What is Electronics

A phone, a radar set, a car's engine computer and a solar inverter look nothing alike, but they all work by doing one thing very well: controlling the movement of tiny particles called electrons. To understand electronics, it helps to start where electrons live, inside the atom, and build up one step at a time.

No background is assumed: every technical word is explained the first time it appears. We go from the atom, to the outer electrons called valence electrons, to why copper conducts electricity and glass does not, then to current, voltage and resistance, and finish with what electronics is and where you find it.

1. The atom: what everything is made of

Matter is anything that has mass and takes up space: the air, a copper wire, you. All ordinary matter is made of atoms. A substance made of only one kind of atom is called an element; copper, silicon and oxygen are elements.

An atom is built from three kinds of smaller particles: protons, neutrons and electrons.^[1]

Electric charge

Electric charge is a basic property of some particles that makes them push or pull on each other. There are only two types, called positive and negative. Like charges repel (push apart) and unlike charges attract (pull together), and the force gets weaker as the charges get farther apart.^[2]

• A proton carries one unit of positive charge.
• An electron carries one unit of negative charge, exactly the same size as the proton's, just opposite in sign.
• A neutron carries no charge at all (it is neutral).^[1]

That single unit of charge is called the elementary charge, written e. Charge is measured in coulombs (symbol C), the standard international unit of charge. Since 2019 the elementary charge has been fixed by definition at exactly 1.602 176 634 × 10⁻¹⁹ coulomb.^[3]​^[4] Turned around, one coulomb is the combined charge of about 6.24 × 10¹⁸ electrons (6.24 billion billion).^[4]

The nucleus and the electrons around it

Protons and neutrons are packed together in a tiny core at the center of the atom called the nucleus. Electrons occupy the much larger space around it. Protons and neutrons are far heavier than electrons, so the nucleus holds almost all of the atom's mass, while the electrons take up almost all of its volume.^[1] A proton weighs about 1,836 times as much as an electron.^[5]

Values from OpenStax and NIST.^[1]​^[5]

The scale is hard to imagine. An atom is about 10⁻¹⁰ meter across (one ten-billionth of a meter), and its nucleus is roughly 100,000 times smaller still. If the nucleus were the size of a blueberry, the whole atom would be about the size of a football stadium.^[1]

The number of protons in the nucleus is the atomic number, and it decides which element the atom is: every atom with 14 protons is silicon, and every atom with 29 is copper. A normal atom is electrically neutral, so it has as many electrons as protons. An atom that has gained or lost electrons has a net charge and is called an ion.^[1]

The electrons are held by the attraction between their negative charge and the positive nucleus. Keep one idea from this section: electrons close to the nucleus are held tightly; electrons far out are held more loosely.

2. Shells and orbitals: where the electrons live

Energy and energy levels

Energy is the capacity to do work, that is, to make something move, heat up or change. It is measured in joules (J). For a sense of size, lifting a small 100-gram apple one meter straight up takes about one joule.

An electron in an atom cannot have just any amount of energy. It can only have certain allowed values, called energy levels. To move to a higher level it must take in exactly the right amount of energy, and it gives that same amount back when it drops down again.^[6]

Shells

The energy levels are grouped into shells, numbered n = 1, 2, 3 and so on, counting outward from the nucleus. This number is called the principal quantum number, or simply the shell number. The higher the shell number, the farther from the nucleus the electrons tend to be, the higher their energy, and the more loosely they are held.^[7]​^[8]

Each shell can hold at most 2n² electrons:^[7]

Subshells and orbitals

Inside each shell, electrons are arranged in subshells labeled s, p, d and f, and each subshell is made of one or more orbitals. An orbital is not a track that the electron runs around. It is a three-dimensional region around the nucleus where the electron is likely to be found, more like a cloud than a racetrack.^[7] The s orbitals are spherical, p orbitals are dumbbell-shaped, and d and f orbitals have more complicated shapes.^[7]

No orbital can hold more than two electrons. This rule is called the Pauli exclusion principle.^[7] The subshells contain 1, 3, 5 and 7 orbitals, so they hold at most 2 (s), 6 (p), 10 (d) and 14 (f) electrons.^[7]​^[8] A shell n has n² orbitals in total, and two electrons per orbital gives the 2n² rule above.^[7]

Electrons fill the lowest-energy places first, though not strictly shell by shell (in larger atoms 4s fills before 3d, for example).^[8] That is why copper's 29 electrons sit 2, 8, 18 and 1 in four shells.

