Key Concepts in Chemistry: Elements, Compounds, Mixtures, and Molecules

Introduction

• Objective: Understand the definitions and differences between elements, compounds, mixtures, and molecules.
• Importance: Fundamental concepts essential for chemistry.

Elements

• Around 100 different elements exist, represented in the Periodic Table.
• Definition: An element consists of atoms that are all the same.
  • Example: Magnesium (Mg) and Sulfur (S) are both elements.
• Symbol: Each element has a symbol starting with a capital letter (e.g., Mg for Magnesium, S for Sulfur).

Compounds

• Formed when atoms of different elements chemically combine.
• Definition: Compounds contain two or more different elements chemically combined in fixed proportions.
  • Example: Magnesium sulfide (MgS) has a fixed 1:1 ratio of magnesium to sulfur atoms.
• Properties of compounds differ significantly from their constituent elements (e.g., Mg is a shiny metal, S is a yellow solid, while MgS forms white crystals).
• Separation requires a chemical reaction.

Mixtures

• Consist of different elements or compounds not chemically combined.
• Separation: Achieved through physical techniques such as filtration, distillation, crystallization, or chromatography.
• Example: A mixture containing magnesium atoms, sulfur atoms, and magnesium sulfide.

Molecules

• Definition: Molecules consist of two or more atoms chemically combined, which can be the same or different elements.
  • Compounds as Molecules: Methane (CH₄), water (H₂O), and ammonia (NH₃) are molecules that are also compounds (contain different elements combined).
  • Non-compound Molecules: Chlorine (Cl₂) and oxygen (O₂) are molecules of a single element and not compounds.

Study Resources

• Recommended: Practice questions available in the revision workbook.