Chemistry dictionary

A substance that donates hydrogen ions (H⁺) to other substances. In water, acids raise the concentration of H⁺ ions and give a pH below 7. Read more →

The minimum energy that colliding particles must have for a reaction to take place, given the symbol Eₐ. Read more →

A single product formed when two or more molecules join directly, with all of their atoms kept. It often forms when one molecule donates a pair of electrons to another. Read more →

Any element in group 1 of the periodic table except hydrogen: lithium, sodium, potassium, rubidium, cesium and francium. Read more →

One of two or more different forms of the same element in the same physical state, with the atoms bonded or arranged differently. Read more →

A mixture of a metal with one or more other elements, usually other metals, such as bronze (copper + tin) or steel (iron + carbon). Read more →

The process of combining a metal with one or more other elements, usually by melting them together, to make an alloy with improved properties. Read more →

Able both to donate and to accept a hydrogen ion (H⁺, a proton). Water and the hydrogen carbonate ion (HCO₃⁻) are common examples. Read more →

Able to react as both an acid and a base. Aluminum oxide, zinc oxide and water are examples. Read more →

A negatively charged ion, formed when an atom or group of atoms gains one or more electrons. Read more →

The smallest particle of an element that still has the chemical properties of that element. It consists of a tiny nucleus of protons and neutrons surrounded by electrons. Read more →

The mass of an atom, usually given in atomic mass units (u), where one unit is one-twelfth of the mass of a carbon-12 atom. The value listed for an element is the average for its natural mix of isotopes. Read more →

The number of protons in the nucleus of an atom, given the symbol Z. It identifies the element: every atom with 6 protons is carbon. Read more →

A measure of the size of an atom, usually taken as half the distance between the nuclei of two identical atoms bonded together. Read more →

The number of particles in one mole of a substance: exactly 6.02214076 × 10²³, usually rounded to 6.022 × 10²³. Read more →

A substance that accepts hydrogen ions (H⁺) or produces hydroxide ions (OH⁻) in water, giving a pH above 7. Bases that dissolve in water are called alkalis. Read more →

One of four regions of the periodic table, the s-, p-, d- and f-blocks, named after the type of subshell that holds each element's highest-energy electrons. Read more →

The temperature at which a liquid's vapor pressure equals the pressure around it, so bubbles of vapor form throughout the liquid. Read more →

A solution that resists changes in pH when small amounts of acid or base are added. It usually contains a weak acid and its conjugate base, or a weak base and its conjugate acid. Read more →

A substance that speeds up a chemical reaction without being used up. Read more →

A positively charged ion, formed when an atom or group of atoms loses one or more electrons. Read more →

A property of matter that causes electrical attraction or repulsion. Protons carry a positive charge and electrons an equal negative charge. Read more →

A lasting attraction between atoms or ions that holds them together in molecules, crystals and metals. Read more →

The state of a reversible reaction in which the forward and reverse reactions happen at the same rate, so the amounts of reactants and products stay constant. Read more →

A way of writing a substance with element symbols and subscript numbers that show which elements it contains and in what proportions. Read more →

A process in which one or more substances (reactants) are changed into new substances (products) by breaking and forming chemical bonds. Read more →

A mixture in which tiny particles, roughly 1 to 1,000 nanometers across, are spread through another substance and do not settle out. Read more →

A fast reaction between a fuel and oxygen that releases energy as heat and usually light. Burning is the everyday word for it. Read more →

A substance made of two or more different elements chemically bonded in a fixed ratio, such as water (H₂O) or sodium chloride (NaCl). Read more →

The amount of solute present in a given amount of solution or solvent. Read more →

The change of a gas or vapor into a liquid, usually when it cools. In chemistry the word also names reactions in which two molecules join and release a small molecule such as water. Read more →

The electrostatic force that pulls oppositely charged particles toward each other, such as a positive nucleus and a negative electron. Read more →

A chemical bond in which two atoms share one or more pairs of electrons. It usually forms between nonmetal atoms. Read more →

The regular, repeating three-dimensional arrangement of particles (atoms, ions or molecules) in a crystalline solid. Read more →

Describes orbitals or other quantum states that have exactly the same energy, such as the three p orbitals of one sublevel. Read more →

The mass of a substance per unit of volume, calculated as density = mass ÷ volume. Read more →

Made of two atoms. Hydrogen, nitrogen, oxygen, fluorine, chlorine, bromine and iodine exist as diatomic molecules: H₂, N₂, O₂, F₂, Cl₂, Br₂ and I₂. Read more →

A conductor through which electric current enters or leaves an electrolyte, as in a battery or an electrolysis cell. Read more →

A substance that conducts electricity when melted or dissolved in water, because it forms ions that are free to move. Read more →

A tiny subatomic particle with a negative electric charge (−1) that occupies the space around an atom's nucleus. Read more →