A note on the "planet" picture

In 1913 Niels Bohr pictured electrons circling the nucleus in fixed orbits, like planets around the sun. His model got the energy levels of hydrogen right, but it failed for helium, the next simplest atom, and it was built on the idea of precise orbits, which turned out to be wrong.^[6]​^[9] The quantum model, with its orbitals, replaced it. The drawings below still use the Bohr-style picture because it is a handy way to count electrons per shell; real electrons are not little balls on circular tracks.

3. The valence shell and valence electrons

The valence shell is an atom's outermost occupied shell, the one with the highest shell number. The electrons in it are its valence electrons. All the electrons in the inner, filled shells are called core electrons.^[8]

Why the valence electrons are the ones that matter

Valence electrons are the farthest from the nucleus and have the highest energy. On top of that, the core electrons sit between them and the nucleus and partly cancel the nucleus's pull, an effect called shielding.^[8] So valence electrons are held most loosely, and they are the ones most easily lost or shared with other atoms.^[8]

So they are the electrons that interact with the outside world. Chemical reactions come down to the loss, gain or sharing of valence electrons, and elements in the same column of the periodic table behave alike because they have the same number of them.^[8] As the next section shows, they also decide whether a material conducts electricity.

Full shells and "eight is enough"

The noble gases (helium, neon, argon and the others in the last column of the periodic table) have completely filled outer shells. For all of them except helium, that means eight valence electrons. In 1916 the chemists Gilbert N. Lewis and Walther Kossel proposed that this "octet" of eight is an especially stable arrangement, and that other atoms tend to gain, lose or share electrons to reach it.^[9] The noble gases themselves rarely react, which is why they were once called "inert." That label is not absolute, though: in 1962 chemists showed that xenon can form compounds.^[9]

When two atoms share a pair of electrons and are both attracted to that shared pair, the link between them is called a covalent bond.^[10] Covalent bonds come up again with silicon.

Four examples

Electron arrangements from OpenStax Chemistry 2e.^[8]

Copper has a single electron in its outer shell (written 4s¹) sitting outside completely filled inner shells.^[8] That lone electron is only loosely held, and in a piece of copper metal it breaks free: physics textbooks model copper wire as having about one free electron per atom.^[18] (Chemists sometimes count copper's ten 3d electrons as valence electrons too, because they can take part in chemical reactions.^[8] For carrying electric current, the single outer electron is the one that matters.)

Silicon and germanium each have four valence electrons: two in an s subshell and two in a p subshell.^[8] Four is exactly half of a full octet, and that halfway position is the key to semiconductors.

Neon has a full outer shell, with nothing it readily gains, loses or shares.^[8]​^[9]

4. Why valence electrons decide: conductor, insulator or semiconductor

• A conductor lets electric charge move through it easily (copper, aluminum, silver).
• An insulator almost completely blocks it (glass, quartz, most plastics).
• A semiconductor is in between, and its ability to conduct is very sensitive to temperature, light and tiny amounts of impurity atoms.^[11]

The gap between the extremes is enormous: good conductors such as aluminum conduct at least a hundred trillion (10¹⁴) times better than insulators such as glass.^[11]

Most solids are crystals: their atoms sit in a regular pattern that repeats in all three directions, a framework called a lattice.^[11] What the valence electrons do once their atoms are locked into a lattice decides which kind of material you get.

