Free Science Lessons: Chemistry Basics

Objectives

• Understand definitions of key chemistry terms: element, compound, mixture, and molecule.

Elements

• Definition: An element consists of all atoms being the same.
• Periodic Table: Contains around 100 different elements. Each element has a unique symbol.
  • Example:
    • Magnesium (Mg): Capital M followed by lowercase g
    • Sulfur (S): Capital S
• Examples:
  • Sample of Magnesium: All atoms are the same.
  • Sample of Sulfur: All atoms are the same.
• Each element is represented by a one or two-letter symbol.

Compounds

• Definition: A compound consists of two or more different elements chemically combined in fixed proportions.
  • Example:
    • Magnesium Sulfide (MgS)
      • Contains 1 atom of magnesium and 1 atom of sulfur (fixed proportion).
• Properties: Compounds usually have different properties than the elements they are composed of.
  • Example:
    • Magnesium (shiny metal), Sulfur (yellow solid) vs Magnesium Sulfide (white crystals).
• Separation: To separate a compound back into its elements, a chemical reaction is required.

Mixtures

• Definition: A mixture has different elements or compounds that are not chemically combined.
  • Example: Mixture of magnesium atoms, sulfur atoms, and magnesium sulfide.
• Separation: Can be separated using physical methods such as:
  • Filtration
  • Distillation
  • Crystallization
  • Chromatography

Molecules

• Definition: A molecule is formed when any elements are chemically combined, regardless if they are the same element or different.
• Examples of Molecules:
  • Compounds:
    • Methane (CH4), Water (H2O), Ammonia (NH3)
  • Elements:
    • Chlorine (Cl2), Oxygen (O2)
• Not all molecules are compounds (e.g., Cl2 and O2 are molecules but not compounds).

Additional Resources

• Revision workbook available for further questions on this topic.