An electron is a negatively charged subatomic particle found in atoms, occupying regions of space around the nucleus called orbitals. Electrons determine much of an atom’s chemical behavior because their arrangement controls how atoms form bonds, exchange energy, and interact with other substances.
In simple terms, electrons help hold atoms together and allow atoms to connect into molecules. They also carry electric current through metals and other conductors, making them central to electricity, electronics, and the study of chemistry and chemical reactions.
For example, a neutral carbon atom contains six electrons, while a neutral oxygen atom contains eight. Their different electron arrangements help explain why carbon and oxygen form different types of chemical bonds, even though both are made of the same basic kinds of subatomic particles.
The Basic Definition of an Electron
An electron is an elementary particle, meaning it has no known smaller internal components. It carries a negative electric charge and has a mass far smaller than that of a proton or neutron. Electrons belong to a family of particles called leptons.
In an atom, electrons are described by quantum mechanics rather than by fixed circular paths. Their positions are represented by probability distributions called orbitals, which describe where an electron is likely to be detected.
Important characteristics of electrons include:
- A negative elementary electric charge.
- A mass of approximately 9.11 × 10⁻³¹ kilograms.
- Quantum behavior described by orbitals and energy states.
- The ability to participate in chemical bonding and electrical conduction.
- An intrinsic quantum property called spin.
Electrons can also exist outside atoms. Free electrons occur in particle beams, plasmas, and other physical systems, so an electron is not defined solely by its location inside an atom.
Where Electrons Are Located in an Atom
Atoms are composed of three main subatomic particles:
- Protons
- Neutrons
- Electrons
These particles differ in charge, mass, and location within the atom. The protons and neutrons form a compact central nucleus, while the electrons occupy a much larger surrounding region.
The Structure of an Atom
Protons have a positive charge and are located in the nucleus.
Neutrons have no charge and are also located in the nucleus.
Electrons have a negative charge and occupy orbitals surrounding the nucleus. The attraction between negatively charged electrons and positively charged protons helps keep electrons bound to atoms.
| Property | Electron | Proton | Neutron |
|---|---|---|---|
| Electric charge | −1 | +1 | 0 |
| Approximate mass | 0.00055 u | 1.0073 u | 1.0087 u |
| Location in an atom | Orbitals around nucleus | Nucleus | Nucleus |
| Main significance | Bonding and chemical behavior | Element identity and nuclear charge | Nuclear stability and isotopes |
The symbol u represents the unified atomic mass unit. Since protons and neutrons are much heavier than electrons, nearly all the mass of an ordinary atom is concentrated in its nucleus.
Important Properties of Electrons
Negative Electric Charge
Every electron has a charge of approximately −1.602 × 10⁻¹⁹ coulombs. A proton has an equal-magnitude positive charge, so an atom with equal numbers of protons and electrons has no overall electric charge.
If an atom gains electrons, it becomes a negatively charged ion called an anion. If it loses electrons, it becomes a positively charged ion called a cation. The number of protons remains unchanged during ordinary ion formation.
Extremely Small Mass
An electron has approximately 1/1836 the mass of a proton. This difference helps explain why electrons can respond readily to electric fields while the nucleus accounts for nearly all an atom’s mass.
Quantum Motion Around the Nucleus
Electrons do not travel around the nucleus in precisely defined planetary orbits. Quantum mechanics describes their behavior using wavefunctions, which provide probabilities for measurements such as position and momentum.
Introductory atomic models sometimes describe electrons as being in constant motion around the atomic nucleus and its electron shells. This is a useful starting picture, but it should not be interpreted as electrons following visible circular tracks. In stationary atomic states, the electron probability distribution can remain unchanged over time.
Electron Shells, Orbitals, and Energy Levels
Electrons in atoms occupy allowed energy states. Chemists organize these states into shells and subshells, which help explain patterns in the periodic table and the chemical behavior of different elements.
The first shell can accommodate up to two electrons, the second up to eight, and the third up to eighteen. However, electrons do not always fill entire shells before entering the next one because the energies of different subshells can overlap.
| Shell number | Maximum electrons | Subshells |
|---|---|---|
| 1 | 2 | 1s |
| 2 | 8 | 2s, 2p |
| 3 | 18 | 3s, 3p, 3d |
| 4 | 32 | 4s, 4p, 4d, 4f |
An orbital can hold a maximum of two electrons with opposite spin states. The arrangement of electrons among orbitals is called an electron configuration. Electrons in the outer occupied region that participate in bonding are called valence electrons.
Worked Example: Counting Electrons in Sodium
Sodium provides a practical example of how electron counts, shell arrangements, and ion formation connect. Its atomic number is 11, meaning every sodium atom has 11 protons.
- Find the atomic number: Sodium has atomic number 11, so its nucleus contains 11 protons.
- Count electrons in the neutral atom: A neutral sodium atom must contain 11 electrons to balance its 11 positive proton charges.
- Arrange the electrons: The first shell contains 2 electrons, the second contains 8, and the third contains 1. The simplified shell arrangement is 2, 8, 1.
- Form a sodium ion: Sodium can lose its single outer electron, producing Na⁺ with 11 protons and 10 electrons.
The charge calculation is +11 − 10 = +1 in elementary charge units. Sodium’s electron configuration changes from 1s² 2s² 2p⁶ 3s¹ to 1s² 2s² 2p⁶ when it forms Na⁺.