Metals: a "sea" of shared electrons

In a metal, the loosely held valence electrons of neighboring atoms overlap so thoroughly that they stop belonging to any one atom. They become delocalized, free to wander through the whole piece of metal: a lattice of positive atomic cores (nucleus plus core electrons) in a "sea" of shared electrons.^[12] Push electrons into one end of a wire and electrons come out the other end, because free electrons are already everywhere in it.^[12] (Chemists note the "sea" is a simplified model; the band picture below is the one physicists use to compare all three kinds of material.^[12]​^[13])

Insulators: every electron has a job

In an insulator the valence electrons are tightly bound, in full shells or in bonds between particular atoms. Nothing is left over to move, and tearing an electron loose takes a lot of energy. Most solid substances are insulators.^[13]

Silicon: four electrons, four neighbors

In a silicon crystal each atom has four neighbors and shares one of its four valence electrons with each of them, forming four covalent bonds.^[11] Every valence electron is busy holding the crystal together, so at very low temperatures pure silicon barely conducts.^[11]

At room temperature the atoms vibrate with heat, and now and then a vibration shakes an electron out of its bond, leaving it free to move. The empty spot left behind is a hole. A neighboring electron can hop into the hole, which moves the hole elsewhere, so a hole acts like a positive charge moving through the crystal.^[11] Pure silicon still conducts poorly, but unlike an insulator, some electrons do break free.^[11]

Energy bands, in brief

An isolated atom has sharp, separate energy levels. When huge numbers of atoms are packed into a crystal, those levels spread into broad ranges of allowed energy called energy bands.^[11]

• The valence band is the highest band filled with electrons (at low temperature); its electrons are tied up in bonds.
• The conduction band is the next band up; an electron that reaches it can move freely and carry current.
• The band gap is the range of energies between them that electrons in the crystal cannot have.^[11]​^[13]

Band gaps are measured in electron volts (eV), a tiny energy unit suited to single electrons: 1 eV = 1.60 × 10⁻¹⁹ joule.^[15] (The name comes from the volt, explained in section 5.)

In a conductor the valence and conduction bands overlap, so valence electrons are effectively free already. In a semiconductor the gap is small enough that heat, light or other energy can lift some electrons across. In an insulator the gap is so large that almost none get across.^[13]

Band gaps shift slightly with temperature, so published values differ in the second decimal place.^[14]

Doping: the doorway to diodes and transistors

Replacing a tiny fraction of the silicon atoms with another element, called doping, changes its conductivity enormously: about 10 boron atoms per million silicon atoms can make it conduct roughly a thousand times better.^[11]

• n-type (negative): add atoms with five valence electrons, such as phosphorus or arsenic. Four form bonds; the fifth is left over as a free electron.^[11]​^[16]
• p-type (positive): add atoms with three valence electrons, such as boron. One bond is an electron short, which creates a hole.^[11]​^[16]

Put p-type and n-type regions side by side and you get a p-n junction, discovered in silicon in 1940 and the basis of junction transistors and solar cells.^[17] Diodes, transistors and integrated circuits are all built from carefully arranged doped regions like these.^[24]​^[28]

Coming next: the next article in this section, Conductors, Insulators, and Semiconductors, covers band theory and doping in depth.

5. Free electrons, current, voltage and resistance

Free electrons

In a metal, some electrons are far enough from their nuclei that the pull on them is weak. These free electrons are not bound to any one atom.^[18] Even with nothing connected, they zip around in nearly random directions, bouncing off atoms like molecules in a gas, so their overall flow in any one direction adds up to zero.^[18]

Current: how much charge flows

Connect a battery and it sets up an electric field in the wire, a push on every charge in it, and the free electrons begin to drift in one direction while still bouncing around.^[18]​^[19] Electric current (symbol I) is the rate at which charge flows past a point:^[18]

I = ΔQ / Δt   (charge that passes ÷ time it takes)

Current is measured in amperes (A, or "amps"). One ampere is one coulomb per second,^[18] roughly six billion billion electrons passing each second.^[4]

The electrons themselves move astonishingly slowly. Their average forward speed, the drift velocity, is only about 0.45 millimeter per second (roughly 1.6 meters per hour) in 12-gauge copper household wire carrying 20 amperes.^[18] Yet a lamp lights the instant you flip the switch, because the signal travels at around 10⁸ meters per second, a large fraction of the speed of light. Each charge pushed into the wire pushes on the charges ahead of it, which push on the ones farther along.^[18] The electrons already in the lamp start moving almost at once.

