Chemistry Key Definitions Module 1

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Isotopes

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are atoms of the same element with different numbers of neutrons

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Atomic (proton) number

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is the number of protons in the nucleus of an atom

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Mass (nucleon) number

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is the number of particles (protons and neutrons) in the nucleus

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An Ion

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is a positively or negatively charged atom or (covalently bonded) group of atoms (a molecular ion)

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Relative isotopic mass

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is the mass of an atom of an isotope compared with one-twelfth of the mass of an atom of carbon-12

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Relative atomic mass

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

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Relative molecular mass

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is the weighted mean mass of a molecule compared with one-twelfth of the mass of an atom of carbon-12

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Relative formula mass

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is the weighted mean mass of a formula unit compared with one-twelfth of the mass of an atom of carbon-12

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Amount of substance

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is the quantity whose unit is the mole. Chemists use 'amount of substance' as a means of counting atoms.

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The Avogadro constant

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is the number of atoms per mole of the carbon-12 isotope (6.02 x 10^23)

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A mole

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is the amount of any substance containing as many particles as there are carbon atoms in exactly 12g of the carbon-12 isotope

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Molar mass

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is the mass per mole of a substance. The units of molar mass are g/mol

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The empirical formula

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is the simplest whole-number ratio of atoms of each element present in a compound

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A molecule

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is a small group of atoms held together by covalent bonds

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The molecular formula

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is the actual number of atoms of each element in a molecule

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Molar volume

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is the volume per mole of a gas. The units of molar volume are dm^3/mol. At room temperature and pressure, the molar volume is approximately 24.0dm^3/mol.

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A standard solution

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is a solution of known concentration. Standard solutions are normally used in titrations to determine unknown information about another substance.

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A species

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is any type of particle that takes part in a chemical reaction

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Stoichiometry

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is the molar relationship between the relative quantities of substances taking part in a reaction

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An acid

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is a species that is a proton donor

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A base

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is a species that is a proton acceptor

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A salt

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is any chemical compound formed from a acid when a H+ ion from the acid has been replaced by a metal ion or another positive ion, such as the ammonium ion, NH4+

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A cation

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is a positively charged ion

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An anion

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is a negatively charged ion

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Hydrated

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refers to a crystalline compound containing water molecules

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Anhydrous

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refers to a substance that contains no water molecules

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Water of crystallisation

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refers to water molecules that form an essential part of the crystalline structure of a compound

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An oxidation number

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is a measure of the number of electrons that an atom uses to bond with atoms of another element. Oxidation numbers are derived from a set of rules

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Oxidation

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is loss of electrons or an increase in oxidation number

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Reduction

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is gain of electrons or a decrease in oxidation number

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A redox reaction

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is a reaction in which both reduction and oxidation take place

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A reducing agent

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is a reagent that reduces (adds electrons to) another species

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An oxidising agent

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is a reagent that oxidises (takes electrons from) another species

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Mole equations

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n=m/Mr

n=CV

n=v/24