Atom

An atom is the smallest constituent unit of ordinary matter that possesses the defining chemical properties of a chemical element. All solid, liquid, gaseous, and plasma matter is composed of neutral or ionised atoms. The word atom derives from the Ancient Greek atomos, meaning "indivisible," reflecting the early philosophical view that atoms could not be subdivided — a view later superseded by the discovery of subatomic particles.

Structure of an atom

An atom consists of a dense central atomic nucleus surrounded by a cloud of negatively charged electrons. The nucleus itself is composed of two types of subatomic particles:

• Protons — positively charged particles whose count defines the atomic number and thus the element.
• Neutrons — electrically neutral particles that, together with protons, contribute to the atom's mass number.

The number of protons in the nucleus is the atomic number, denoted Z. In a neutral atom, the number of electrons equals the number of protons, balancing the overall electric charge. Atoms of the same element with different neutron counts are called isotopes.

The electron cloud

Electrons occupy regions of space described by quantum mechanics as orbitals — mathematical functions expressing the probability of finding an electron at a given location. Orbitals are grouped into shells and subshells, characterised by quantum numbers. The arrangement of electrons in these shells, known as the electron configuration, determines how an atom interacts chemically with other atoms.

Nuclear forces

Protons, bearing the same positive charge, would repel one another electrostatically. The strong nuclear force, acting at very short range, overcomes this repulsion and holds the nucleus together. Neutrons contribute to nuclear stability by increasing the distance-averaged separation of protons and by participating directly in the strong force.

Size and mass

Atoms are extraordinarily small. A typical atomic radius ranges from about 30 to 300 picometres (trillionths of a metre), depending on the element and its bonding state. The nucleus is far smaller still — roughly 100,000 times smaller than the atom as a whole — yet contains nearly all of the atom's mass. The atomic mass unit (amu, or dalton) is defined as one-twelfth the mass of a carbon-12 atom, approximately 1.66 × 10⁻²⁷ kilograms.

Historical development

Ancient philosophy

The concept of indivisible units of matter was proposed independently by ancient Greek thinkers, most notably Democritus (c. 460 – c. 370 BCE) and his teacher Leucippus. Their atomic theory was philosophical rather than experimental and was largely overshadowed for centuries by Aristotle's rejection of the void necessary for atoms to move.

Early modern atomic theory

The scientific atomic theory was placed on an empirical footing by John Dalton in the early 19th century. Between approximately 1803 and 1808, Dalton proposed that each element consists of unique atoms of characteristic mass, and that chemical reactions are rearrangements of those atoms. This framework explained the law of definite proportions and the law of multiple proportions.

Discovery of subatomic structure

The discovery that atoms are themselves divisible proceeded in stages:

1. 1897 — J. J. Thomson identified the electron through cathode-ray experiments, demonstrating the existence of a subatomic particle with negative charge.
2. 1909–1911 — Ernest Rutherford and colleagues conducted the gold-foil experiment, revealing that atomic mass is concentrated in a tiny, positively charged nucleus, replacing Thomson's "plum-pudding" model.
3. 1913 — Niels Bohr introduced a quantised model of the hydrogen atom, explaining its spectral lines by postulating discrete electron orbits.
4. 1932 — James Chadwick discovered the neutron, completing the basic picture of nuclear composition.

Quantum mechanical model

The Bohr model was superseded by the full quantum mechanical treatment developed through the 1920s by Erwin Schrödinger, Werner Heisenberg, and others. The modern model describes electrons not as particles in fixed orbits but as wave functions distributed across probabilistic orbitals. This framework, part of quantum mechanics, accurately predicts atomic spectra, chemical bonding, and many physical properties of matter.

Chemical behaviour

Atoms bond with one another to form molecules and extended structures through several mechanisms:

• Covalent bonds — sharing of electron pairs between atoms.
• Ionic bonds — transfer of electrons from one atom to another, creating oppositely charged ions that attract.
• Metallic bonds — delocalised electrons shared across a lattice of metal atoms.
• Van der Waals interactions — weaker, transient dipole-based attractions.

The periodic table of the elements organises all known elements by atomic number and groups elements with similar electron configurations and chemical behaviour in the same columns.

Ions and excited states

When an atom gains or loses electrons, it becomes an ion — negatively charged (anion) if electrons are gained, and positively charged (cation) if electrons are lost. Ionisation energies vary systematically across the periodic table. Separately, an atom can absorb a photon and promote an electron to a higher energy orbital, entering an excited state; when the electron returns to the ground state, a photon of characteristic wavelength is emitted, giving rise to atomic emission spectra used in spectroscopy and astrophysics to identify elements remotely.

Frequently asked questions

Is an atom the smallest particle of matter?

An atom is the smallest unit of matter that retains the chemical identity of an element. Atoms are themselves composed of subatomic particles — protons, neutrons, and electrons — and protons and neutrons are further composed of quarks, so atoms are not the smallest particles in an absolute sense.

Is it possible to see an atom?

Atoms cannot be seen with visible light because they are smaller than the wavelength of visible photons. However, modern instruments such as scanning tunnelling microscopes and transmission electron microscopes can image individual atoms indirectly by detecting tunnelling currents or electron scattering.

Are all atoms of the same element identical?

Atoms of the same element share the same number of protons but may differ in the number of neutrons; such variants are called isotopes. Most elements occur naturally as a mixture of isotopes, and isotopes of the same element have nearly identical chemical properties but different masses and nuclear stabilities.

Can atoms be created or destroyed in chemical reactions?

In ordinary chemical reactions, atoms are neither created nor destroyed — they are rearranged into different molecules. This principle, conservation of matter, is a cornerstone of chemistry. Atoms can, however, be transformed through nuclear reactions, such as fission, fusion, or radioactive decay, which alter the nucleus itself.

Is the nucleus of an atom mostly empty space?

Yes. The nucleus occupies only a tiny fraction of the atom's total volume; most of an atom's volume is the diffuse electron cloud. If an atom were scaled to the size of a large sports stadium, the nucleus would be roughly the size of a marble at the centre.