The Electron: Discovery, Charge, Mass and Why It Matters

If chemistry had a main character, it would be the electron. Protons decide which element an atom is, but electrons decide almost everything an atom does: how it bonds, what colour it glows, whether it conducts electricity, and whether it reacts violently or not at all. This article tells the story of how the electron was discovered and explains the properties that make it so important.

The electron at a glance

Property	Value
Symbol	e⁻
Charge	−1.602 × 10⁻¹⁹ C (relative charge −1)
Mass	9.109 × 10⁻³¹ kg (about 1/1836 of a proton)
Location in atom	outside the nucleus, in orbitals
Discovered	1897, J.J. Thomson
Size	no measurable size; treated as a point particle

For how electrons compare with protons and neutrons, see protons, neutrons and electrons.

Discovery: the cathode ray experiments

In the late 1800s, physicists were fascinated by cathode rays: mysterious glowing beams produced inside glass tubes from which most of the air had been pumped out, when a high voltage was applied between two metal plates. The rays travelled from the negative plate (the cathode) to the positive one. But what were they?

In 1897, the English physicist J.J. Thomson carried out a series of experiments that answered the question:

1. He showed that cathode rays were deflected by electric fields towards the positive plate, so they must carry a negative charge.
2. They were also bent by magnetic fields.
3. By balancing the electric and magnetic deflections, he measured the charge-to-mass ratio (e/m) of the particles.

The ratio was enormous, about 1,800 times larger than that of a hydrogen ion. Either the particles carried a huge charge, or they had a tiny mass. Thomson argued for the tiny mass. Crucially, he got the same value no matter which metal the cathode was made from, or which gas was in the tube. So these particles must be a universal part of all atoms.

Thomson called them “corpuscles”; the name electron (suggested earlier for a unit of electric charge) soon took over. This was the first subatomic particle discovered, and it proved that atoms aren’t indivisible. Thomson received the Nobel Prize in Physics in 1906. See the history of atomic models.

Measuring the charge: Millikan’s oil drops

Thomson had measured only the ratio of charge to mass. In 1909, the American physicist Robert Millikan measured the charge itself.

He sprayed tiny oil droplets between two horizontal metal plates. Some droplets picked up extra electrons and became charged. By adjusting the voltage on the plates, he could make a droplet hang motionless, with the upward electric force exactly balancing gravity. From the droplet’s size and the voltage, he calculated its charge.

The key result: every droplet’s charge was a whole-number multiple of one tiny value, about 1.6 × 10⁻¹⁹ coulombs. Charge comes in fixed packets, and this smallest packet is the charge on one electron.

Combining Millikan’s charge with Thomson’s charge-to-mass ratio gave the electron’s mass: about 9.1 × 10⁻³¹ kg.

How light is an electron?

• About 1/1836 the mass of a proton.
• A hydrogen atom is 99.95% proton by mass.
• In a typical atom, electrons contribute less than 0.03% of the mass.

That’s why electrons are ignored in the mass number. See mass number explained.

Where electrons are in an atom

After Ernest Rutherford showed in 1911 that atoms have a tiny positive nucleus, the obvious question was: where are the electrons?

• Bohr’s model (1913): electrons move in fixed circular orbits (energy levels) around the nucleus. It explained the line spectrum of hydrogen and introduced the idea that electron energies are quantised.
• Quantum mechanical model (1920s onwards): electrons don’t follow orbits. Instead, they occupy orbitals, three-dimensional regions where there’s a high probability of finding them. Orbitals are grouped into shells and subshells (s, p, d, f).

For practical chemistry, the idea of shells holding up to 2, 8, 18, … electrons is often enough, while electron configuration rules and quantum numbers describe the full picture.

Why electrons control chemistry

Bonding

Chemical bonds are made of electrons:

• Ionic bonds: electrons are transferred from one atom to another, forming ions.
• Covalent bonds: pairs of electrons are shared between atoms.
• Metallic bonds: outer electrons are delocalised across a lattice of metal ions.

See ionic vs covalent bonds and metallic bonding.

Reactivity

The number of valence (outer) electrons determines how an atom reacts. Sodium, with one outer electron, loses it easily; chlorine, with seven, gains one readily. Noble gases, with full outer shells, barely react. See how to find valence electrons.

Redox

Oxidation is loss of electrons; reduction is gain of electrons. Every battery, every rusting nail and every breath you take involves electrons moving between substances. See oxidation and reduction.

Colour and light

When electrons absorb energy, they jump to higher energy levels; when they fall back, they emit light of particular colours. This explains flame test colours, neon signs, fireworks and the colours of many compounds.

Electricity

An electric current in a metal wire is a flow of electrons. Metals conduct because their outer electrons are free to move. See best conductor of electricity.

Particle and wave

In 1924, Louis de Broglie proposed that electrons, like light, can behave as waves. This was confirmed in 1927 when beams of electrons were shown to produce diffraction patterns when passed through crystals, just as X-rays do.

This wave nature explains why electrons in atoms can only have certain energies (standing waves fit only certain patterns), and it’s put to practical use in the electron microscope, which uses electron waves with wavelengths far shorter than visible light to image tiny structures.

Electrons in the wider world

• Beta radiation is a stream of fast electrons emitted from nuclei when a neutron turns into a proton. See alpha, beta and gamma radiation.
• Static electricity comes from electrons transferred between surfaces by friction.
• Electronics (the clue is in the name) is the technology of controlling electron flow in devices such as transistors. See semiconductors explained.
• Positrons are the antimatter counterparts of electrons, with the same mass but a positive charge, used in PET medical scans.

Common misconceptions

• “Electrons are tiny balls orbiting the nucleus.” They’re better described by orbitals, regions of probability.
• “Electricity is electrons racing through wires.” Individual electrons drift slowly (millimetres per second); it’s the electrical signal that travels almost at the speed of light.

Key takeaways

• The electron was the first subatomic particle discovered, by J.J. Thomson in 1897 using cathode rays.
• Millikan’s oil-drop experiment measured its charge: 1.6 × 10⁻¹⁹ C.
• Electrons are about 1,836 times lighter than protons and are found in orbitals around the nucleus.
• Electrons control bonding, reactivity, redox, colour and electrical conduction.
• Electrons behave as both particles and waves, a property used in electron microscopes.