What is an electron? The particle behind electricity

An electron is a fundamental particle, one with no known smaller parts, that carries a negative electric charge and has about 1/1836 of the mass of a proton. Electrons make up the outer part of every atom. How they are arranged decides how an atom bonds with others, and when they move through a wire they are what an electric current is made of. Almost everything chemical or electrical comes down to what electrons do.

In short

• The electron has a fixed charge of −1.602 × 10⁻¹⁹ coulombs and a mass of 9.109 × 10⁻³¹ kg, and as far as anyone can measure it has no size and no parts.
• In an atom, electrons occupy shells and orbitals, and the outermost ones, the valence electrons, decide how the atom bonds.
• One ampere of current is about 6.24 billion billion electrons passing a point each second, even though each one drifts along a wire slower than a snail.

What exactly is an electron?

The electron belongs to a family of particles called leptons, and in the Standard Model of particle physics it is elementary: experiments that probe it at the smallest distances reached have found no structure inside it. Protons and neutrons, the particles in an atom's nucleus, are by contrast made of quarks. The electron's key numbers, as fixed by the scientific committee CODATA and published by the US National Institute of Standards and Technology (NIST), are:

Property	Value	What it means
Charge	−1.602176634 × 10⁻¹⁹ coulombs, exactly	The smallest free unit of charge, called e; a proton carries +e, equal to it as far as any experiment can tell
Mass	9.109 × 10⁻³¹ kg	About 1/1836 of a proton's mass
Spin	½	A built-in quantum property that makes each electron act like a tiny magnet
Size	None measured	It behaves as a point in every experiment so far

Since 2019, the charge has been exact by definition: the International System of Units now fixes the value of e, and the ampere, the unit of current, is defined from it. The electron also has an antimatter twin, the positron, with the same mass and the opposite charge.

How was the electron discovered?

In 1897 J. J. Thomson, at the Cavendish Laboratory in Cambridge, was studying cathode rays, the glowing beams that flow through a glass tube emptied of air when a high voltage is applied. He bent the rays with magnets and, once he had pumped out nearly all the gas, with an electric field as well. From how much they bent, he worked out the ratio of their mass to their charge and found it more than a thousand times smaller than for a charged hydrogen atom, the lightest thing known.

Thomson concluded that the rays were "charges of negative electricity carried by particles of matter," and that these particles were far smaller than any atom and the same whatever metal or gas he used. He called them corpuscles; the name electron, coined earlier for the unit of charge, stuck instead. It was the first particle smaller than an atom ever found, and it showed that atoms have parts. Thomson received the Nobel Prize in Physics in 1906. In 1909 the American physicist Robert Millikan measured the charge on its own by watching charged oil droplets float between metal plates, which, combined with Thomson's ratio, gave the electron's mass.

Where are the electrons in an atom?

Electrons are held near the nucleus by the electric attraction between their negative charge and the protons' positive charge. They don't orbit like planets. Each electron occupies an orbital, a region of space shaped by quantum mechanics where it is likely to be found, and orbitals are grouped by energy into shells.

The rules of filling work like seats in a theater, cheapest first:

1. Each orbital holds at most two electrons, and those two must have opposite spins. This is the Pauli exclusion principle.
2. The first shell has one orbital, so it holds 2 electrons. The second has four orbitals and holds 8.
3. Higher shells hold more (18, then 32), but they overlap in energy, so the outer shells of the lighter elements tend to stop at 8 before the next shell begins.

Take sodium, with 11 electrons: 2 in the first shell, 8 in the second, and 1 left over in the third. That single outer electron is loosely held, which is why sodium gives it up so easily. Chlorine, with 17, has 2, 8 and 7, one short of a full outer shell. The outermost electrons are called valence electrons, and how many an atom has, and how tightly it holds them, decides what it bonds with. That is the subject of the chemical bond note, and it is also why the columns of the periodic table behave alike.

The orbital picture also explains light. When an electron drops from a higher-energy orbital to a lower one, it gives off a photon whose energy equals the difference, so each element glows with its own set of colors. The green of the northern lights is oxygen atoms doing exactly this after being struck high in the atmosphere.

How many electrons flow in an electric current? A worked example

An electric current is a flow of charge, and in a metal wire the charge is carried by electrons. A current of 1 ampere means 1 coulomb of charge passing a point every second. How many electrons is that?

1. One electron carries 1.602 × 10⁻¹⁹ coulombs.
2. Electrons per second = 1 coulomb ÷ 1.602 × 10⁻¹⁹ coulombs = 6.24 × 10¹⁸.
3. That is about 6 billion billion electrons every second, a little more than the current a 100-watt bulb draws from a 120-volt US outlet.

Now the surprise. How fast does each electron move? Copper has about 8.5 × 10²⁸ free electrons in every cubic meter. In a wire 1 square millimeter in cross-section (10⁻⁶ m²) carrying 1 ampere, the average drift speed is the current divided by the number of electrons per meter of wire times their charge: 1 ÷ (8.5 × 10²⁸ × 10⁻⁶ × 1.602 × 10⁻¹⁹), which comes to about 0.07 millimeters a second, or roughly 26 centimeters an hour.

A light still comes on the instant you flip the switch because the wire is already full of electrons. Closing the circuit sets up an electric field along the whole wire at a large fraction of the speed of light, and every electron starts drifting at once, like water in a full hose. The energy travels quickly; the electrons themselves barely move. That is the core idea of electricity.

Is the electron a tiny ball?

No, and the picture fails in a precise way. An electron has a definite mass and charge, but it also behaves as a wave. Fire electrons one at a time through two narrow slits and they build up a striped interference pattern that only waves produce, which quantum mechanics describes as a wave of probability. The electron microscope uses this: the wavelength of the fast electrons in it can be about 100,000 times shorter than visible light's, so it resolves details far smaller than any light microscope can.

So an electron is neither a ball nor a ripple but a quantum object that shows particle-like or wave-like behavior depending on how it is measured. The tiny-planet image is a good way to remember that electrons surround the nucleus, and a poor guide to anything else about them.

Why do electrons matter outside physics?

Chemistry is mostly the study of where electrons go. A chemical reaction moves electrons between atoms or into new shared arrangements, and the energy released or absorbed comes from how those arrangements differ. Living cells harvest energy from food by passing electrons along a chain of molecules. Every computer chip is a set of switches that steer electrons, and every solar panel works by light knocking electrons loose.

Check yourself

• Without looking back, explain how Thomson could tell that cathode rays were made of particles far smaller than atoms.
• Why does sodium give up an electron so readily, while chlorine takes one?
• If electrons drift along a wire at centimeters an hour, why does a lamp light the moment the switch is closed?

Sources

• Elementary charge — NIST, CODATA 2022
• Electron mass — NIST, CODATA 2022
• Proton-electron mass ratio — NIST, CODATA 2022
• Ampere: Introduction — National Institute of Standards and Technology
• The Discovery of the Electron: 1897 experiments — American Institute of Physics
• J. J. Thomson, Nobel Prize in Physics 1906 — Nobel Prize Outreach