LC Chemistry key terms

The number of protons in the nucleus of an atom, which uniquely identifies the element and determines its position in the periodic table.

The total count of protons and neutrons in one atom's nucleus, written as a superscript beside the element symbol and used to distinguish isotopes.

Atoms of the same element with identical proton counts but different neutron counts, so they share chemical properties while differing in mass and sometimes nuclear stability.

The weighted average mass of an element's naturally occurring isotopes compared with one-twelfth the mass of a carbon-12 atom; it is dimensionless and rarely a whole number.

The rule that electrons occupy the lowest-energy available orbitals first, giving the sequence 1s, 2s, 2p, 3s and so on when building up electron configurations.

The arrangement of an atom's electrons among orbitals and subshells, commonly written such as 1s² 2s² 2p⁶ and used to explain periodic trends and bonding behaviour.

The guideline that electrons fill degenerate orbitals singly with parallel spins before any pairing occurs, minimising electron-electron repulsion within a subshell.

A measure of how strongly an atom attracts the shared electrons of a bond; values rise across a period and fall down a group, predicting bond polarity.

The electrostatic attraction between oppositely charged ions formed when one atom transfers electrons to another, typically between a metal and a non-metal.

A bond in which shared electrons are pulled toward the more electronegative atom, creating partial charges and often a molecular dipole.

A quantity describing the separation of charge in a molecule, equal to charge times distance; symmetrical molecules may have polar bonds yet a zero overall value.

A model predicting molecular shape by assuming electron pairs around a central atom repel and spread as far apart as possible, with lone pairs occupying more space than bonding pairs.

The mixing of atomic orbitals on a central atom to form equivalent new orbitals, such as sp³ in methane, explaining observed bond angles and geometries.

The amount of substance containing 6.022 × 10²³ specified particles, linking countable particles to measurable masses through molar mass.

The mass in grams of one mole of a substance, numerically equal to its relative molecular mass and used to convert between mass and amount in calculations.

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

The actual number of atoms of each element in one molecule, found by multiplying an empirical formula by a whole number derived from molar mass.

The reactant that is completely consumed first and therefore fixes the maximum amount of product obtainable from a reaction.

The actual mass of product obtained expressed as a percentage of the theoretical maximum predicted by stoichiometry, reflecting losses and side reactions.

A volumetric procedure in which a solution of known concentration is added from a burette to a measured volume of analyte until the equivalence point is signalled.

A solution whose concentration is accurately known, prepared by weighing a primary standard or by standardising against one.

A highly pure, stable, non-hygroscopic substance used to standardise solutions, such as anhydrous sodium carbonate or potassium hydrogenphthalate.

The stage of a titration where the amount of titrant added exactly matches the stoichiometric amount required to react with all the analyte.

A weak acid or base whose colour changes over a narrow pH range, chosen so the endpoint coincides with the equivalence point of the titration.

A mixture of a weak acid and its conjugate base, or a weak base and its conjugate acid, that resists pH change on addition of small amounts of acid or alkali.

A hypothetical gas whose particles occupy negligible volume and exert no intermolecular forces, obeying PV = nRT under all conditions.

Equal volumes of gases at the same temperature and pressure contain equal numbers of particles, allowing volume ratios to serve as mole ratios.

The concentration of a solution expressed as moles of solute per cubic decimetre (litre) of solution, the standard unit for volumetric work.

A physical property of a solution that depends on the number of dissolved particles rather than their identity, such as boiling point elevation and freezing point depression.

The net movement of solvent through a semipermeable membrane from a dilute solution to a more concentrated one, driven by differences in solvent chemical potential.

The heat energy exchanged with the surroundings at constant pressure during a reaction, negative for exothermic and positive for endothermic processes.

The principle that the total enthalpy change of a reaction depends only on initial and final states, so indirect routes can be summed to find an unknown value.

The minimum energy colliding particles need to overcome the energy barrier and form products, lowered by a catalyst and reflected in the reaction rate's temperature dependence.

A substance that increases reaction rate by providing an alternative lower-energy pathway without being consumed, so it does not alter equilibrium position.

When a system at equilibrium is disturbed by a change in concentration, pressure or temperature, it shifts in the direction that partly opposes the disturbance.

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

A specific atom or group of atoms that determines the characteristic reactions of an organic molecule, such as hydroxyl, carbonyl or carboxyl groups.

A family of organic compounds sharing the same functional group and general formula, with successive members differing by a CH₂ unit and showing graded physical properties.

The existence of compounds with the same molecular formula but different arrangements of atoms, giving distinct structures and often different physical and chemical properties.

A loss of electrons by a species, accompanied by an increase in oxidation number; it is always paired with reduction in a redox reaction.

A gain of electrons by a species, accompanied by a decrease in oxidation number, occurring at the cathode in electrolysis or a galvanic cell.

A bookkeeping value assigned to an atom assuming bonds are fully ionic, used to identify oxidation and reduction in complex species.

The potential of a half-cell measured against the standard hydrogen electrode under standard conditions, used to predict the direction of redox reactions.

The use of an external electrical current to drive a non-spontaneous chemical change in an electrolyte, occurring at inert or reactive electrodes.

A purification technique in which a solid is dissolved in a minimum of hot solvent, then cooled so pure crystals form while impurities remain in solution.

Separation of an insoluble solid from a liquid by passing the mixture through filter paper, either under gravity or with suction.

A technique for assessing purity: a pure solid melts sharply over a narrow range close to its literature value, while impurities broaden and depress the range.

Separation of liquids by differences in boiling point, or purification of a liquid from non-volatile contaminants, using evaporation followed by condensation.