Which way does current flow?

By convention, current is drawn in the direction positive charge would move: out of a battery's positive terminal, through the circuit, and back to the negative terminal. This is conventional current.^[18] In a metal wire the moving charges are negative electrons, so they actually drift the opposite way.^[18] The convention dates to Benjamin Franklin in the 1700s, who named the two kinds of charge long before electrons were known.^[18] Both descriptions give the same answers if you stay consistent; most textbooks and circuit diagrams use conventional current, while some technical training teaches electron flow. Figure 3 shows both.

Voltage: the energy behind the push

A rock at the top of a hill has stored energy that it releases as it rolls down; stored energy of position is called potential energy. Charges have electrical potential energy in the same way, depending on where they sit in an electric field.^[15] Voltage, properly called potential difference, is the change in potential energy per unit of charge between two points, measured in volts (V):^[15]

1 volt = 1 joule per coulomb

So every coulomb that travels around a circuit from one terminal of a 9-volt battery to the other delivers 9 joules of energy along the way. A voltage is always measured between two points.^[15] (The electron volt is simply the energy one electron gains crossing 1 volt.^[15]) Batteries, generators and wall outlets are voltage sources: their potential difference creates the electric field in the conductor that pushes charges along. Voltage is the cause; current is the effect.^[19]

Resistance: what holds the current back

As electrons drift along, they keep colliding with the atoms of the lattice and hand them some of their energy, which is why wires and components warm up.^[18] This opposition to current is resistance (R), measured in ohms (Ω); one ohm is one volt per ampere.^[19] Resistance depends on:^[20]

• Material: each material has a resistivity (ρ, "rho"), its built-in resistance regardless of shape. Copper's, 1.72 × 10⁻⁸ ohm-meter, is lower than that of any common metal except silver.
• Length: a longer wire means more collisions, so more resistance.
• Cross-sectional area: a thicker wire has less resistance.
• Temperature: in metals, hotter atoms vibrate more and cause more collisions, so resistance rises.

For a uniform wire, R = ρL/A (length L, cross-sectional area A).^[20] One meter of 12-gauge copper wire (about 3.31 square millimeters) works out to only about 0.005 ohm.^[18]​^[20] Semiconductors go the other way with temperature: heat frees more electrons and holes, so they conduct better when warm.^[11]​^[13]

Ohm's law

Georg Simon Ohm (1787–1854) showed by experiment that the current through a metal wire is proportional to the voltage across it: double the voltage, double the current.^[19] With resistance included, that is Ohm's law:^[19]

V = I × R   (voltage = current × resistance)

Worked example. Connect a 450-ohm resistor (a component made to have a set resistance) across a 9-volt battery, ignoring the small resistance of the wires and battery:

I = V / R = 9 V / 450 Ω = 0.02 A = 20 milliamperes (mA)

That is about 1.25 × 10¹⁷ electrons every second (0.02 ÷ 1.602 × 10⁻¹⁹).^[3]

Power (P) is the rate at which energy is delivered, measured in watts (W), one joule per second. For electricity, P = V × I.^[21] In the example, 9 V × 0.02 A = 0.18 W, which the resistor gives off as a little warmth.

Ohm's law is an observed rule, not one that holds everywhere. Many materials follow it closely, but many important electronic parts do not.^[19] A diode, for example, passes current easily one way and hardly at all the other. Parts like that are what make electronics possible.

