Elements, Compounds and Mixture

• Element is a pure substance that cannot be broken down into two or more simpler substances by chemical methods
• Elements can appear as:

  • atoms, if they are not chemically bonded to each other (e.g. helium)
  • molecules, if they are covalently bonded (e.g. oxygen molecules, O2)
  • a lattice of positive ions with a “sea of delocalised electrons” (e.g. copper)
• A compound is a pure substance containing two or more different elements that are chemically bonded in a fixed ratio
• Compounds can be separated into their constituent elements using chemical methods.

  1. Electrolysis (breaking down compounds by passing electric current through it)

  2. Thermal decomposition

  (breaking down compounds by

  exposing it to strong heat)
• A molecule is a group of two or more non-metal atoms that are chemically bonded together
• There are two types of molecules

  1. Diatomic molecules

  (two atoms chemically combined) Polyatomic molecules

  (three or more atoms chemically combined)

  H2
• Molecules can consist of either atoms of the same element (e.g. H2) or atoms of

  different elements (e.g. CO2).

  • H2 is an element, while CO2 is a compound.
• A mixture is made up of two or more elements and/or compounds that are not chemically combined.
• Mixtures can be made up of:

  • elements only
  • compounds only
  • both elements and compounds
• Characteristics of elements: Only one element

  Mostly natural occurring Fixed melting and boiling point