Important AS definitions (Part I)

Atomic number or proton number is the total number of protons in the nucleus of an atom.

Mass Number or nucleon number is the total number of protons and neutrons in the nucleus of an atom.

Isotopes are atoms with the same number of protons but different numbers of neutrons.

The relative isotopic mass is the mass of one isotope of an element compared to 1/12 the mass of the 12C isotope.

The relative atomic mass is the average mass of one atom compared to 1/12 the mass of the 12C isotope.

First Ionisation Energy is the energy required to remove one mole of electrons from 1 mole of gaseous atoms to produce 1 mole of gaseous unipositive ions.

First Electron Affinity is the energy liberated when one electron is added to each atom in 1 mole of gaseous atoms to form 1 mole of gaseous ions with a uninegative charge.

Electronegativity is the power of an atom to attract bonding electrons.

Oxidation is loss of electrons. Remember OIL RIG
Reduction is gain of electrons.

Oxidising agent is a substance which removes electrons from another substance and itself is reduced.

Reducing agent is a substance which gives electrons to another substance and itself is oxidised.

Disproportionation is simultaneous oxidation and reduction of one species in a chemical reaction.

Metallic bond is the electrostatic attractions between the positive ions and a 'sea' of delocalised valence electrons surrouding the positive ions.

Ionic bond is the attraction between the oppositely charged ions formed by transfer of one or more electrons from one atom to another.

Covalent bond is the attraction of the nuclei for the shared pair of electrons between two atoms.

Sigma bond is the covalent bond formed by ‘head on’ overlap of atomic orbitals.

Pi Bond is the covalent bond formed by sideways overlap of atomic orbitals.

Valence Shell Electron Pair Repulsion (VSEPR) Theory states that the shape of a molecule is determined by the repulsion between the electron pairs in the valence shell. According to the VSEPR , the electron pairs will arrange themselves so as to be as far apart as possible.

Polarising power is the ability of a positive ion to attract electrons away from a negative ion.

Dipole-Dipole Forces are intermolecular attractions between molecules which are permanently polarised.

Hydrogen Bond is the electrostatic attraction between a hydrogen atom which is bonded to a very electronegative atom and an electronegative atom of a neighbouring molecule.

Van Der Waals Forces are intermolecular forces of attraction which arise from temporary
dipoles in molecules.

Standard Enthalpy Of Formation is the enthalpy change when 1 mole of the substances is formed from its elements at their natural states at 298K and 1 atm pressure.

Standard Enthalpy Of Combustion is the enthalpy change when 1 mole of an element or compound is completely burned in oxygen at 298K and 1 atm pressure.

Standard Enthalpy Of Neutralisation is the enthalpy change when 1 mole of water is formed from the reaction between an acid and a base at 298K and 1 atm pressure.

Hess’s Law states that the total energy change resulting from a chemical reaction is dependant only on the initial and final states of the reactants and is independent of the reaction route.

Bond energy is the energy required to break 1 mole of bonds from gaseous molecules into gaseous atoms.

Reference : Stephen Oakes