6. So, what is electronics?

Encyclopaedia Britannica defines electronics as the branch of physics and electrical engineering that deals with the emission, behavior and effects of electrons, and with electronic devices.^[22] In practical terms, electronics is controlling the flow of electrons with components, above all semiconductor devices such as diodes and transistors, to process signals, information and power. A signal is a voltage or current that changes over time to carry information: the audio in a radio broadcast, the echo in a radar receiver, the ones and zeros in a computer.

Electrical versus electronic

Ordinary electrical wiring mostly moves energy around: wires, switches and breakers carry power to lights, heaters and motors, and the current changes only when someone flips a switch. Electronic devices let one electrical signal control another. In a vacuum tube, small voltage changes on a wire grid control a much larger current.^[22] In the first transistor, the voltage on one contact controlled the current through another, amplifying the signal up to 100 times.^[25] That ability to amplify (make a weak signal stronger) and switch (turn current on and off under electrical control) is the heart of electronics.

The line can blur. A home solar inverter is power equipment built from electronics: transistors switch very rapidly to turn the panels' one-way direct current (DC) into the back-and-forth alternating current (AC) the grid uses. The U.S. Department of Energy calls such devices power electronics.^[30]

A short history

• 1904: John Ambrose Fleming builds a two-electrode vacuum tube, the "Fleming valve," and uses it to detect wireless signals.^[23]
• 1906: Lee De Forest adds a control grid, creating the Audion, a tube that can amplify radio signals.^[22] Tubes go on to enable radio broadcasting, long-distance telephony, television and early computers such as ENIAC (1946), but they are fragile, wear out and need time to warm up.^[22]
• World War II: microwave radar receivers need detectors faster than vacuum tubes allow. Millions of silicon and germanium crystal diodes are made for Allied radar, and researchers learn to purify and dope both elements.^[24]
• December 1947: John Bardeen and Walter Brattain at Bell Labs demonstrate the first transistor, a germanium point-contact device.^[25] With William Shockley they share the 1956 Nobel Prize in Physics.^[26]
• 1958–1959: Jack Kilby at Texas Instruments builds a working circuit whose transistor, resistor and capacitor are all formed from semiconductor material (September 1958),^[27] and Robert Noyce at Fairchild files a patent on a practical way to build complete circuits on a single silicon chip (1959).^[28] This is the integrated circuit, or "chip." Kilby shares the 2000 Nobel Prize in Physics.^[29]

Since the early 1960s silicon has dominated; silicon devices make up more than 95 percent of all semiconductor products sold.^[11]

Where electronics is used

Semiconductor devices are the key parts of most electronic systems for communications, signal processing, computing and control.^[11] A few examples:

• Phones and computers: processors and memory are integrated circuits, alongside radios for Wi-Fi, Bluetooth and cellular.
• Cars: engine control, anti-lock brakes, airbags, sensors and infotainment.
• Medicine: patient monitors, ultrasound and MRI imaging, pacemakers and hearing aids.
• Radar and aviation: radar helped launch the semiconductor era,^[24] and aircraft depend on electronic radios, navigation aids, weather radar and instruments. This site's Research Projects section has deep dives into several radar and navigation systems.
• Energy: solar inverters and battery systems that convert and manage power.^[30]
• Home automation: smart sensors, switches and small computers; see this site's Home Assistant and Microcontrollers sections.

The whole story in brief

1. Atoms have a positive nucleus surrounded by negative electrons in shells; the farther out, the more loosely held.
2. The outermost, valence electrons are the ones that interact with the world.
3. How tightly they are held (the band gap) makes a material a conductor, insulator or semiconductor, and doping tunes semiconductors precisely.
4. Voltage pushes free electrons into a slow drift we measure as current; resistance opposes it: V = I × R.
5. Electronics uses semiconductor devices to amplify, switch and process signals, information and power.

Sources

All links were checked in October 2026. External links open in a new tab.

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