II · THE DISCOVERY · HISTORY OF SCIENCE

Electron

1897 · Joseph John Thomson · Cavendish Laboratory

Before 1897, the atom was thought to be indivisible. Thomson's discovery of the electron showed that atoms themselves had parts, and one of those parts could move.

At a glance

Discovered: 1897, at the Cavendish Laboratory of Experimental Physics
Discoverer: Joseph John Thomson
What it is: A subatomic particle with negative electric charge
Named after: Amber — the Greek word for it is electron

Look closer

The charge is exactly one unit
The electron's electric charge is negative one elementary charge — meaning it carries the smallest unit of negative charge that exists in nature. That precision matters because it means every electron in the universe has exactly the same charge as every other. The elementary charge is a fixed quantity, and the electron defines what negative one of those quantities looks like.
It is much lighter than the rest of the atom
An electron has almost two thousand times less mass than a proton or neutron, the other particles that make up atoms. That enormous difference in mass means electrons can move and rearrange much more easily than the heavy nucleus at the centre of an atom. The lightness is why electrons are the mobile part of matter — the part that flows as electricity, forms chemical bonds, and responds to voltage.
Named after amber, not discovered in it
The electron is named after amber because the Greek word for amber is elektron, and amber was known in antiquity for producing static charge when rubbed. The name connects the particle discovered in 1897 to an electrical phenomenon observed thousands of years earlier, though the particle itself was not found inside amber.

The story

The electron is a subatomic particle — a piece of matter smaller than an atom — with a negative electric charge. Every electron carries exactly one elementary charge, the smallest unit of charge that exists, and every electron has the same charge and the same mass as every other.

Atoms are made of electrons, protons, and neutrons, but the electrons are far lighter than the other two. An electron has almost two thousand times less mass than a proton or neutron, which means it can move and shift position much more easily. In an atom, electrons occupy regions called atomic orbitals around the positively charged nucleus at the centre. The arrangement of those electrons — which orbitals they occupy and how much energy they have — determines the atom's chemical properties: what it will bond with, how it will react, what colour it might produce.

Electrons are bound to the nucleus, but not all equally tightly. The outermost electrons, called valence electrons, are the least tightly held. These are the ones that form chemical bonds between atoms, creating molecules and crystals. They are also the ones transferred or shared in chemical reactions. The inner electrons, closer to the nucleus, make up the atomic core and are much harder to dislodge.

In metals, the outermost electrons are not bound to any single atom. They are delocalised — free to move through the material — and that mobility is what makes metals conduct electricity and heat so well. In semiconductors, the number of mobile charge carriers — electrons and also holes, which are gaps where an electron could be — can be adjusted precisely by adding impurities, changing temperature, applying voltage, or exposing the material to radiation. That fine control is the basis of modern electronics.

Electrons can also exist as free particles, not bound to any atom. In a vacuum, free electrons can be accelerated to high speeds, focused into beams, and used in devices like cathode ray tubes, electron microscopes, and electron beam welding. Particle accelerators can accelerate electrons to very high energies, and when those electrons are deflected they emit synchrotron radiation — intense light used in research.

Joseph John Thomson discovered the electron in 1897 at the Cavendish Laboratory of Experimental Physics. The name comes from the Greek word for amber, elektron, because amber produces static charge when rubbed — a phenomenon known for millennia before anyone knew what a charge carrier was.

Why it mattered then

Before Thomson's discovery in 1897, the atom was considered indivisible — the smallest unit of matter. Finding the electron showed that atoms themselves had internal structure, that they were made of smaller parts, and that one of those parts could be removed and moved around. That was a fundamental shift in how matter was understood. The discovery also explained electricity. For the time, there was a physical object — a particle — that could be identified as the carrier of electric charge. Current in a wire was not an abstract fluid or force but the movement of electrons through the material. Static electricity, the spark from a doorknob, the glow in a discharge tube: all were the behaviour of the same particle. Within a few years, the electron became central to the emerging model of the atom. Experiments on how electrons scatter, how they orbit the nucleus, and how they absorb and emit light led to quantum mechanics in the decades that followed.

Why it matters now

The electron is the particle that makes almost all modern technology work. Every electronic device — every computer, phone, sensor, and circuit — operates by controlling the movement of electrons through semiconductors. The ability to adjust how many electrons can move, and where, by doping silicon or applying voltage is what allows a transistor to switch on and off, and transistors are the building blocks of all digital logic. Chemistry is the behaviour of electrons. A chemical bond forms when atoms share or transfer valence electrons. A reaction happens when electrons rearrange. The colour of a substance, its conductivity, its reactivity: all are determined by the configuration and energy of its electrons. Understanding the electron made it possible to predict and design chemical behaviour rather than simply catalogue it. Electron beams are used to image materials at scales far smaller than light microscopes can resolve, because electrons can be focused to much finer points. Electron microscopy is a standard tool in biology, materials science, and nanotechnology. Synchrotron radiation from accelerated electrons is used to study the structure of proteins, crystals, and complex materials. The electron is also one of the fundamental particles in the standard model of particle physics — an elementary particle, meaning it is not made of smaller parts. Its charge, mass, and quantum properties are constants that appear throughout physics, and measuring those properties more precisely has been a continuing effort for more than a century.

The surprising detail

The electron is named after amber — the Greek word elektron — because amber was known in antiquity to produce static charge when rubbed with cloth. The phenomenon was observed for thousands of years before anyone knew what was moving or why. When Thomson discovered the particle responsible for electric charge in 1897, the name connected a laboratory measurement to an effect that had been noticed, without explanation, since ancient times. The particle and the ancient observation were finally linked by a word.

What is disputed

The reference material describes the electron as an elementary particle, meaning it is not made of smaller parts, but that is a statement from current theory rather than a direct observation from 1897. Thomson's discovery showed that the electron was smaller than an atom and carried charge; the claim that it is truly elementary rests on later evidence and the standard model of particle physics, which the verified facts do not describe.

Remember this

The electron is the particle that carries electric charge and forms chemical bonds. It was discovered in 1897, and it is in every atom.

Test yourself

Why does the enormous difference in mass between an electron and a proton matter for how materials behave?

Go deeper

Image: IAEA Imagebank, CC BY-SA 2.0. Licence: CC BY-SA 2.0. Source.

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