As Chemistry Definitions

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acid

Back 1

A species that is a proton donor.

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alkali

Back 2

A type of base that dissolves in water forming hydroxide ions, OH–(aq) ions.

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amount of substance

Back 3

The quantity whose unit of the mole. Chemists use ‘amount of substance’ as a means of counting atoms.

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anhydrous

Back 4

A substance that contains no water molecules.

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atomic orbital

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A region within an atom that can hold up to two electrons, with opposite spins.

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atomic (proton) number

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The number of protons in the nucleus of an atom.

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Avogadro constant, NA

Back 7

The number of atoms per mole of the carbon-12 isotope (6.02 × 1023 mol–1).

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base

Back 8

A species that is a proton acceptor.

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compound

Back 9

A substance formed from two or more chemically bonded elements in a fixed ratio, usually shown by a chemical formula

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concentration

Back 10

The amount of solute, in mol, per 1 dm3 (1000 cm3) of solution.

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coordinate bond

Back 11

A shared pair of electrons which has been provided by one of the bonding atoms only; also called a dative covalent bond.

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covalent bond

Back 12

A bond formed by a shared pair of electrons.

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dative covalent

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A shared pair of electrons which has been provided by one of the bonding atoms only; also called a coordinate bond.

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dehydration

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An elimination reaction in which water is removed from a saturated molecule to make an unsaturated molecule.

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delocalised electrons

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Electrons that are shared between more than two atoms.

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electron configuration

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The arrangement of electrons in an atom.

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electronegativity

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A measure of the attraction of a bonded atom for the pair of electrons in a covalent bond.

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electron shielding

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The repulsion between electrons in different inner shells. Shielding reduces the net attractive force from the positive nucleus on the outer-shell electrons.

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elimination reaction

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The removal of a molecule from a saturated molecule to make an unsaturated molecule.

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empirical formula

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The simplest whole-number ratio of atoms of each element present in a compound.

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giant covalent lattice

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A three-dimensional structure of atoms, bonded together by strong covalent bonds.

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giant ionic lattice

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A three-dimensional structure of oppositely charged ions, bonded together by strong ionic bonds.

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giant metallic lattice

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A three-dimensional structure of positive ions and delocalised electrons, bonded together by strong metallic bonds.

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group

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A vertical column in the Periodic Table. Elements in a group have similar chemical properties and their atoms have the same number of outer shell electrons.

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hydrated

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Crystalline and containing water molecules.

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hydrolysis

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A reaction with water or aqueous hydroxide ions that breaks a chemical compound into two compounds.

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ion

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A positively or negatively charge atom or (covalently bonded) group of atoms (a molecular ion).

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ionic bonding

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The electrostatic attraction between oppositely charged ions.

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(first) ionisation energy

Back 29

The energy required to remove one electron from each ion in one mole of gaseous atoms to form one mole of gaseous 1+ ions.

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(second) ionisation energy

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The energy required to remove one electron from each ion in one mole of gaseous 1+ ions to form one mole of gaseous 2+ ions.

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(successive) ionisation energy

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A measure of the energy required to remove each electron in turn, e.g. the second ionisation energy is the energy required to remove one electron from each ion in one mole of gaseous 1+ ions to form one mole of gaseous 2+ ions.

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isotopes

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Atoms of the same element with different numbers of neutrons and different masses.

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limiting reagent

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The substance in a chemical reaction that runs out first.

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lone pair

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An outer-shell pair of electrons that is not involved in chemical bonding.

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mass (nucleon) number

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The number of particles (protons and neutrons) in the nucleus.

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metallic bond

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The electrostatic attraction between positive metal ions and delocalised electrons.

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molar mass, M

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The mass of a mole of a substance. The units of molar mass are g mol–1.

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molar volume

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The volume per mole of a gas. The units of molar volume are dm3 mol–1. At room temperature and pressure the molar volume is approximately 24.0 dm3 mol–1.

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mole

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The amount of any substance containing as many particles as there are carbon atoms in exactly 12 g of the carbon-12 isotope.

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molecular formula

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The number of atoms of each element in a molecule.

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molecular ion, M+

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The positive ion formed in mass spectrometry when a molecule loses an electron.

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molecule

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A small group of atoms held together by covalent bonds.

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nomenclature

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A system of naming compounds.

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period

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A horizontal row of elements in the Periodic Table. Elements show trends in properties across a period.

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periodicity

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A regular periodic variation of properties of elements with atomic number and position in the Periodic Table.

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precipitation reaction

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The formation of a solid from a solution during a chemical reaction. Precipitates are often formed when two aqueous solutions are mixed together.

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principal quantum number, n

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A number representing the relative overall energy of each orbital, which increases with distance from the nucleus. The sets of orbitals with the same n value are referred to as electron shells or energy levels.

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rate of reaction

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The change in concentration of a reactant or a product in a given time.

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relative atomic mass, Ar

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The weighted mean mass of an atom of an element compared with one-twelfth of the mass of an atom of carbon-12.

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relative formula mass

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The weighted mean mass of a formula unit compared with one-twelfth of the mass of an atom of carbon-12.

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relative isotopic mass

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The mass of an atom of an isotope compared with one-twelfth of the mass of an atom of carbon-12.

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relative molecular mass, Mr

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The weighted mean mass of a molecule compared with one-twelfth of the mass of an atom of carbon-12.

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salt

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A chemical compound formed from an acid, when a H+ ion from the acid has been replaced by a metal ion or another positive ion, such as the ammonium ion, NH4+.

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shell

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A group of atomic orbitals with the same principal quantum number, n. Also known as a main energy level.

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simple molecular lattice

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A three-dimensional structure of molecules, bonded together by weak intermolecular forces.

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skeletal formula

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A simplified organic formula, with hydrogen atoms removed from alkyl chains, leaving just a carbon skeleton and associated functional groups.

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species

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Any type of particle that takes part in a chemical reaction.

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spectator ions

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Ions that are present but take no part in a chemical reaction.

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standard conditions

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A pressure of 100 kPa (1 atmosphere), a stated temperature, usually 298 K (25 °C), and a concentration of 1 mol dm–3 (for reactions with aqueous solutions).

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standard solution

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A solution of known concentration. Standard solutions are normally used in titrations to determine unknown information about another substance.

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standard state

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The physical state of a substance under the standard conditions of 100 kPa (1 atmosphere) and 298 K (25 °C).

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stoichiometry

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The molar relationship between the relative quantities of substances taking part in a reaction.

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sub-shell

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A group of the same type of atomic orbitals (s, p, d or f) within a shell.

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water of crystallisation

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Water molecules that form an essential part of the crystalline structure of a compound.