The words you need to know

The smallest unit of an element that retains that element's chemical properties. Made of a nucleus containing protons and neutrons, surrounded by electrons. The number of protons determines which element it is: one proton is hydrogen, six is carbon, 79 is gold. Atoms are mostly empty space: if the nucleus were the size of a marble, the atom would be roughly the size of a football stadium. Everything you can touch is made of them.

A substance made entirely of atoms with the same number of protons. You cannot break an element down into simpler chemical substances. There are 118 known elements, 94 of which occur naturally. Hydrogen, carbon, oxygen, gold, iron: all elements. The periodic table organises them by atomic number (proton count) and groups elements with similar properties into columns. Everything in the universe is made of these 118 building blocks in varying combinations.

Two or more atoms bonded together. A molecule of water is two hydrogen atoms bonded to one oxygen atom (H₂O). A molecule of oxygen gas is two oxygen atoms bonded to each other (O₂). Molecules can be simple (two atoms) or enormously complex (DNA molecules contain billions of atoms). The properties of a molecule are often entirely different from the properties of the atoms it contains: hydrogen and oxygen are both gases; together they make liquid water.

A substance made of atoms from two or more different elements, chemically bonded in fixed ratios. Water (H₂O), salt (NaCl), carbon dioxide (CO₂): all compounds. Unlike a mixture, the components of a compound cannot be separated by physical means: you cannot filter the sodium out of salt. You need a chemical reaction to break a compound apart. The properties of a compound are distinct from its constituent elements: sodium is a reactive metal, chlorine is a toxic gas, sodium chloride is table salt.

The three subatomic particles that make up atoms. Protons carry a positive charge and sit in the nucleus. Neutrons carry no charge and also sit in the nucleus. Electrons carry a negative charge and occupy the space around the nucleus in shells or orbitals. The number of protons (atomic number) defines the element. The number of neutrons can vary, producing isotopes. The number of electrons determines the atom's charge and its chemical behaviour.

A variant of an element with the same number of protons but a different number of neutrons. Carbon-12 has 6 protons and 6 neutrons. Carbon-14 has 6 protons and 8 neutrons. Both are carbon. Carbon-14 is radioactive and decays at a known rate, which makes it useful for dating organic material (radiocarbon dating). Isotopes of the same element behave almost identically chemically but can have very different nuclear properties.

An atom or molecule that has gained or lost one or more electrons, giving it an electric charge. A cation has lost electrons and carries a positive charge. An anion has gained electrons and carries a negative charge. Salt dissolves in water as sodium cations (Na⁺) and chloride anions (Cl⁻). Ions are how many substances conduct electricity in solution. The movement of ions is also fundamental to nerve signals in the body.

The arrangement of all known elements by atomic number, with elements grouped into columns based on their chemical properties. Devised by Dmitri Mendeleev in 1869, who left gaps for elements not yet discovered and correctly predicted their properties from the table's logic. Elements in the same column (group) have the same number of outer electrons and behave similarly. Reading across a row (period), the atomic number increases by one each step. One of science's most elegant organising structures.

The force holding atoms together in molecules and compounds. Covalent bonds involve atoms sharing electrons: both atoms get to count the shared electrons as their own, giving each a full outer shell. Ionic bonds involve one atom donating an electron to another, forming oppositely charged ions that attract. Metallic bonds involve electrons moving freely through a lattice of positive metal ions, which is why metals conduct electricity. The type of bond determines the physical and chemical properties of the substance.

A weak attraction between a hydrogen atom in one molecule and a strongly electronegative atom (usually oxygen or nitrogen) in another. Not a true bond: no electrons are shared or transferred. But hydrogen bonds are significant in aggregate: water's unusually high boiling point, surface tension, and the fact that ice floats are all consequences of hydrogen bonding. DNA's double helix is held together by hydrogen bonds, which is why it can be unzipped and copied.

A process in which substances (reactants) are transformed into different substances (products) by breaking and forming chemical bonds. Mass is conserved: the total mass of reactants equals the total mass of products. What changes is how the atoms are arranged. Combustion, digestion, photosynthesis, rusting: all chemical reactions. The rate of a reaction depends on temperature, concentration, and the presence of catalysts. Reactions can release energy (exothermic) or absorb it (endothermic).

Exothermic reactions release energy, usually as heat. Combustion is exothermic: burning wood produces heat and light. Endothermic reactions absorb energy from the surroundings, making them feel cold. Dissolving ammonium nitrate in water is endothermic: the mixture gets cold, which is how instant cold packs work. Whether a reaction is exothermic or endothermic depends on whether the energy released by forming new bonds is greater or less than the energy required to break the old ones.

A substance that increases the rate of a chemical reaction without being consumed by it. A catalyst provides an alternative reaction pathway with a lower activation energy (the energy needed to start the reaction). Catalysts are essential to industrial chemistry: the Haber process for making ammonia (and therefore fertilisers, and therefore feeding most of humanity) uses an iron catalyst. Enzymes are biological catalysts. Your body runs on them.

Oxidation is the loss of electrons. Reduction is the gain of electrons. They always happen together: one substance loses electrons (is oxidised) and another gains them (is reduced). The classic memory aid: OIL RIG (Oxidation Is Loss, Reduction Is Gain). Rusting is oxidation: iron loses electrons to oxygen. Charging a battery forces electrons onto one electrode (reduction) while removing them from another (oxidation). Redox reactions underlie corrosion, combustion, respiration, and electrochemistry.

