Elements, Compounds and Mixtures

• Every chemical substance falls into one of three classes:
  • Element
  • Compound
  • Mixture
• Edexcel expects you to classify a substance from its name, formula, or particle diagram

Element

• Built entirely from atoms that all share the same proton number
• Each element has its own characteristic proton number (e.g. gold = 79, oxygen = 8)
• Cannot be reduced to anything simpler by ordinary chemical or physical methods
• 118 elements are known, all set out in the Periodic Table
• Examples: iron (Fe), copper (Cu), magnesium (Mg), oxygen (O₂), nitrogen (N₂), sulfur (S₈)
• Diatomic and polyatomic forms are still elements
  • O₂ is an element (not a compound), because every atom in it is oxygen

Compound

• A pure substance formed when atoms of two or more different elements join in a fixed ratio through chemical bonds
• Has its own chemical properties, different from those of its elements
• Can only be broken back into its elements by another chemical reaction (not by physical means)
• Millions of different compounds exist
• Composition by mass is fixed
  • Water is always H₂O, never H₃O or H₂O₂
  • Sodium chloride is always one Na per one Cl
• Examples: water (H₂O), carbon dioxide (CO₂), calcium carbonate (CaCO₃), copper(II) sulfate (CuSO₄)

Mixture

• Two or more substances (any combination of elements and compounds) sharing the same space, with no chemical bonds between them
• Each component keeps its own chemical identity and properties
• Proportions can vary (a salt solution can be weak or strong)
• Components can be separated again by physical methods
• Examples:
  • Air (mostly N₂ and O₂, plus Ar, CO₂, water vapour)
  • Seawater (water + dissolved salts)
  • Iron filings + sulfur powder
  • Brass (copper + zinc)

Recognising the class from a formula