Chemistry AS level Definition (Chp 1 to Chp 18)

CHEMISTRY DEFINITIONS:

Atomic Number: Number of protons in the nucleus of an atom.

Isotope: Atoms of the same element with different mass numbers.

Energy Levels: Each electron in an atom has its particular average amount of energy. The further away the electron is from the nucleus, the more energy it has. Each principal energy level corresponds to an electron shell at a certain distance from the nucleus.

Electronic Configuration: A way of representing the arrangement of electrons in atoms showing the principal quantum shells, sub-shells and number of electrons present.

First Ionization Energy, IE: Energy needed to remove 1 mole of electrons from 1 mole of atoms of an element in the gaseous state to form 1 mole of gaseous ions.

Relative Atomic Mass, Ar: Weighted average mass of atoms in a given sample of an element compared to the value of the unified atomic mass unit.

Mole: Amount of substance which contains 6.02 x 10^23 specified particles (atoms, molecules, ions or electrons).

Oxidation Number (Oxidation State): Number given to an atom or ion in a compound that describes how oxidised or reduced it is.

Ionic Bonding: Electrostatic attraction between oppositely charged ions (cations and anions).

Covalent Bonding: Electrostatic attraction between the nuclei of 2 atoms and a shared pair of electrons.

Lone Pairs (of electrons): Pairs of electrons in the outer shell of an atom that are not involved in bonding.

Bond Energy: Energy required to break 1 mole of a particular covalent bond in the gaseous state.

Bond Length: Distance between the nuclei of 2 covalently bonded atoms.

Electronegativity: The power of a particular atom that is covalently bonded to another atom to attract the bonding pair of electrons towards itself.

Ideal Gas: A gas whose volume varies in proportion to the temperature and in inverse proportion to the pressure. Noble gases such as Helium and Neon approach ideal behaviour because of their low intermolecular forces.

Enthalpy Change ΔH: Heat energy transferred during a chemical reaction.

Reaction Pathway Diagram: Shows the relative enthalpies of the reactants (on the left) and the products (on the right), and the enthalpy change as an arrow. It may also include the activation energy.

Activation Energy E_A: Minimum energy colliding particles must possess to break bonds to start a chemical reaction.

Standard Enthalpy Change of Reaction ΔH_r: Enthalpy change when the amounts of reactants shown in the stoichiometric equation react to give products under standard conditions.

Standard Enthalpy Change of Formation ΔH_f: Enthalpy change when one mole of a compound is formed from its elements under standard conditions.

Standard Enthalpy Change of Combustion ΔH_c: Enthalpy change when one mole of a substance is burnt in excess oxygen under standard conditions.

Standard Enthalpy Change of Neutralisation ΔH_neut: Enthalpy change when one mole of water is formed by the reaction of an acid with an alkali under standard conditions.

Oxidation: Loss of electrons from an atom, ion or molecule.

Reduction: Gain of electrons by an atom, ion or molecule.

Redox Reaction: A reaction in which oxidation and reduction take place at the same time.

Oxidising Agent: A substance which brings about oxidation by removing electrons from another atom or ion.

Reducing Agent: A substance which brings about reduction by donating (giving) electrons to another atom or ion.

Disproportionation: Simultaneous oxidation and reduction of the same species in a chemical reaction.

Le Chatelier's Principle: If 1 or more factors that affect a dynamic equilibrium are changed, the position of equilibrium moves to minimise this change.

Bronsted-Lowry Acid: Proton donor

Bronsted-Lowry Base: Proton acceptor

Rate of Reaction: Change in amount or concentration of a particular reactant or product per unit time.

Structural Formula: A formula that shows how many and the symbols of atoms are bonded to each carbon atom in an organic molecule.

Displayed Formula: 2D representation of an organic molecule, showing all its atoms (by their symbols) and their bonds (by short single, double or triple lines between the symbols).

Skeletal Formula: Simplified displayed formula with all carbon and hydrogen atoms and C-H bonds removed.

Homologous Series: A group of organic compounds having the same functional group, the same general formula, and similar chemical properties.

Functional Group: An atom or group of atoms in an organic molecule which determines the characteristic chemical reactions.

General Formula: A formula that represents a homologous series of compounds using letters and numbers.

Aliphatic Compounds: Straight chain or branched chain organic compounds, and also include cyclic organic compounds that do not contain benzene rings.