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Allotrope = Forms of the same element with differing structures.

Bond Angle = The angle between two covalent bonds.

Bond Length = The distance between the nuclei of two bonded atoms.

Bonding Pair = A pair of electrons involved in a covalent bond.

Brittle = A property of a material describing the fact that it shatters when hit.

Covalent Bond = The attraction between two atoms caused by the overlapping of electron orbitals resulting in the sharing of one or more pairs of electrons between the atoms.

Crystal = A solid structure in which the constituent atoms or ions have a regular arrangement that extends out in three dimensions.

Dative Covalent Bond = A covalent bond formed by the donation of a lone pair of electrons from one atom into an empty orbital on another atom. Also called a co-ordinate bond.

Delocalised Electrons = Electrons that are not attached to any particular atom in a structure and are free to move between the atoms. They are present in metallic lattices.

Dipole = The unequal sharing of electron density in a molecule resulting in regions of partial positive charge (δ+) and regions of partial negative charge (δ−). Dipoles may be permanent, temporary or induced.

Ductile = A property of a material, such as some metals, describing the fact that it can be drawn into wires.

Electron Density A measure of how likely it is that an electron will be found at a specific location within a molecule.

Electronegativity (χ) The power of an atom to withdraw electron density and attract the pair of bonding electrons in a covalent bond.

Giant Covalent A regular lattice structure wherein all the atoms are covalently bonded.

Hydrogen Bonding A special example of a permanent dipole-dipole intermolecular force that occurs when hydrogen is bonded to nitrogen, oxygen or fluorine.

A hydrogen bond is the attraction between a lone pair on a nitrogen, oxygen or fluorine atom on one molecule, and the δ+ hydrogen atom (bonded to nitrogen, oxygen or fluorine) on a neighbouring molecule.

Induced Dipole The result of a non-polar molecule moving close to a molecule with a dipole. The dipole in one molecule distorts the electron density in the other, creating a dipole in that molecule. This occurs in the formation of London Forces.

Intermolecular Force An attraction between two or more covalent molecules. The three main types are London Forces, permanent dipole-dipole forces and Hydrogen Bonds.

Ionic Bond The electrostatic attraction between oppositely-charged ions. This force holds the ions together in an ionic lattice.

Ionic Lattice A regular arrangement of positively and negatively-charged ions held together by electrostatic forces acting between them.

Lattice A regular, three dimensional, arrangement of atoms or ions.

Linear The shape a molecule adopts to minimize mutual repulsion when it has two bonding pairs of electrons around its central atom.

London Forces Weak intermolecular forces of attraction that occur when temporary dipoles cause induced dipoles in neighbouring atoms.

Lone Pair A pair of electrons that is not involved in a covalent bond.

Malleable A property of a material, such as some metals, describing the fact that its shape can be altered by hammering.

Metallic Bond The attraction between positively-charged metal ions and a sea of electrons from their highest energy shell that have become delocalised. This force holds metal atoms together in a giant metallic lattice.

Metallic Lattice A regular arrangement of positively-charged metal ions surrounded by a sea of delocalised electrons and held together by the electrostatic attraction between them.

Molecule A substance containing atoms which are covalently bonded together. Each molecule has a defined and finite number of atoms given by its molecular formula. The molecules are held together by intermolecular forces.

Mutual Repulsion The repulsion between pairs of electrons.

Non-Polar Bond A covalent bond in which the electron density is distributed symmetrically around both atoms.

Non-Polar Molecule A molecule in which either there are no polar bonds, or the polar bonds are arranged symmetrically so that the partial charges cancel out and the molecule has no overall permanent dipole.

Octahedral The shape a molecule adopts to minimize mutual repulsion when it has six bonding pairs of electrons around its central atom.

Permanent Dipole-Dipole Forces Intermolecular forces between regions of partial positive and partial negative charge on molecules with permanent dipoles.

Polar Bond A covalent bond between two atoms with different electronegativities, which causes an asymmetrical distribution (unequal sharing) of the electron density in the bond. This, in turn, creates a slight positive charge (δ+) on one atom and a slight negative charge (δ−) on the other.

Polar Molecule A molecule containing polar bonds arranged asymmetrically so that the partial charges do not cancel out. The molecule therefore has an overall dipole.

Temporary Dipole A spontaneous and transient dipole resulting from the chance uneven distribution of electron density of a molecule.

Tetrahedral The shape a molecule adopts to minimize mutual repulsion when it has four bonding pairs of electrons around its central atom.

Trigonal Bipyramidal The shape a molecule adopts to minimize mutual repulsion when it has five bonding pairs of electrons around its central atom.

Trigonal Planar The shape a molecule adopts to minimize mutual repulsion when it has three bonding pairs of electrons around its central atom.

Trigonal Pyramidal The shape a molecule adopts to minimize mutual repulsion when it has three bonding pairs and one lone pair of electrons around its central atom.

V-Shaped The shape a molecule adopts to minimize mutual repulsion when it has two lone pairs and two bonding pairs about its central atom. Also called bent