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| Question | Answer |
| Relative isotopic mass | The mass of an atom of an isotope compared with one-twelfth of the mass of an atom of carbon-12. |
| Relative atomic mass (Ar) | The weighted mean mass of an atom of an element, compared with one-twelfth of the mass of an atom of carbon-12. |
| Isotopes | Atoms of an element with different numbers of neutrons and different masses. |
| Relative molecular (Mr)/formula mass | The weighted mean mass of a molecule/formula unit, compared with one-twelfth of the mass of an atom of carbon-12. |
| Molar mass (M) | of a substance is the mass of one mole of a substance. It has units of gmol-1 |
| Avogadro constant (NA) | The number of entities per mole (6.02 x 10^23 mol-1 - number given on data sheet) i.e number of atoms per mole of the carbon-12 isotope. |
| Amount of substance | The quantity whose unit is the mole. Chemists use 'amount of substance' as a means of counting atoms. |
| Mole (symbol 'mol') | The unit for amount of substance (a mole is the amount of any substance containing as many particles as there are carbon atoms in exactly 12g of C-12). |
| Empirical formula | The simplest whole number ratio of atoms of each element present in a compound. |
| Molecular formula | The ACTUAL number of atoms of each element in a molecule. |
| "Anhydrous" | Refers to a substance that contains no water molecules. |
| "Hydrated" | Refers to a crystalline compound containing water molecules. |
| "Water of crystallisation" | Refers to water molecules that form an essential part of the crystalline structure of a compound. |
| Oxidation | Loss of electrons/increase in oxidation number. |
| Reduction REMEMBER "OIL RIG" | Gain of electrons/decrease in oxidation number. |
| First ionisation energy | The energy required to remove one electron from each atom in one mole of GASEOUS ATOMS, to form one mole of GASEOUS 1+ IONS. |
| Successive ionisation energy | The energy required to remove each electron in turn e.g. second ionisation energy - the energy required to remove one electron from each ION in one mole of gaseous 1+ IONS, to form one mole of gaseous 2+ IONS. |
| Covalent bond | A shared pair of electrons |
| Electronegativity | The ability of an atom to attract the bonding electrons in a covalent bond. |
| Hydrogen bond | A strong dipole-dipole attraction between an electron-deficient H atom on one molecule, and a lone pair of electrons on a highly electronegative atom on a DIFFERENT MOLECULE. (between molecules containing -OH (e.g. water) and -NH (e.g. NH3) groups. |
| Metallic bonding | The attraction of positive ions to delocalised electrons. |
| Ionic bonding | Electrostatic attraction between oppositely-charged ions. |
| Atomic Number | is the number of protons in an atom of an element |
| Mass number | number of proton +neutrons in the nucleus of an atom |
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