The energy change when a neutral atom in the gas phase gains an electron to form a negative ion. Halogens have especially high electron affinities. Read more →

The arrangement of an atom's electrons in shells, subshells and orbitals, written as a list such as 1s² 2s² 2p⁶ for neon. Read more →

How strongly an atom attracts the shared electrons in a chemical bond. Fluorine is the most electronegative element. Read more →

A pure substance made of only one kind of atom, defined by the number of protons in its nucleus (its atomic number). Read more →

The simplest whole-number ratio of the atoms of each element in a compound. Read more →

Describes a process or reaction that absorbs energy, usually as heat, from its surroundings, so the surroundings get colder. Its enthalpy change (ΔH) is positive. Read more →

A thermodynamic quantity, H, that describes the heat content of a system; its change, ΔH, equals the heat absorbed or released by a reaction at constant pressure. Read more →

A measure of how spread out, or dispersed, the energy and particles of a system are, often described loosely as disorder. Its symbol is S. Read more →

The change of a liquid into a gas (vapor) at its surface, at temperatures below its boiling point. Read more →

Describes a process or reaction that releases energy, usually as heat, to its surroundings, so the surroundings get warmer. Its enthalpy change (ΔH) is negative. Read more →

Separating a substance from a mixture, often by dissolving it selectively in a solvent. The word is also used for obtaining a metal from its ore. Read more →

The electric charge carried by one mole of electrons, about 96,485 coulombs. This value is called the Faraday constant (F). Read more →

The number of wave cycles that pass a point per second, measured in hertz (Hz). Read more →

The unwanted rock and minerals mixed with the valuable mineral in an ore, which must be removed when the metal is extracted. Read more →

The lowest-energy arrangement of the electrons in an atom, ion or molecule. Read more →

A vertical column of the periodic table. Elements in the same group usually have the same number of outer electrons and similar chemical properties. Read more →

The time it takes for half of the radioactive nuclei in a sample to decay, given the symbol t½. Read more →

Any element in group 17 of the periodic table: fluorine, chlorine, bromine, iodine, astatine and tennessine. The name means salt-former. Read more →

Not uniform — the different parts can be seen or separated, as in sand mixed with water. Read more →

Uniform throughout — every sample has the same composition and appearance, as in salt water or air. Read more →

Electrons fill orbitals of equal energy one at a time, with parallel spins, before any of them pair up. Read more →

The mixing of atomic orbitals on one atom to form new, equivalent hybrid orbitals used in bonding, such as the sp³ orbitals of carbon in methane. Read more →

Solvation in which the solvent is water, or a reaction in which water is added to a compound. Read more →

Describes an atom or ion that, like hydrogen, has only one electron, such as He⁺ or Li²⁺. Such a species is also called a hydrogen-like atom. Read more →

An imaginary gas whose particles take up no space and neither attract nor repel each other, so it obeys the ideal gas law, PV = nRT, exactly. Read more →

A substance that changes color to show a change in conditions, most often whether a solution is acidic or basic. Read more →

Very unreactive. The noble gases are often called inert gases, although some of them can form compounds. Read more →

An atom or group of atoms that has gained or lost electrons and so carries an overall electric charge. Read more →

A chemical bond formed by the attraction between oppositely charged ions, usually after a metal atom transfers electrons to a nonmetal atom. Read more →

The minimum energy needed to remove the most loosely held electron from a neutral atom in the gas phase: X(g) → X⁺(g) + e⁻. Read more →

Atoms of the same element with different numbers of neutrons, such as carbon-12 and carbon-14. Read more →

The amount of matter in an object. Unlike weight, it does not change with gravity. Read more →

The total number of protons and neutrons in an atom's nucleus, given the symbol A. Read more →

The temperature at which a solid turns into a liquid at a given pressure. For a pure substance it is the same as the freezing point. Read more →

An element, or a mixture of elements such as an alloy, that is typically shiny, conducts heat and electricity well and can be hammered or bent without breaking. Read more →

The attraction between positively charged metal ions and the sea of delocalized electrons that moves freely among them. Read more →

An element with properties in between those of metals and nonmetals. Boron, silicon, germanium, arsenic, antimony and tellurium are usually counted as metalloids. Read more →

Two or more substances physically combined, each keeping its own properties. Mixtures can usually be separated by physical means such as filtering, evaporation or distillation. Read more →

A simplified representation of something too small, large or complex to observe directly, used to explain observations and make predictions. Models can be physical objects, diagrams, equations or ideas. Read more →

The concentration of a solution expressed as moles of solute per liter of solution, written mol/L or M. Read more →

The SI unit for amount of substance. One mole contains exactly 6.02214076 × 10²³ particles, often rounded to 6.022 × 10²³ (Avogadro's number). Read more →

A formula that shows the actual number of atoms of each element in one molecule of a substance, such as C₆H₁₂O₆ for glucose. Read more →

