QCE Chemistry key terms

The strong force holding atoms together within a single molecule or formula unit, formed by transferring or sharing electrons. It determines the identity of the substance and is broken only during chemical change.

A bond formed when two atoms share one or more pairs of electrons, usually between nonmetals. The shared electrons are attracted to both nuclei, holding the atoms together.

The electrostatic attraction between oppositely charged ions in a lattice, formed when electrons are transferred from a metal to a nonmetal. It is strong and non-directional.

The attraction between a lattice of metal cations and a sea of delocalised valence electrons. It explains the conductivity, malleability and generally high melting points of metals.

A measure of how strongly an atom attracts the shared electrons in a covalent bond. Values increase across a period and decrease down a group.

The uneven sharing of bonding electrons caused by a difference in electronegativity, giving one end of the bond a partial negative and the other a partial positive charge.

A separation of charge within a molecule or bond, with a partial positive end and a partial negative end. Molecular dipoles result from polar bonds that do not cancel by symmetry.

A model predicting molecular shape from the repulsion between electron pairs around a central atom. Lone pairs repel more strongly than bonding pairs, so they compress bond angles.

The angle between two adjacent bonds at a central atom, determined by electron-pair repulsion. It is measured in degrees and helps identify molecular geometry.

A diagram showing valence electrons as dots and bonds as lines, used to represent how atoms are connected and where lone pairs sit in a molecule or ion.

The description of a species as a hybrid of two or more valid Lewis structures when a single structure cannot show the true delocalised electron arrangement, such as in the carbonate ion.

Electrons not confined to a single bond or atom but spread over several atoms or an entire structure, allowing charge or energy to be shared, as in metals and benzene.

The energy released when one mole of an ionic solid forms from its gaseous ions, or the energy needed to separate it. It reflects the strength of ionic bonding.

The relatively weak attraction between separate molecules or ions, such as dispersion forces, dipole-dipole attraction and hydrogen bonding. These forces control boiling point and solubility.

A weak, temporary attraction caused by instant fluctuations in electron distribution that create short-lived dipoles. These forces increase with the number of electrons and molecular size.

A particularly strong dipole-dipole attraction between a hydrogen atom bonded to N, O or F and a lone pair on a nearby N, O or F atom. It raises boiling points noticeably.

The ease with which an electron cloud can be distorted to form a temporary dipole. Larger atoms and molecules are more polarisable, strengthening dispersion forces.

The ability of two liquids to mix and form a single uniform phase. It is favoured when the substances have similar intermolecular forces and similar polarity.

The maximum amount of a solute that dissolves in a given quantity of solvent at a stated temperature. It depends on the balance of solute-solvent interactions.

Describing a molecule with both a polar, water-loving region and a nonpolar, water-repelling region, such as a soap ion. Such molecules form micelles in water.

A state in a reversible reaction where the forward and reverse rates are equal, so concentrations stay constant even though both reactions continue at the molecular level.

The ratio of product concentrations to reactant concentrations, each raised to its stoichiometric coefficient, at equilibrium at a given temperature. Its size indicates the extent of reaction.

The same ratio used for the equilibrium constant but calculated with concentrations at any moment, not just at equilibrium. Comparing it with the equilibrium constant predicts the direction of change.

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 until equilibrium is restored.

A substance that increases the concentration of hydrogen ions when dissolved in water. An Arrhenius base similarly increases hydroxide ion concentration in water.

A proton donor. Its conjugate base is what remains after it loses a proton, and together the pair forms a conjugate acid-base pair linked by transfer of one hydrogen ion.

Two species that differ by a single proton, such as ammonia and the ammonium ion. Each acid has a matching conjugate base and vice versa.

Able to act as either an acid or a base by donating or accepting a proton, depending on what it reacts with. Water and the hydrogen carbonate ion are common examples.

The negative base-10 logarithm of the hydrogen ion concentration in mol per litre. It gives a convenient scale from acidic values below 7 to basic values above 7 at 25 degrees Celsius.

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

The loss of electrons by a species, shown by an increase in its oxidation number. It always occurs together with reduction in a redox reaction.

The gain of electrons by a species, shown by a decrease in its oxidation number. The species that is reduced acts as the oxidising agent.

A number assigned to an atom in a compound that tracks its electron ownership, using agreed rules. Changes in these numbers reveal which species are oxidised or reduced.

A reaction in which electrons are transferred from one species to another, so oxidation and reduction happen together. Examples include displacement and combustion reactions.

A device that converts chemical energy into electrical energy, or the reverse, using two electrodes in electrolytes. Electrons flow through the external circuit while ions move in the electrolyte.

The voltage of a half-cell measured against the standard hydrogen electrode under standard conditions. More positive values indicate a greater tendency to be reduced.

A specific group of atoms within an organic molecule that gives it characteristic chemical properties and reactions, such as the hydroxyl group in alcohols.

A family of organic compounds with the same functional group, similar chemical properties and a regular gradation in physical properties, differing by one CH2 unit between members.

One of two or more compounds with the same molecular formula but different arrangements of atoms. Structural isomers differ in connectivity; stereoisomers differ in spatial arrangement.

The property of a molecule that cannot be superimposed on its mirror image, usually because a carbon atom is bonded to four different groups. Such molecules rotate plane-polarised light.

A reaction in which two molecules combine to form one product, typically when a molecule adds across a carbon-carbon double or triple bond, converting it to a single bond.

A reaction in which one atom or group in a molecule is replaced by another, common in alkanes and haloalkanes. No bond saturation change is required.

A polymer formed when monomers join with the loss of a small molecule such as water, often between a diacid and a diol or diamine. Polyesters and polyamides are examples.

A representation showing which atoms are bonded to which in a molecule, including functional groups, without necessarily drawing every bond as a separate line.