ORT Chemistry key terms

A region of space around a nucleus where the probability of finding an electron is high, described by quantum numbers n, l, m and spin; each orbital holds at most two electrons of opposite spin.

One of a set of integers or half-integers (principal, angular momentum, magnetic, spin) that specifies the state of an electron in an atom and constrains which orbitals and energies are allowed.

The arrangement of electrons among atomic orbitals, written following the Aufbau order and Hund's rule, which explains periodic trends in size, ionisation energy and reactivity.

No two electrons in one atom may share all four quantum numbers, so a single orbital accommodates at most two electrons, which must have opposed spins.

The regular variation across a period or down a group of properties such as atomic radius, ionisation energy, electronegativity and metallic character, explained by nuclear charge and shielding.

A relative measure of how strongly an atom in a bond attracts shared electron density; large differences give ionic character, small differences give covalent or polar covalent bonds.

A covalent bond in which electron density is unevenly shared because the bonded atoms differ in electronegativity, producing partial charges and often a molecular dipole moment.

The mixing of atomic orbitals of similar energy to form equivalent hybrid orbitals (sp, sp2, sp3) whose geometry explains observed bond angles and molecular shapes.

A model predicting molecular shape by arranging electron pairs around a central atom as far apart as possible, treating lone pairs as occupying more space 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, raising boiling points and shaping biomolecules.

A hydroxide that reacts both with acids to form salts and with strong alkalis to form hydroxo complexes, typical of aluminium, zinc and some transition metals.

A charged species in which ligands donate electron pairs to a central metal ion; its colour, stability and geometry depend on the metal and the ligands present.

An ion or molecule that donates one or more lone pairs to a central metal ion, forming a coordinate bond; examples include water, ammonia, chloride and cyanide.

The amount of substance containing 6.022 x 10^23 specified entities; it links mass in grams to numbers of particles through molar mass.

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

The reactant that is completely consumed first in a reaction and therefore determines the maximum amount of product formed; other reactants remain in excess.

The number written before a species in a balanced equation, giving the molar ratio in which reactants combine and products form, from which yields are calculated.

The splitting of an ionic compound or acid in solution into freely moving ions, which allows the solution to conduct electricity and determines its colligative behaviour.

The fraction of dissolved molecules or formula units that have separated into ions, ranging from near zero for weak electrolytes to essentially one for strong electrolytes.

The negative base-ten logarithm of hydrogen ion activity in a solution; values below 7 indicate acidity and above 7 alkalinity at standard temperature.

A solution that resists large pH change on adding small amounts of acid or base, usually a weak acid with its conjugate base or a weak base with its conjugate acid.

The reaction of ions from a dissolved salt with water, producing acidic, alkaline or neutral solutions depending on the strengths of the parent acid and base.

The formal charge an atom would carry if all its bonds were treated as fully ionic, calculated from electronegativity rules and used to track redox changes.

A reaction in which oxidation states change because one species loses electrons (is oxidised) while another gains them (is reduced), the two processes always occurring together.

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

The use of external electrical energy to drive a non-spontaneous redox reaction in an electrolyte, producing substances at the electrodes according to Faraday's laws.

An atom or group of atoms that gives a class of organic compounds its characteristic reactions and properties, such as hydroxyl, carbonyl, carboxyl or amino groups.

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

The existence of compounds with the same molecular formula but different arrangements of atoms, giving structural isomers or stereoisomers with distinct properties.

The property of a molecule that is not superimposable on its mirror image, usually because a carbon atom carries four different substituents, giving two optically active enantiomers.

A reaction in which a nucleophile replaces a leaving group on a saturated carbon, proceeding by a one-step SN1 or a concerted SN2 mechanism depending on substrate and conditions.

A reaction in which an electron-rich double or triple bond attacks an electrophile, forming a carbocation intermediate that is then captured, typical of alkenes and alkynes.

In the addition of HX to an unsymmetrical alkene, the hydrogen adds to the carbon bearing more hydrogens, because the more stable carbocation intermediate is formed.

The atom or group that departs with its bonding electron pair during substitution or elimination; good leaving groups are weak bases such as halide ions.

The joining of many small monomer units into a large molecule, either by addition across double bonds or by condensation with loss of a small molecule such as water.

A small molecule containing a double bond or two reactive functional groups that can link repeatedly to form a polymer chain or network.

The amide linkage formed by condensation between the carboxyl group of one amino acid and the amino group of another, giving the repeating backbone of proteins.

The identification of the ions or elements present in a sample using characteristic reactions, flame colours and precipitations, without measuring amounts.

A procedure in which a clean wire dipped in a sample is held in a flame; characteristic colours identify certain metal cations such as sodium, potassium, calcium and copper.