OCR AS CHEMISTRY A DEFINITIONS

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Isotope	Atoms of the same element with a different number of neutrons.
Atomic Number	The number of protons in the nucleus of an atom.
Mass Number	The total number of protons and neutrons in the nucleus.
Ion	A positively or negatively charged atom or covalently bonded group of atoms.
Relative Isotopic Mass	The mass of an atom of an isotope compared with 1/12th the mass of an atom of Carbon-12.
Relative Atomic Mass	The weighted mean mass of an atom of an element compared with 1/12th the mass of an atom of Carbon-12.
Relative Molecular Mass	The weighted mean mass of a molecule compared with 1/12th the mass of an atom of Carbon-12.
Amount of Substance	The quantity whose unit is the mole.
Avagadro's Constant	The number of atoms per mole of the Carbon-12 isotope. (6.022x10^23)
A Mole	The amount of any substance containing as many particles as there are carbon atoms in exactly 12g of the Carbon-12 isotope.
Molar Mass	The mass per mole of a substance.
Empirical Formula	The simplest whole number ratio of atoms of each element present in a compound.
Molecule	A small group of atoms held together by covalent bonds.
Molecular Formula	The actual number of atoms of each element in a molecule.
Concentration	The amount of solute, in mol, dissolved per 1dm^3 of solution.
Standard Solution	A solution of known concentration.
Species	Any type of particle that takes part in a chemical reaction.
Stoichiometry	The molar relationship between the relative quantities of substances taking part in a reaction, basically mole ratio.
Acid	A proton donor
Base	A proton acceptor
Alkali	A type of base that dissolves in water forming hydroxide ions, OH-.
Salt	Any chemical compound formed from an acid when a H+ ion from the acid has been replaced by a metal ion or another positive ion.
Cation	A positively charged ion.
Anion	A negatively charged ion.
Hydrated	A crystalline compound containing water molecules.
Anhydrous	A substance that contains no water molecules.
Water of Crystalisation	Water molecules that form an essential part of the crystalline structure of a compound.
Oxidation Number	A measure of the number of electrons that an atom uses to bond with atoms of another element.
Oxidation	A loss of electrons or an increase in oxidation number.
Reduction	A gain of electrons or a decrease in oxidation number.
Reducing Agent	A reagent that reduces (adds electrons to) another species.
Oxidising Agent	A reagent that oxidises (takes electrons from) another species.
First Ionisation Energy	The energy required to remove one electron from each atom in one mole of gaseous atoms to form one mole of gaseous 1+ ions.
Electron Shielding	The repulsion between electrons in different inner shells. It reduces the net attractive force from the positive nucleus on the outer shell.
Successive Ionisation Energies	A measure of the energy required to remove each electron in turn.
Shell	A group of atomic orbitals with the same principal quantum number, n.
Principal Quantum Number, n	A number representing the relative overall energy of each orbital, which increases with distance from the nucleus. (2n^2)
Atomic Orbital	A region within an atom that can hold up two electrons with opposite spins.
Sub-shell	A group of the same type of atomic orbitals (s, p, d or f) within a shell.
Electron Configuration	The arrangement of electrons in an atom.
Compound	A substance formed from two or more chemically bonded elements in a fixed ratio, usually shown by a chemical formula.
Ionic Bond	The electrostatic attraction between oppositely charged ions.
Giant Ionic Lattice	A 3D structure of oppositely charged ions held together by strong ionic bonds.
Group	A vertical column in the periodic table. Elements in the same group have similar chemical properties and their atoms have the same number of outer shell electrons.

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