A group of two or more atoms held together by covalent bonds, such as O₂ or H₂O. It is the smallest unit of a covalent substance that keeps its chemical properties. Read more →

A unit of length equal to one billionth of a meter (10⁻⁹ m), with the symbol nm. Read more →

Having no overall electric charge, or, for a solution, being neither acidic nor basic (pH 7 at 25 °C). Read more →

A reaction between an acid and a base that produces a salt and, usually, water, canceling out their acidic and basic properties. Read more →

An electrically neutral subatomic particle found in the nucleus, with a mass slightly greater than that of a proton. Read more →

Any element of group 18: helium, neon, argon, krypton, xenon and radon, along with the synthetic element oganesson. The naturally occurring ones are colorless, odorless gases that rarely react. Read more →

An element that lacks the properties of metals: nonmetals are usually poor conductors of heat and electricity, and their solids are dull and brittle. Read more →

The tiny, dense, positively charged center of an atom, made of protons and neutrons. It holds almost all of the atom's mass. Read more →

A region around a nucleus where an electron is most likely to be found. Each orbital holds at most two electrons, and they must have opposite spins. Read more →

Loss of electrons by a substance during a reaction, which increases its oxidation number. It can also mean gain of oxygen or loss of hydrogen. Read more →

A tiny piece of matter, such as an atom, molecule or ion, or a subatomic particle such as a proton, neutron or electron. Read more →

A horizontal row of the periodic table. There are seven periods, and the elements in each are listed in order of increasing atomic number. Read more →

A scale that measures how acidic or basic a water-based solution is, based on its concentration of hydrogen ions. At 25 °C, pH 7 is neutral, values below 7 are acidic and values above 7 are basic. Read more →

An uneven distribution of electric charge in a bond or molecule, giving one end a partial negative charge (δ−) and the other a partial positive charge (δ+). Read more →

An insoluble solid that forms from a solution during a chemical reaction. As a verb, to precipitate means to come out of solution as a solid. Read more →

The substances formed by a chemical reaction, written on the right side of a chemical equation. Read more →

A positively charged subatomic particle (+1) found in the nucleus of every atom. The number of protons is the atomic number and decides which element an atom is. Read more →

The spontaneous emission of radiation by unstable atomic nuclei as they decay into more stable ones. Read more →

The starting substances in a chemical reaction, written on the left side of a chemical equation. Read more →

A reaction in which electrons are transferred from one substance to another, so that one substance is oxidized and another is reduced. Read more →

Gain of electrons by a substance during a reaction, which decreases its oxidation number. It can also mean loss of oxygen or gain of hydrogen. Read more →

An ionic compound made of positive ions (cations) and negative ions (anions), typically formed when an acid reacts with a base. Table salt, sodium chloride, is the best-known example. Read more →

A group of electrons in an atom that share the same principal energy level, numbered n = 1, 2, 3 and so on outward from the nucleus. It is also called an energy level. Read more →

The substance that is dissolved in a solution, usually the component present in the smaller amount. Read more →

A homogeneous mixture in which one or more substances (solutes) are dissolved evenly in another (the solvent). Read more →

The process of solvent molecules surrounding and interacting with dissolved particles such as ions or molecules. Read more →

The substance that dissolves a solute to form a solution, usually the component present in the larger amount. Read more →

A solution whose concentration is known accurately, used as a reference to measure the concentration of another solution. Read more →

The calculation of the quantities of reactants and products in chemical reactions, based on the mole ratios in a balanced equation. Read more →

A subdivision of an electron energy level (shell), labeled s, p, d or f. These sublevels hold at most 2, 6, 10 and 14 electrons. Read more →

The change of a solid directly into a gas, without first becoming a liquid. Read more →

A set of orbitals within a shell that all have the same shape, labeled s, p, d or f. It is another name for a sublevel. Read more →

Matter with a uniform, definite composition and fixed properties — either an element or a compound. Also called a pure substance. Read more →

A heterogeneous mixture in which particles larger than about 1,000 nanometers are spread through a liquid or gas but settle out when left to stand. Read more →

A measure of how hot or cold something is, related to the average kinetic energy of its particles. Read more →

A laboratory method for finding the concentration of a solution by reacting it with a measured volume of a solution of known concentration. Read more →

A metal in the d-block of the periodic table, groups 3 to 12. Strictly, it is an element whose atoms or common ions have a partly filled d subshell. Read more →

Describes a substance, nucleus or particle that tends to change, decompose, react or decay rather than stay as it is. Read more →

The electrons in the outermost shell of an atom; they are the ones that take part in chemical bonding. Read more →

Describes a substance that evaporates easily at normal temperatures, like rubbing alcohol or acetone. Read more →

The amount of three-dimensional space that a substance or object takes up. Read more →

A repeating disturbance that carries energy from one place to another, described by its wavelength, frequency and amplitude. Read more →