, so me = 9.109 × 10⁻³¹ ÷ 1.66054 × 10⁻²⁷ = 5.486 × 10⁻⁴ u.
Mass to energy: using E = m c², E = 9.109 × 10⁻³¹ × (2.998 × 10⁸)² = 8.187 × 10⁻¹⁴ J. Dividing by 1.602 × 10⁻¹³ J per MeV gives 0.511 MeV. That is why particle physicists simply say “the electron mass is 0.511 MeV”.
How was the mass of the electron measured?
Nobody weighed an electron directly. Its mass was worked out from two separate measurements:
- J. J. Thomson (1897) deflected cathode rays with electric and magnetic fields and measured their charge-to-mass ratio, e/m. The modern value is 1.7588 × 10¹¹ C/kg.
- Robert Millikan (1909 onwards) measured the charge of the electron with his oil-drop experiment, e = 1.602 × 10⁻¹⁹ C.
Dividing the charge by the charge-to-mass ratio gives the mass: m = e ÷ (e/m) = 1.602 × 10⁻¹⁹ ÷ 1.7588 × 10¹¹ = 9.11 × 10⁻³¹ kg.
Today the most precise values come from Penning traps, which hold a single electron (or an ion with one electron) in electric and magnetic fields and measure how fast it circles. Those frequencies can be measured so precisely that the electron mass is known to about 11 significant figures.
Electron vs proton vs neutron
| Particle | Mass (kg) | Mass (u) | Charge |
|---|---|---|---|
| Electron | 9.109 × 10⁻³¹ | 0.000549 | −1.602 × 10⁻¹⁹ C |
| Proton | 1.6726 × 10⁻²⁷ | 1.00728 | +1.602 × 10⁻¹⁹ C |
| Neutron | 1.6749 × 10⁻²⁷ | 1.00866 | 0 |
Because the electron is so light, almost all of an atom’s mass sits in its nucleus. In a hydrogen atom the single electron accounts for only about 0.05% of the total mass. This is why chemists usually ignore electrons when they add up atomic masses.
Solved examples
1. Mass of one mole of electrons. One mole contains 6.022 × 10²³ electrons. Mass = 6.022 × 10²³ × 9.109 × 10⁻³¹ kg = 5.486 × 10⁻⁷ kg, or about 0.55 mg. A whole mole of electrons weighs less than a grain of salt.
2. Number of electrons in 1 kg. N = 1 ÷ 9.109 × 10⁻³¹ = 1.098 × 10³⁰ electrons.
3. Speed of an electron accelerated through 100 V. Kinetic energy gained = eV = 1.602 × 10⁻¹⁹ × 100 = 1.602 × 10⁻¹⁷ J. From ½ m v² = eV, v = √(2eV/m) = √(2 × 1.602 × 10⁻¹⁷ ÷ 9.109 × 10⁻³¹) ≈ 5.93 × 10⁶ m/s. That is about 2% of the speed of light, so the non-relativistic formula is fine here.
Rest mass vs “relativistic mass”
The value 9.109 × 10⁻³¹ kg is the rest mass, the mass of an electron that is not moving relative to you. Older textbooks talk about a “relativistic mass” that grows with speed, γme. Modern physics keeps the mass fixed and puts the speed dependence into energy and momentum instead: E = γmec² and p = γmev. Either way, the number quoted as “the mass of an electron” is always the rest mass.
Common mistakes students make
- Writing 9.1 × 10⁻³¹ g instead of kg. In grams the exponent is −28.
- Using 1.6 × 10⁻¹⁹ (the electron’s charge) where the mass is needed. They are different quantities.
- Saying the electron has “no mass”. It is very light, not massless.
- Forgetting that 0.511 MeV is an energy equivalent; the full unit is MeV/c².
Frequently asked questions
What is the mass of one electron in kg?
The mass of one electron is 9.109 × 10⁻³¹ kg. The CODATA recommended value is 9.1093837 × 10⁻³¹ kg.
What is the mass of an electron in grams?
It is 9.109 × 10⁻²⁸ g, because one kilogram is 1,000 grams.
What is the mass of an electron in amu?
The electron mass is 5.486 × 10⁻⁴ u, which is roughly 1/1823 of one atomic mass unit.
How many times heavier is a proton than an electron?
A proton is about 1,836 times heavier than an electron (the exact ratio is 1836.15).
What is the mass of an electron in MeV?
Its rest energy is 0.511 MeV, so physicists write the electron mass as 0.511 MeV/c².