European Baccalaureate Chemistry key terms

The number of protons in the nucleus of an atom, which uniquely identifies the element and determines the number of electrons in a neutral atom.

The total count of protons and neutrons in one nucleus of a given atom; it is an integer and differs between isotopes of the same element.

One of two or more forms of an element whose atoms have the same number of protons but different numbers of neutrons, so they differ in mass but share chemical behaviour.

The weighted mean mass of the atoms of an element on a scale where one atom of carbon-12 has a mass of exactly 12; it accounts for natural isotopic abundance.

The minimum energy needed to remove one electron from each atom in one mole of gaseous atoms of an element, forming singly positive gaseous ions.

The repeating pattern of physical and chemical properties shown by elements when arranged in order of increasing atomic number, explained by recurring outer electron configurations.

A measure of how strongly an atom in a molecule attracts the shared electron pair of a covalent bond; values rise across a period and fall down a group.

A covalent bond in which the shared electrons are drawn towards the more electronegative atom, producing partial positive and partial negative charges at the two ends.

A quantity describing the separation of charge in a molecule, equal to charge times distance; it is a vector, so molecular shape decides whether bond dipoles cancel overall.

A model predicting molecular shape by assuming electron pairs around a central atom repel and arrange themselves as far apart as possible, with lone pairs repelling more strongly than bonding pairs.

A strong intermolecular attraction between a hydrogen atom bonded to N, O or F and a lone pair on a nearby electronegative atom, much weaker than a covalent bond but stronger than other dipole forces.

The amount of substance containing exactly the Avogadro number of specified entities, where the Avogadro constant is approximately 6.02 × 10^23 particles per mole.

The mass of one mole of a substance, expressed in grams per mole and numerically equal to the relative molecular or formula mass on the carbon-12 scale.

The simplest whole-number ratio of the atoms of each element present in a compound, obtained from percentage composition or combustion data.

The reactant that is completely consumed first in a chemical reaction and therefore determines the maximum amount of product that can be formed.

The actual mass of product obtained expressed as a percentage of the theoretical maximum mass calculated from the limiting reactant; it is always at most 100 per cent.

The proportion of the total mass of reactants that ends up in the desired product, used as a measure of how efficient and sustainable a reaction route is.

The heat energy transferred at constant pressure when a reaction occurs in the molar quantities shown in the equation; negative for exothermic, positive for endothermic reactions.

The enthalpy change when one mole of a compound forms from its elements in their standard states under standard conditions of 298 K and 100 kPa.

The total enthalpy change of a reaction depends only on the initial and final states, not on the route taken, so enthalpy cycles can be used to calculate unknown values.

A measure of the dispersal of energy and the number of ways particles can be arranged; it increases when solids melt, liquids vaporise or gases are produced in greater amount.

The quantity ΔH − TΔS that decides whether a reaction is feasible: a negative value indicates a spontaneous process at that temperature.

The minimum kinetic energy that colliding particles must possess for a reaction to occur, corresponding to the energy barrier between reactants and the transition state.

The slowest elementary step in a multi-step mechanism; it controls the overall reaction rate and therefore appears in the experimentally derived rate equation.

A substance that increases the rate of a reaction by providing an alternative pathway of lower activation energy, without being consumed and without changing the position of equilibrium.

The state of a reversible reaction in a closed system where forward and reverse rates are equal, so concentrations stay constant even though both reactions continue.

When a system at equilibrium is disturbed by a change in concentration, pressure or temperature, the position of equilibrium shifts to oppose that change.

The ratio of the product concentrations, each raised to its stoichiometric coefficient, to the reactant concentrations similarly raised, measured at equilibrium; it varies only with temperature.

A species that donates a proton (hydrogen ion) to another species; a base is the corresponding proton acceptor, and the two form a conjugate acid–base pair.

The negative logarithm to base ten of the hydrogen ion concentration in mol dm⁻³; values below 7 are acidic and above 7 alkaline at 298 K.

A solution that resists large changes in pH when small amounts of acid or alkali are added, usually containing a weak acid with its conjugate base in comparable amounts.

The loss of electrons by a species, accompanied by an increase in oxidation number; it must be matched by reduction of another species in the same redox reaction.

A reaction in which oxidation and reduction occur together, because electrons transferred from one species are accepted by another.

The electromotive force of a cell made of the half-cell under study and a standard hydrogen electrode, measured at 298 K, 100 kPa and 1 mol dm⁻³ ion concentrations.

The decomposition of a compound driven by an external direct current passing through it, either molten or in solution, producing chemical change at the electrodes.

An electron-pair donor, often negatively charged or containing a lone pair, that attacks an electron-deficient carbon atom in substitution or addition reactions.

An electron-pair acceptor, often positively charged or polarised, that attacks regions of high electron density such as a carbon–carbon double bond or an aromatic ring.

A step-by-step description of bond breaking and forming in which curved arrows show the movement of electron pairs from a source of electrons to an electron-deficient site.

An atom or group of atoms within an organic molecule that gives the compound its characteristic reactions, such as hydroxyl, carbonyl, carboxyl or amine groups.

A family of organic compounds with the same functional group and general formula, in which successive members differ by one CH2 unit and show gradually changing physical properties.

One of two or more compounds with the same molecular formula but different arrangements of atoms, giving different physical properties and often different chemical behaviour.

In chromatography, the time taken for a component to travel from injection to detector; it is characteristic of a substance under fixed conditions and aids identification.

The distance travelled by a spot divided by the distance travelled by the solvent front in thin-layer or paper chromatography; it helps identify a substance when compared with standards.

A technique that identifies functional groups by the absorption of infrared radiation at frequencies matching the vibration of specific bonds, such as O–H, C=O or C–H.

The design of chemical products and processes that reduce or eliminate the use and generation of hazardous substances, favouring high atom economy, low energy use and renewable feedstocks.