Pancyprian Chemistry key terms

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

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

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

The weighted mean mass of an atom of an element on a scale where one atom of carbon-12 has exactly twelve units, reflecting natural isotopic abundance.

The minimum energy needed to remove one mole of electrons from one mole of gaseous atoms or ions, giving a measure of how tightly a shell holds its electrons.

A relative measure of the tendency of an atom in a bond to attract the shared electron pair towards itself; it increases across a period and decreases down a group.

An unusually strong intermolecular attraction between a hydrogen atom bonded to N, O or F and a lone pair on a nearby electronegative atom, raising melting and boiling points markedly.

Weak, short-lived attractions caused by instantaneous fluctuations in electron distribution, present in all molecules and the dominant force in non-polar substances.

A covalent bond in which the shared electrons are unequally attracted, producing partial charges on the bonded atoms because the two atoms differ in electronegativity.

Attractions between the permanent partial positive and partial negative ends of polar molecules, weaker than hydrogen bonds but stronger than dispersion forces of similar molecular size.

The amount of substance containing exactly the Avogadro number of specified particles, linking countable particles to measurable masses through molar mass.

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

The reactant that is completely consumed first in a reaction, fixing the maximum amount of product that can form when other reactants remain in excess.

The actual mass of product obtained expressed as a percentage of the theoretical maximum calculated from the balanced equation, always less than or equal to 100%.

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

The relationship pV = nRT connecting pressure, volume, amount in moles and absolute temperature, valid for gases at low pressure and moderate temperature.

The pressure a single gas in a mixture would exert if it alone occupied the whole container; it equals its mole fraction times the total pressure.

The number of moles of one component divided by the total number of moles in the mixture; all mole fractions sum to one.

The concentration of a solution expressed as moles of solute per cubic decimetre of solution, commonly written mol dm-3.

The enthalpy change when one mole of a substance burns completely in excess oxygen under standard conditions, usually measured with a calorimeter and always exothermic for fuels.

The enthalpy change when one mole of a compound forms from its elements in their standard states at 298 K and 1 bar, defined as zero for elements in those states.

The total enthalpy change of a reaction is independent of the route taken, allowing unknown enthalpy changes to be calculated from known ones using an energy cycle.

The minimum kinetic energy that colliding particles must possess for a reaction to occur, corresponding to the top of the energy barrier on a reaction profile.

The slowest step in a multi-step reaction mechanism, which controls the overall rate because the sequence cannot proceed faster than this step.

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 product concentrations to reactant concentrations, each raised to its stoichiometric power, at equilibrium for a given temperature; its value does not depend on initial amounts.

A substance that increases reaction rate by providing an alternative pathway of lower activation energy, being chemically unchanged at the end and not shifting the equilibrium position.

A species that donates a proton to another species in a reaction; every acid has a conjugate base formed after the proton is lost.

Two species differing by exactly one proton, such as ammonia and the ammonium ion, related by the transfer of a hydrogen ion in an acid-base equilibrium.

A mixture of a weak acid and its conjugate base, or a weak base and its conjugate acid, that resists large pH changes when small amounts of acid or alkali are added.

A bookkeeping charge assigned to an atom in a compound by applying agreed rules, used to identify oxidation and reduction in reactions.

The potential of a half-cell measured against the standard hydrogen electrode at 298 K and unit activity, used to predict cell voltage and direction of redox change.

The decomposition of a substance by passing a direct electric current through it in molten or aqueous form, driving a non-spontaneous redox reaction at the electrodes.

The regular repetition of physical and chemical properties of elements as atomic number increases, explained by recurring outer electron configurations across periods.

A d-block element that forms at least one stable ion with a partially filled d subshell, typically showing variable oxidation states and coloured complex ions.

An ion or molecule that donates a lone pair of electrons to a central metal ion to form a coordinate bond within a complex ion.

An atom or group of atoms within an organic molecule that determines its characteristic chemical reactions, such as the hydroxyl group in alcohols or the carbonyl group in aldehydes.

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

A reaction in which a nucleophile attacks an electron-deficient carbon and replaces a leaving group, typical of halogenoalkanes reacting with hydroxide or cyanide ions.