Elements, Compounds, and Mixtures

Overview

• Topic: Atoms, Elements, Compounds, and Mixtures (Cambridge O Level 5070 / IGCSE 0620 syllabus).
• Objective: Describe differences between elements, compounds, and mixtures.
• Focus: Definitions, classification, particle types, examples, and simple properties.

Elements

• Definition: A pure substance that cannot be broken down into simpler substances by chemical means.
• Classification: Metals (left side of periodic table) and non-metals (right side).
• Known elements: About 116 known; ~92 naturally occurring, ~24 synthesized.
• Arrangement: Elements are arranged in the periodic table by increasing proton number.
• Properties:
  • Metals: Generally good conductors of electricity.
  • Non-metals: Generally poor conductors; graphite is a non-metal that conducts electricity.
• Particles:
  • Atom: Smallest unit of an element that retains the element’s properties.
  • Molecule: Group of two or more atoms chemically joined; can be of the same element (e.g., Cl2, Br2, I2, O2, H2).
• Symbols: Each element has a chemical symbol (e.g., copper → Cu).

Compounds

• Definition: A pure substance formed by the chemical combination of two or more elements.
• Examples: Sodium chloride (NaCl), water (H2O).
• Forms:
  • Molecular compounds: Discrete molecules made of different elements (e.g., H2O).
  • Ionic compounds: Composed of ions (cations and anions) and often solid (e.g., Na+ and Cl− in NaCl).
• Ions:
  • Cation: Positively charged ion (e.g., Na+).
  • Anion: Negatively charged ion (e.g., Cl−).
• Note: Compounds have fixed chemical formulas and specific properties different from their constituent elements.

Mixtures

• Definition: Two or more substances not chemically joined together.
• Types:
  • Homogeneous mixture: Components mix uniformly and form a single-phase solution (uniform composition).
  • Heterogeneous mixture: Composition not uniform; different components can be observed.
• Examples:
  • Homogeneous: Salt dissolved in water (solution), air (treated as uniform at large scale for many purposes).
  • Heterogeneous: Chalk in water, suspensions where particles are visible.
• Special mixture — Alloys:
  • Definition: Mixture of metals, sometimes with non-metals (e.g., carbon in iron to make steel).
  • Purpose: Improve mechanical properties (hardness, strength). Examples: Brass, bronze.

Key Terms and Definitions

• Matter: Substance with mass that occupies space.
• Pure Substance: Material with a fixed composition (elements or compounds).
• Atom: Smallest unit of an element with its chemical properties.
• Molecule: Two or more atoms chemically bonded.
• Element: Pure substance made of one type of atom.
• Compound: Pure substance of two or more elements chemically combined.
• Mixture: Physical combination of substances, not chemically bonded.
• Homogeneous: Uniform composition throughout.
• Heterogeneous: Non-uniform composition.
• Cation: Positively charged ion.
• Anion: Negatively charged ion.
• Alloy: Mixture of metals (sometimes with non-metals) to improve properties.

Summary Table: Elements, Compounds, Mixtures

Action Items / Next Steps

• