Compounds

• When elements react, atoms combine with other atoms to form compounds.
• Compounds are substances formed from two or more elements. The atoms of each are in fixed proportions and held together by chemical bonds.
• Making bonds involves giving atoms away, taking or sharing electrons. The nuclei aren’t affected at all when a bond is made.
• A compound which is formed from a metal and a non-metal consists of ions. This is called ionic bonding.
• In ionic bonding, the metal atoms lose electrons to form positive ions, and the non-metal atoms gain electrons to form negative ions.
• During ionic bonding, the opposite charges mean that they’re strongly attracted to each other.
• A compound formed of non-metals consists of molecules. This is covalent bonding.
• In covalent bonding, each atom shares an electron with another atom.
• Examples of covalent bonding include:

  • water
  • carbon monoxide
  • hydrogen chloride gas
• The properties of a compound are usually totally different from the properties of the original elements.
• An example of compound properties changing is that iron (a lustrous magnetic metal) and sulphur (a yellow powder) form a dull grey solid lump (iron sulfide).
• A formula shows what atoms are in a compound.
• Formulas that might come in handy!

  • Carbon dioxide - CO2
  • Ammonia - NH3
  • Water - H2O
  • Sodium chloride - NaCl
  • Carbon monoxide - CO
  • Hydrochloric acid - HCl
  • Calcium chloride - CaCl2
  • Sodium carbonate - Na2CO3
  • Sulfuric acid - H2SO4