Unit 1 Definitions

Simply by learning these definitions you can give yourselves a huge boost in your A-Level Chemistry Unit 1!

  • Isotope

Isotopes are atoms of the same element (same number of protons) with different numbers of neutrons

  • Relative atomic mass, Ar

Is the weighted mean mass of an atom of an element compared with one-twelfth of the mass of carbon-12

  • Relative isotopic mass

Is the weighted mean mass of an atom of an isotope compared with one-twelfth of the mass of carbon-12

  • Relative molecular mass

Is the weighted mean mass of a molecule, compared with one-twelfth of the mass of carbon-12

  • Relative formula mass

A term used for the relative masses of ionic compounds or ions. The term is used to avoid the suggestion that their formula represents molecules. The formula mass is relative to the mass of an atom of carbon-12 which is defined as exactly 12.

  • Mole

Is the SI unit for the amount of substance. A mole has as many specified entities (atoms, molecules, ions, electrons and so on) as there are atoms in exactly 12g of the isotope Carbon 12. The number of entities per mole is the Avogadro constant.

  • Avogadro constant

The number of atoms, molecules, ions or other entities in one mole of a substance. The Avogadro constant is equal to 6.02 x 1023

  • Molar mass

The number of atoms, molecules, ions or other entities in one mole of a substance.

  • Empirical formula

The simplest whole number molar ratio of the amounts of elements in a compound.

  • Molecular formula

A formula which shows the number of atoms of each element in a molecule.

  • Anhydrous

Anhydrous salts are the compounds left after removing the water from a hydrated salt. Anhydrous means ‘without water’

  • Hydrated

Contains water

  • Water of crystallisation

Water molecules which make up part of the crystal structure of a compound

  • Acid

A Proton Donor

  • Base

A Proton acceptor

  • Alkali

A soluble base

  • Oxidation state

The states of oxidation or reduction shown by an element in its chemistry.
Describing the oxidation state of an entity according to its oxidation state is a useful way of making sense of a large number of compounds and reactions

  • First ionisation energy

The energy required to remove 1 electron from every atom in 1 mole of gaseous atoms to form a mole of gaseous 1+ ions
X –> X+ + e

  • Electron Orbital

a subdivision of a sub shell and shell. Each orbital is defined by its energy, shape and direction in space. Each orbital contains up to two electrons.

  • Electron Shell

An energy level representing the distance of a group of electrons from the nucleus of an atom.

  • Ionic bond

A bond formed by the electrostatic forces of attraction between a positively charged ion and a negatively charged ion

  • covalent bond

Formed when atoms share a pair of electrons. The atoms are held together by the attraction between positive charges in the nuclei and the negative charge on the shared electron pair.

  • Dative covalent bond

A bond formed when one atom contributes both electrons in a covalent bond ( forms a shared pair of electrons)

  • Electronegativity

The measure of the pull of an atom on a pair of electrons in a chemical bond.

  • Permanent dipole

An intermolecular force between two polar molecules.

  • Polar covalent bond

When two atoms with different electronegativities bond covalently. Electrons are drawn toward the more electronegative element. This causes the bond to polarise with one end becoming δ⁻ and the other becoming δ⁺

  • Intermolecular forces

Attractive forces between molecules.

  • Hydrogen bond

A type of attraction between molecules which is much stronger than other types of intermolecular forces, but much weaker than covalent bonding.
Molecules which can exhibit hydrogen bonding must contain a hydrogen directly bonded to a N, O or F containing a lone pair of electrons.

  • Metallic bond

Bonding between atoms in a metal crystal. Each metal atom contributes electrons from its outer shell to a sea of delocalised electrons.

  • Periodicity

A repeating pattern of properties shown across different periods

  • Element

All the atoms of an element have the same number of protons in the nucleus. The atomic number identifies an element and fixes its position in the periodic table.

Whilst it looks like a very long list, don’t forget that you already know many of these and will be continuing to develop your knowledge of all of them over the term!

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When have these definitions appeared in the OCR exam papers?
Definition Jan 2009 June 2009 Jan 2010 June 2010 Jan 2011 June 2011 Jan 2012
Isotopes Y I
Relative Isotopic Mass Y Y I Y
Relative Atomic Mass Y Y Y
Amount of Substance (the Mole) Y
Covalent Bond Y
Ionic Bond Y  Y Y
Metallic Bond I
Van der Waals’ Forces Y
Oxidation  I I I I I I
Reduction I I I I I I
Disproportionation Reaction I I I
Thermal Decomposition Reaction Y
First Ionisation Energy Y  I I Y Y
Atomic Number Y
Base  I
Salt  Y  I  I  Y
Water of crystallisation  I  I  Y
Periodicity y
Orbital  Y I I  Y
Shell I
Sub Shell I
 Electronegativity  Y