This explains why sodium commonly forms a positive ion in compounds such as sodium chloride (NaCl). Chlorine can gain an electron to form Cl⁻, and the oppositely charged ions attract each other to form an ionic solid.
How Electrons Form Chemical Bonds
Chemical bonding depends strongly on how electrons are distributed between atoms. Three commonly taught bonding models are ionic, covalent, and metallic bonding.
Ionic Bonds
Ionic bonding occurs through the electrostatic attraction between positively and negatively charged ions. Electron transfer can produce these ions, as illustrated by sodium and chlorine forming sodium chloride.
Covalent Bonds
In covalent bonding, atoms share electron density. A hydrogen molecule (H₂), for example, contains two hydrogen atoms joined by a shared pair of electrons. Water (H₂O) contains covalent bonds between oxygen and hydrogen.
Metallic Bonds
In metals, some electrons occupy states extending throughout the material rather than belonging exclusively to one atom. These mobile electrons contribute to electrical conductivity and the bonding that holds metallic solids together.
How Electrons Produce Electric Current
Electric current is the rate at which electric charge passes through a surface. In metal wires, mobile electrons carry that charge. When a voltage is applied across a closed circuit, an electric field causes the electrons to develop a small net drift in one direction.
The conventional direction of electric current is opposite to electron drift because conventional current was defined as the direction positive charge would move. In other materials, including electrolytes and semiconductors, charge can also be transported by ions or positively charged carriers called holes.
A current of one ampere means one coulomb of charge passes a point each second. Since each electron carries approximately 1.602 × 10⁻¹⁹ coulombs of charge, one ampere corresponds to about 6.24 × 10¹⁸ electron charges passing a cross-section per second in a conductor where electrons carry the current.
Electric circuits make possible lighting, computers, communication equipment, and household appliances. Their operation depends not just on electrons existing, but on controlling charge movement and energy transfer through suitable materials.
How J. J. Thomson Discovered the Electron
In 1897, physicist J. J. Thomson reported experimental evidence for negatively charged particles smaller than atoms. His work with cathode rays helped establish that atoms were not indivisible objects.
Thomson studied beams produced inside partially evacuated glass tubes. By observing their deflection in electric and magnetic fields, he determined that the particles had a remarkably large charge-to-mass ratio compared with known ions.
He concluded that the particles were constituents of matter rather than features of one particular gas or electrode. Later experiments, including Robert Millikan’s oil-drop measurements, helped determine the electron’s charge, allowing its mass to be calculated.
How Electron Behavior Explains Chemical Reactions
During ordinary chemical reactions, atomic nuclei generally remain unchanged while electrons are redistributed among atoms and molecules. Breaking and forming chemical bonds involves changes in electron arrangements and the energies of the reacting substances.
Electrons also participate in oxidation and reduction reactions. Oxidation involves loss of electrons, while reduction involves gain of electrons when described in terms of electron transfer. These processes are central to batteries, corrosion, and many biological energy-conversion reactions.
Despite their tiny size, electrons play an enormous role in the structure of matter and the functioning of modern technology. Understanding their arrangement allows chemists to explain periodic trends, predict bonding patterns, and investigate how materials respond to light and electricity.
Common Misconceptions About Electrons
Electrons Orbit Like Planets
Planetary drawings are simplified teaching models. Actual electrons in atoms are described by quantum states and probability distributions, not definite circular paths around the nucleus.
Electrons Have No Mass
Electrons have a small but measurable rest mass. They are much lighter than protons and neutrons, but they are not massless particles like photons.
All Electric Current Is Moving Electrons
Electrons carry current in metal wires, but ions can carry current in solutions, and both electrons and holes can contribute in semiconductors. Electric current refers broadly to the movement of electric charge.
Atoms Always Have Equal Numbers of Protons and Electrons
Neutral atoms have equal numbers of protons and electrons. Ions do not: gaining electrons creates a negative ion, while losing electrons creates a positive ion.
Frequently Asked Questions
What is the charge of an electron?
An electron has a charge of approximately −1.602 × 10⁻¹⁹ coulombs, equivalent to −1 elementary charge. Its negative charge is equal in magnitude to the positive charge of a proton.
Where are electrons found in an atom?
Electrons occupy orbitals around the atomic nucleus. These orbitals represent probability distributions rather than fixed paths, and their energies determine how electrons are arranged within an atom.
How many electrons does an atom have?
A neutral atom has the same number of electrons as protons, equal to its atomic number. For example, neutral oxygen has eight electrons and neutral carbon has six. Ions have different electron counts because they have gained or lost electrons.
Are electrons smaller than protons?
Electrons are far lighter than protons, with about 1/1836 of a proton’s mass. Both are treated as elementary particles in the Standard Model, so comparing their physical sizes is not as straightforward as comparing ordinary objects.
Can electrons leave an atom?
Yes. An atom can lose an electron through ionization when sufficient energy is supplied or during certain chemical reactions. The atom then becomes a positively charged ion, while the removed electron may move freely or become associated with another particle.
Electrons connect atomic structure to observable chemical and electrical behavior. Their negative charge, small mass, and quantum-mechanical arrangement explain how atoms bond, how ions form, and how current travels through conductors. Understanding electrons is therefore a foundation for studying chemistry, electricity, and the properties of materials.
Sources: OpenStax, Chemistry 2e; OpenStax, University Physics Volume 3; National Institute of Standards and Technology (NIST), CODATA Recommended Values of the Fundamental Physical Constants; Encyclopaedia Britannica, Electron.
