Compounds, Formulae & Equations

• Covalent bond


  The sharing of a pair of electrons between two atoms
• Metallic bond

  The attraction of positive metal ions to delocalised electrons
• Ionic bond


  The electrostatic attraction between oppositely charged ions
• Ways atoms form chemical bonds

  • Sharing a pair of e- (generally non-metal + non-metal atoms) covalent bonding e.g. CO₂
  • Transfer of e- (generally metal + non-metal atoms) ionic bonding e.g. NaCl
  • Delocalisation of e- (metals) metallic bonding e.g. Na
• Atoms in Group 14 have 4 electrons in their outermost shell and they usually share a pair of electrons to form covalent bonds
• For a positive elemental ion (Groups 1, 2, 13 and d-block) the name of the ion is the same as the name of the element but for negative elemental ion (Groups 15, 16 and 17) elemental ion usually loses the latter part of the elemental name and it is replaced with 'ide'
• Polyatomic ions to learn

  • Ammonium, NH₄⁺
  • Nitrate (V), NO₃⁻
  • Sulfate (VI), SO₄²⁻
  • Phosphate (V), PO₄³⁻
  • Hydroxide, OH⁻
  • Carbonate, CO₃²⁻

  Useful to know:

  • Hydrogen sulfate, HSO₄⁻
  • Hydrogen carbonate, HCO₃⁻
• A symbol equation is a shorthand way of describing a chemical reaction using chemical symbols to show the number and type of each atom in the reactants and products
• A word equation is a longer way of describing a chemical reaction using only words to show the reactants and products
• During chemical reactions, atoms cannot be created or destroyed. The number of each atom on each side of the reaction must therefore be the same
• Steps to write a chemical equation

  1. Write the correct formulae of the given reactants (on left) and products (on right) with an arrow in between (left to right)

  2. Add state symbols (if required)

  3. Balance the equation for each atom in turn
• State symbols for elements

  • Gas - H2(g), N2(g), O2(g), F2(g), Cl2(g)
  • Liquid - Br2(l), Hg(l)
  • Solid - all others e.g. Na(s)
• State symbols for compounds

  • Gases - CO2(g), CH4(g), NH3(g)
  • Liquids - H2O(l), C2H5OH(l)
  • Solids - All ionic compounds e.g. NaCl(s)
• Ionic equation


  Shows only the ions or other particles taking part in a reaction, and not the spectator ions
• Solubility rules


  • Compounds of sodium, potassium and ammonium are soluble (aq)
  • Salts (nitrates, chlorides, sulfates, phosphates) are soluble (aq)
  • Bases (carbonates, oxides, hydroxides) are insoluble (s)
• Names + symbols of common ions