2.1. Atoms and Reactions: Module 2: Foundations In Chemistry: Chemistry OCR A A Level

Isotopes

Isotopes are atoms of the same element with the same atomic number, but with a different number of neutrons, resulting in a different mass number.

Relative atomic mass (Ar)

The weighted mean mass of an atom of an element, relative to one twelfth of the mean mass of an atom of the carbon-12 isotope.

Relative isotopic mass

The isotopic mass of an isotope relative to one twelfth of the mean mass of an atom of the carbon-12 isotope.

Relative molecular mass (Mr)

The weighted mean mass of a molecule of a compound, relative to one twelfth of the mean mass of an atom of the carbon-12 isotope.

Mass Spectrometry

Mass spectrometry is used to identify different isotopes and find the overall relative atomic mass of an element. Ions are very useful in the analytical technique of mass spectrometry.

Ionic bonding

Ionic bonding occurs between a metal and a nonmetal. Electrons are transferred from the metal to the non-metal to achieve full outer shells.

Amount of substance

Amount of substance defines the number of particles in a substance. It is measured in moles.

Molar mass

Molar mass is the mass per mole and has units g⋅mol⁻¹.

Molar gas volume

Molar gas volume is the gas volume per mole and has units dm³⋅mol⁻¹.

Empirical formula

Empirical formula is the simplest whole number ratio of atoms of each element present in a compound.

Molecular formula

Molecular formula is the number and type of atoms of each element in a molecule. It is the true number of each atom in the molecule.

Water of crystallisation

Water of crystallisation is water that is part of the crystalline structure. The molecules are stoichiometrically chemically bonded into the crystal structure.

Anhydrous

An anhydrous substance contains no water of crystallisation.

Hydrated

A hydrated substance contains water of crystallisation.

Standard solution

A standard solution is a solution of known concentration. This can be prepared by dissolving a known mass of solid in solvent and making it up to an exact volume.

Limiting reagent

The limiting reagent in a reaction is the reactant that is not in excess. It will be used up first, causing the reaction to stop. In calculations, you must use the moles of the limiting reagent

along with the stoichiometric ratios of reactant to product to calculate the moles of product formed.

Roman numerals

Roman numerals are used to indicate the magnitude of an oxidation number. For example, iron has two common oxidation states, iron(II) and iron(III).