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Glossary
acid
A species that is a proton donor.
alkali
A type of base that dissolves in water forming hydroxide ions,
OH–(aq) ions.
anhydrous
A substance that contains no water molecules.
anion
A negatively charged ion.
atomic orbital
A region within an atom that can hold up to two electrons, with
opposite spins.
atomic (proton)
number
The number of protons in the nucleus of an atom.
Avogadro constant, NA The number of atoms per mole of the carbon-12 isotope
(6.02 × 1023 mol–1).
base
A species that is a proton acceptor.
cation
A positively charged ion.
concentration
The amount of solute, in mol, per 1 dm3 (1000 cm3) of solution.
coordinate bond
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
A bond formed by a shared pair of electrons.
dative covalent
A shared pair of electrons which has been provided by one of
the bonding atoms only; also called a coordinate bond.
delocalised electrons
Electrons that are shared between more than two atoms.
displacement reaction
A reaction in which a more reactive element displaces a less
reactive element from an aqueous solution of the latter’s ions.
disproportionation
The oxidation and reduction of the same element in a redox
reaction.
electron configuration
The arrangement of electrons in an atom.
electronegativity
A measure of the attraction of a bonded atom for the pair of
electrons in a covalent bond.
electron shielding
The repulsion between electrons in different inner shells.
Shielding reduces the net attractive force from the positive
nucleus on the outer-shell electrons.
empirical formula
The simplest whole-number ratio of atoms of each element
present in a compound.
giant covalent lattice
A three-dimensional structure of atoms, bonded together by
strong covalent bonds.
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giant ionic lattice
A three-dimensional structure of oppositely charged ions,
bonded together by strong ionic bonds.
giant metallic lattice
A three-dimensional structure of positive ions and delocalised
electrons, bonded together by strong metallic bonds.
group
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.
hydrated
Crystalline and containing water molecules.
hydrocarbon
A compound of hydrogen and carbon only.
hydrogen bond
A strong dipole–dipole attraction between an electron-deficient
hydrogen atom (O–Hδ+ or N–Hδ+) on one molecule and a lone
pair of electrons on a highly electronegative atom (H–O:δ– or H–
N:δ–) on a different molecule.
intermolecular force
An attractive force between neighbouring molecules.
Intermolecular forces can be van der Waals’ forces (induced
dipole–dipole forces), permanent dipole–dipole forces or
hydrogen bonds.
ion
A positively or negatively charge atom or (covalently bonded)
group of atoms (a molecular ion).
ionic bonding
The electrostatic attraction between oppositely charged ions.
(first) ionisation
energy
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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(second) ionisation
energy
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.
(successive)
ionisation energy
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.
isotopes
Atoms of the same element with different numbers of neutrons
and different masses.
limiting reagent
The substance in a chemical reaction that runs out first.
lone pair
An outer-shell pair of electrons that is not involved in chemical
bonding.
mass (nucleon)
number
The number of particles (protons and neutrons) in the nucleus.
metallic bond
The electrostatic attraction between positive metal ions and
delocalised electrons.
molar mass, M
The mass mole of a substance. The units of molar mass are
g mol–1.
molar volume
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.
mole
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
The number of atoms of each element in a molecule.
molecule
A small group of atoms held together by covalent bonds.
oxidation
Loss of electrons or an increase in oxidation number.
oxidation number
A measure of the number of electrons that an atom uses to
bond with atoms of another element. Oxidation numbers are
derived from a set of rules.
oxidising agent
A reagent that oxidises (takes electrons from) another species.
period
A horizontal row of elements in the Periodic Table. Elements
show trends in properties across a period.
periodicity
A regular periodic variation of properties of elements with atomic
number and position in the Periodic Table.
permanent dipole
A small charge difference across a bond resulting from a
difference in electronegativities of the bonded atoms.
permanent dipole–
dipole force
An attractive force between permanent dipoles in neighbouring
polar molecules.
pi-bond (π-bond)
The reactive part of a double bond formed above and below the
plane of the bonded atoms by sideways overlap of p-orbitals.
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polar covalent bond
A bond with a permanent dipole.
polar molecule
A molecule with an overall dipole, taking into account any
dipoles across bonds.
precipitation reaction
The formation of a solid from a solution during a chemical
reaction. Precipitates are often formed when two aqueous
solutions are mixed together.
principal quantum
number, n
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.
redox reaction
A reaction in which both reduction and oxidation take place.
reducing agent
A reagent that reduces (adds electron to) another species.
reduction
Gain of electrons or a decrease in oxidation number.
relative atomic mass,
Ar
The weighted mean mass of an atom of an element compared
with one-twelfth of the mass of an atom of carbon-12.
relative formula mass
The weighted mean mass of a formula unit compared with onetwelfth of the mass of an atom of carbon-12.
relative isotopic mass
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
The weighted mean mass of a molecule compared with onetwelfth of the mass of an atom of carbon-12.
Salt
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+.
shell
A group of atomic orbitals with the same principal quantum
number, n. Also known as a main energy level.
simple molecular
lattice
A three-dimensional structure of molecules, bonded together by
weak intermolecular forces.
spectator ions
Ions that are present but take no part in a chemical reaction.
standard solution
A solution of known concentration. Standard solutions are
normally used in titrations to determine unknown information
about another substance.
sub-shell
A group of the same type of atomic orbitals (s, p, d or f) within a
shell.
thermal
decomposition
The breaking up of a chemical substance with heat into at least
two chemical substances.
van der Waals’ forces
Very weak attractive forces between induced dipoles in
neighbouring molecules.
volatility
The ease that a liquid turns into a gas. Volatility increases as
boiling point decreases.
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water of crystallisation Water molecules that form an essential part of the crystalline
structure of a compound.
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