Last reviewed 30 Sept 2026 · 8 min read
Matter
Matter is anything that has mass and occupies space. It exists in three common states — solid (definite shape and volume), liquid (definite volume, no fixed shape) and gas (neither) — with plasma (ionised gas, as in the Sun and stars, lightning, neon signs) and Bose–Einstein condensate as further states.
- Pure substances: elements (one kind of atom: hydrogen, iron) and compounds (two or more elements chemically combined in a fixed ratio: water H₂O, carbon dioxide CO₂).
- Mixtures: homogeneous (solutions, air, brass) and heterogeneous (sand and water, milk). The components of a mixture retain their properties and can be separated by physical methods (filtration, distillation, evaporation, sublimation, chromatography, magnetic separation, sedimentation and decantation, centrifugation).
- A molecule is the smallest particle of a substance capable of independent existence; an atom is the smallest particle of an element that takes part in a chemical reaction.
Laws of chemical combination
- Law of conservation of mass (Lavoisier): mass is neither created nor destroyed in a chemical reaction.
- Law of constant (definite) proportions (Proust): a compound always contains the same elements in the same proportion by mass (water is always 1 part hydrogen to 8 parts oxygen by mass).
- Law of multiple proportions (Dalton) and Gay-Lussac's law of gaseous volumes.
- Avogadro's law: equal volumes of all gases at the same temperature and pressure contain equal numbers of molecules. One mole of any substance contains particles (Avogadro's number); one mole of a gas occupies 22.4 litres at STP.
Development of atomic models
| Scientist | Contribution | Model |
|---|---|---|
| John Dalton (1808) | atoms are indivisible, tiny particles; atoms of one element are identical; atoms combine in simple ratios | solid sphere (billiard-ball) model |
| J. J. Thomson (1897–1904) | discovered the electron; atom is a sphere of positive charge with electrons embedded | plum-pudding (watermelon) model |
| E. Goldstein | discovered anode (canal) rays — the proton | |
| Ernest Rutherford (1911) | gold-foil (alpha-scattering) experiment: most alpha particles passed straight, a few deflected, very few rebounded → a tiny, dense, positively charged nucleus with electrons around it | nuclear model |
| James Chadwick (1932) | discovered the neutron | |
| Niels Bohr (1913) | electrons move in fixed circular orbits (energy levels) without radiating; energy is emitted or absorbed when an electron jumps between levels | Bohr model |
| Schrödinger, Heisenberg, de Broglie | electrons show wave-particle duality; only the probability of finding an electron in a region (orbital) can be given | quantum mechanical model |
Drawbacks of Rutherford's model: it could not explain the stability of the atom (an orbiting electron should radiate energy and spiral into the nucleus). Bohr's model explained the hydrogen spectrum but failed for larger atoms.
Subatomic particles
| Particle | Symbol | Charge | Relative mass | Location | Discoverer |
|---|---|---|---|---|---|
| Electron | e⁻ | −1 (−1.6 × 10⁻¹⁹ C) | 1/1836 (about 9.1 × 10⁻³¹ kg) | outside the nucleus | J. J. Thomson |
| Proton | p⁺ | +1 | 1 u (about 1.67 × 10⁻²⁷ kg) | nucleus | Rutherford / Goldstein |
| Neutron | n | 0 | 1 u | nucleus | Chadwick |
- The nucleus is about 10⁻¹⁵ m across, about 100,000 times smaller than the atom, but holds almost all the mass.
- Protons and neutrons are nucleons; they are made of quarks. Hydrogen (protium) is the only atom with no neutron.
- An atom is electrically neutral: number of protons = number of electrons.
Atomic number, mass number, isotopes
- Atomic number () = number of protons. It identifies the element.
- Mass number () = protons + neutrons (nucleons). Number of neutrons .
- Notation: ; e.g. : 6 protons, 6 neutrons, 6 electrons.
| Term | Meaning | Examples |
|---|---|---|
| Isotopes | same , different (same element, different neutrons; same chemical properties) | protium, deuterium, tritium; ¹²C and ¹⁴C; ³⁵Cl and ³⁷Cl; ²³⁵U and ²³⁸U |
| Isobars | same , different | ⁴⁰Ar, ⁴⁰K, ⁴⁰Ca |
| Isotones | same number of neutrons | ¹⁴C and ¹⁶O |
| Isoelectronic | same number of electrons | Na⁺, Mg²⁺, Ne, F⁻ |
Uses of isotopes: ¹⁴C for radiocarbon dating; ⁶⁰Co for cancer treatment; ¹³¹I for thyroid disorders; ²³⁵U as nuclear fuel; deuterium (heavy water) as a moderator in reactors; ²⁴Na for detecting blood clots; isotopes as tracers in agriculture and industry.
An atom of sodium has and . It has 11 protons, 11 electrons and neutrons.