Chem definitions

• Atomic (proton) number


  The number of protons in the nucleus of an atom
• Electron


  A fundamental particle found in the energy levels of an atom with a relative mass of 1/2000 and a relative charge of -1
• Isotope


  Atoms of the same element with different numbers of neutrons
• Mass (nucleon) number


  The number of particles (protons and neutrons) in the nucleus
• Nucleus


  The central part of the atom in which the nucleons (neutrons and protons) are found
• Neutron


  A fundamental particle found in the nucleus of

  atoms with a relative mass of 1 and a relative

  charge of 0
• Proton


  A fundamental particle found in the nucleus of

  atoms with a relative mass of 1 and a relative

  charge of +1
• Anion


  A negatively charged ion
• Cation 


  A positively charged ion
• Empirical Formula


  The simplest whole number ratio of each type of atom in a compound
• Molecular Formula


  The actual number of each type of atom in a compound
• Molecule


  A small group of atoms held together by covalent bonds e.g. H2O, Cl2 , CH4 etc
• Concentration 


  The moles of solute dissolved per 1 dm3
• Molar mass 


  The mass in grams per mole of a substance (g mol -1 )
• Molar volume


  The volume that 1 mole of a gas occupies at room temperature and pressure (24dm3 or 24000cm3 )
• Relative atomic mass


  The weighted mean mass of an atom compared with 1/12 of the mass of an atom of carbon-12
• Relative formula mass


  The weighted mean mass of a formula unit compared with 1/12 of the mass of an atom of carbon-12
• Relative isotopic mass


  The mass of an atom of an isotope compared with 1/12 of the mass of an atom of carbon-12
• Relative molecular mass


  The weighted mean mass of a molecule compared with 1/12 of the mass of an atom of carbon-12
• Standard solution


  A solution of known concentration
• Stoichiometry


  The molar relationship between the substances involved in a chemical reaction
• The Avogadro Constant (NA)


  The number of atoms per mole of the carbon-12 isotope (6.02 x 10 23 mol -1 )
• Acid


  A proton (H + ) donor
• Alkali


  A soluble base which dissolves in water to form hydroxide ions (OH - ) e.g. NaOH, Ca(OH)2 (used to neutralise acid soils), Mg(OH)2 (used to cure acid indigestion)
• Anhydrous Salt


  Salt not containing water of crystallisation e.g. CuSO4
• Base


  A proton (H + ) acceptor
• Hydrated Salt


  Salt containing water of crystallisation e.g. CuSO4 .5H2O
• Salt


  A compound formed when an H + ion from an acid is replaced by a metal ion or ammonium (NH4 + )ion
• Water of crystallisation


  Water molecules contained within crystals of a hydrated salt (separated from the anhydrous part by a dot in the formula e.g. MgSO4 .7H2O)
• Disproportionation


  A reaction where the same element is oxidised and reduced
• Oxidation


  The loss of electrons or an increase in oxidation number
• Oxidation number


  A measure of the number of electrons an atom uses to bond with atoms of another element
• Oxidising agent


  A reagent which oxidises (takes electrons from) another species. An electron acceptor
• Redox reaction


  A reaction which involves both reduction and oxidation
• Reducing agent


  A reagent which reduces (adds electrons to) another species. An electron donor
• Reduction


  The gain of electrons or an decrease in oxidation number
• Electron configuration


  The arrangement of electrons in an atom
• Electron shielding


  The repulsion between electrons in different inner shells
• (Atomic) Orbital


  A region of space around the nucleus of an atom which holds up to two electrons with opposite spins (s, p, d, f)
• Sub shell


  A group of the same type of atomic orbital (s,

  p, d, f)

  s sub-shells contain ONE s-orbital (2

  electrons)

  p sub-shells contain THREE p-orbitals (6

  electrons)

  d sub-shells contain FIVE d-orbitals (10

  electrons)

  f sub-shells contain SEVEN f-orbitals (14

  electrons)