Chemistry Key Definitions (AS AQA)

Key definitions for the January exams. (AQA Chemistry new spec.) Layed out simply so that you can stay on top of the definitions. Many definitions are taken from the official AQA textbook and so are accurate to this course. There are other sources too, which provide a simpler view on the definitions. I hope I've managed to condense the key words down well enough for good revision.


Unit 1.1 (Foundation)

Key definitions which get asked a lot in exams are underlined and in bold.

What is atomic mass?

The number of protons in the nucleus of an atom.

What is the mass number?

The total number of protons and neutrons in the nucleus of an atom.

What is an isotope?

Atoms with the same atomic number but different mass numbers OR atoms with the same number of protons but different numbers of neutrons.

James Rushworth

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Unit 1.1 continued

What is relative atomic mass?

The average mass of an atom in relation to 1/12 of the mass of one atom of carbon-12.

What is relative molecular mass?

The average mass of a molecule in relation to 1/12 of the mass of one molecule of carbon-12.

Explain what first ionisation energy means...

The energy required to remove one electron from each of a mole of free gaseous atoms in that element.

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Unit 1.2 (Amount of substance)

What is Avogadro's number (definition)?

The number of elementary particles that there are in 12.0 grams of carbon-12. [6.022 x 10^23]

Define the term mole...

A quantity of a substance which contains the Avogadro number [6.022 x 10^23] particles (e.g. atoms, molecules or ions.)

What is the empirical formula?

The simplest whole number ratio in which atoms in a compound combine together.

Molecular formula tells us the numbers of atoms of each different element that make up a molecule of a compound.

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Unit 1.3 (Bonding)

What is an ionic bond?

A chemical bond in which an electron/electrons is transferred from one atom to another, resulting in the formation of oppositely charged ions with electrostatic forces of attraction between them. Happens between a metal and a non metal, e.g sodium and chlorine ionically bonds to get sodium chloride.

What is a covalent bond?

Describes a chemical bond in which electrons are shared between two atoms. Usually between non-metals. e.g methane, CH4

What is a metallic bond?

A chemical bond in which outer electrons are spread over a lattice of metal ions in a delocalised system.

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Unit 1.3 continued

What is a simple covalent bond (or molecular crystal)?

Consist of molecules held in a regular array by one or more of three types of intermolecular forces: van der Waals, dipole-dipole or hydrogen bonding. e.g Iodine.

What is a macromolecular crystal (macromolecule)?

A substance where covalent bonds extend throughout the compound, forming a giant covalent structure. e.g diamond or graphite

What is meant by the term polar?

Describes a molecule in which the charge is not symmetrically distributed so that one area if slightly positively charged and another is slightly negatively charged.

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Unit 1.3 continued

Define electronegativity...

The power of an atom to attract the electrons in a covalent bond.

What is a dative covalent bond (co-ordinate bond)?

Covalent bonding in which both the electrons in the bond come from one of the atoms in the bond.

What is meant by the term Dipole-dipole force?

An intermolecular force that results from the attraction between molecules with permanet dipoles. These are a result of the polarity in a particular bond. Acts only between certain types of molecule.

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Unit 1.3 continued

What are van der Waals forces?

An intermolecular force of attraction that is caused by instantaneous dipoles. It acts between all atoms and molecules. It is also known as temporary dipole-temporary dipole bonding.

What is meant by the term hydrogen bonding (H-bonding)?

A type of intermolecular force in which a hydrogen atom, covalently bonded to an electronegative atom, interacts with another electronegative atom. e.g. ammonia, NH3, has hydrogen bonds.

These are the strength of the intermolecular forces, from weakest to strongest:

  • van der Waals forces
  • Dipole- dipole forces
  • Hydrogen bonding
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Unit 1.4 (Periodicity)


  • s block element
  • p- block element
  • d- block element

An s-block element has s-electrons only it's outer shell.

A p-block element has p-electrons in it's outer shell.

A d- block element has s and d-electrons but no p-electrons in it's outer shell.

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Unit 1.5 (Introduction to organic chemistry)

What is the homologous series?

A set of organic compounds with the same functional group. The compounds differ in the length of their hydrocarbon chains.

What is meant by an isomer?

One of two (or more) compounds with the same molecular formula but different structural formula, i.e. the same atoms are arranged differently in space.

There are 3 different types of isomerism, what are they?

  • Structural
  • Chain
  • Functional
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Unit 1.5 continued

What is a functional group?

An atom or group of atoms in an organic molecule which is responsible for the characteristic reactions of that molecule.

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Great thanks =]



Pretty good. :)



Thank you! really helpful for last minute revision



this is awesome thank you :)




Ray Sunshine


how about unit 2 definitions

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