A substance that donates hydrogen ions (H⁺, also called protons) in solution. The more hydrogen ions, the more acidic. Vinegar, lemon juice, stomach acid, battery acid. In the Brønsted-Lowry definition (the most widely used), an acid is any proton donor. In the Lewis definition, an acid is an electron-pair acceptor. Acids react with bases in neutralisation reactions, producing water and a salt. Strong acids (hydrochloric, sulfuric, nitric) dissociate completely in water; weak acids (acetic, citric) dissociate partially.

A base is a substance that accepts hydrogen ions. An alkali is a base that dissolves in water to produce hydroxide ions (OH⁻). All alkalis are bases; not all bases are alkalis. Baking soda, bleach, ammonia, and drain cleaner are all alkaline. Bases and acids neutralise each other. The concept of pH describes how acidic or alkaline a solution is. Bases taste bitter (do not taste them) and feel soapy.

A measure of the concentration of hydrogen ions in a solution, on a scale from 0 to 14. pH 7 is neutral (pure water). Below 7 is acidic. Above 7 is alkaline. The scale is logarithmic: pH 4 is ten times more acidic than pH 5, and a hundred times more acidic than pH 6. Blood is maintained at pH 7.35 to 7.45. Small deviations from this range are life-threatening. The pH of the ocean has fallen by 0.1 units since industrialisation, which represents a 26% increase in acidity.

The unit chemists use to count atoms and molecules. One mole contains exactly 6.022 × 10²³ particles (Avogadro's number). This sounds arbitrary but is not: one mole of any element weighs as many grams as its atomic mass in unified atomic mass units. One mole of carbon (atomic mass 12) weighs 12 grams. The mole allows chemists to work with practical quantities while knowing exactly how many atoms they have. Atoms are too small to count individually; the mole solves this.

The amount of a substance dissolved in a given volume of solution. Typically measured in moles per litre (mol/L, also written M for molarity). A 1M solution contains one mole of solute per litre of solution. Concentration determines how quickly reactions proceed and is critical in medicine: a drug administered at the wrong concentration is either ineffective or dangerous. When a solution is diluted, the same number of molecules occupies a larger volume, reducing concentration.

The total heat content of a system at constant pressure. What matters in chemistry is usually the change in enthalpy during a reaction: how much heat is released or absorbed. A negative enthalpy change means the reaction is exothermic (releases heat). A positive enthalpy change means it is endothermic (absorbs heat). Enthalpy change is measured in joules or kilojoules per mole. The enthalpy of combustion tells you how much energy you get from burning a fuel.

A state in a reversible reaction where the rate of the forward reaction equals the rate of the reverse reaction, so the concentrations of reactants and products remain constant. Not a static state: both reactions are happening, just at the same rate. Le Chatelier's principle: if you disturb an equilibrium (by changing temperature, pressure, or concentration), the system shifts to oppose the change. The Haber process exploits this: by removing ammonia as it forms, the equilibrium shifts to produce more.

The study of carbon-containing compounds. Carbon is uniquely able to form four bonds and to chain together into long, stable molecules of almost unlimited complexity. Organic chemistry covers everything from methane (one carbon atom) to proteins and DNA (millions). The word "organic" here is historical: it once meant compounds derived from living organisms, before chemists discovered that carbon compounds could be synthesised in the lab. Plastics, pharmaceuticals, fuels, and all living matter are organic chemistry.

A compound containing only hydrogen and carbon. Methane, ethane, propane, butane: the simplest hydrocarbons. Crude oil is a mixture of hydrocarbons. When hydrocarbons burn in oxygen, they produce carbon dioxide and water. The simplest hydrocarbons are gases at room temperature; longer chains are liquids (petrol, diesel); very long chains are solids (wax, tar). The carbon chain length and the degree of branching determine physical properties.

A large molecule made of many repeating smaller units (monomers) joined in a chain. Natural polymers: cellulose, proteins, DNA, rubber. Synthetic polymers: polyethylene, nylon, PVC. The properties of a polymer depend on the monomer, chain length, and how the chains are arranged. Plastics are synthetic polymers. The extraordinary versatility of polymers (rigid, flexible, transparent, opaque, conductive, insulating) comes from the enormous range of possible monomers and chain structures.

Molecules with the same molecular formula but different structural arrangements of atoms. Ethanol (drinking alcohol) and dimethyl ether have the same formula (C₂H₆O) but very different structures and properties: ethanol is a liquid you can drink; dimethyl ether is a gas used as a propellant. Structural isomers have the same atoms arranged differently. Stereoisomers have the same connectivity but differ in three-dimensional arrangement. The same atoms, differently arranged, can be a nutrient or a poison.

Using electrical current to drive a non-spontaneous chemical reaction. A current passed through molten aluminium oxide extracts aluminium metal: that is how all commercial aluminium is produced. Electrolysis of water splits it into hydrogen and oxygen. Electroplating uses electrolysis to deposit a thin layer of one metal onto another. The electrode where reduction happens is the cathode; oxidation happens at the anode. Electrolysis is energy-intensive, which is why aluminium recycling saves 95% of the energy of primary production.

The physical forms matter takes: solid, liquid, gas, and plasma. In a solid, particles are tightly packed in a fixed arrangement. In a liquid, particles are close but can move past each other. In a gas, particles are far apart and move freely. Plasma is an ionised gas (like the sun and lightning). Changes of state (melting, freezing, evaporation, condensation, sublimation) involve energy input or release. They are physical changes, not chemical ones: the substance's identity does not change.