Ideal Gas: See Perfect Gas

Ideal Solution: A solution in which there are no interactions between the solvent and the solute particles.

Induction: A more electronegative element will inductively withdraw electron density along a σ bond, polarising the bond. This will stabilise carbanions, because the negative charge is somewhat pulled away from the carbon centre and spread a bit more.

Inert-Pair Effect: The instability of compounds with a p-block element in the oxidation state N relative to the N-2 state for elements beyond the first transition series, due to the low lying nature of electrons in the 4s orbital.

Inert: A species is inert if it has a low kinetic reactivity.

Infinite Square Well: Special case of the particle in a box where a particle is confined to a region by two potential walls that rise instantaneously to infinity. Within these walls, the potential energy is zero.

Initiation: In radical reactions, the initiation of the reaction by the creation of a radical species that will begin the chain reaction.

Inner-Sphere Mechanism: An electron transfer reaction which proceeds with the two ions forming bonds to a common shared ligand.

Insulator: This is a compound with negligible electrical conductivity, typically with a large band gap between the highest fully occupied band and the lowest fully unoccupied band.

Integrand: The function in an integral that is actually integrated; the portion that comes between the integral sign and the “dvariable”.

Intensity: The intensity of radiation can be measured as the number of photons that collide with a surface one metre square per unit time. As such, it is quite distinct from the energy of the radiation, each photon having an energy hν.

Intercalation Compound: A compound formed when a guest molecule or ion is inserted into cavities or other spaces, particularly between layers, in a host compound, resulting in little change of structure in the host compound.

Interchange Mechanism: The mechanism for the substitution of ligands of a complex where the rate determining step is the concerted addition of the entering ligand and loss of the leaving ligand, such that the entering and leaving ligands are both bonded to the metal ion in the transition state complex.

Intermediate: An intermediate in a reaction can be isolated. It is a species that is in a local energy minimum, i.e. it needs to be further activated to form the end products. It is not the same as a transition state.

Internal Energy: The total energy of a system. It is a state function, so depends solely upon the current state of the system (determined by variables such as pressure, temperature, etc).

Interstitial: This is a hole in the lattice which is not occupied in the ideal structure, an example being the octahedral holes in the fluorite structure.

Intrinsic Defects: These are defects brought about in a solid due to favourable thermodynamics of formation, and without any external influence.

Intrinsic Semiconductor: A semiconductor whose conduction behaviour is brought about only by the thermal promotion of electrons from the valence band to the conduction band.

Inversion: If a chiral centre changes configuration during the course of a reaction, inversion has occurred. See also retention.

Ionic Atmosphere: The Coulombic interaction between particles in an ionic solution leads to a tendency for particles to be surrounded by an ionic atmosphere in which particles of the opposite charge predominate.

Ionic Model: This treats solids as being made up of rigid, oppositely charged spheres.

Ionic Radius: This gives the size of an ion when considered as a rigid sphere in the Ionic Model.

Ionization Energy: The energy change accompanying the loss of an electron from a species.

Ion: An atom or group of atoms which has a charge imbalance due to a gain or loss of electrons.

Ipso: A position on a phenyl ring – see ortho.

Irreducible Representation: The basic types of behaviour that orbitals may show when the symmetry operations of the group are applied to them.

Irving-Williams Series: The series of divalent metal cations when arranged in order of the magnitude of the stability constants for the metal ion complex with a wide range of ligands.

Isoelectronic: Two species are isoelectronic if they have the same electronic configurations.

Isolated system: A system that can exchange neither energy nor matter with its surroundings.

Isomerise: To convert to a different isomer.

Isomer: Compounds with the same composition and molecular weight, but differing structure are said to be isomers.

Isotope: Atoms with the same atomic number but differing mass numbers are isotopes. They differ only in the number of neutrons in the nucleus.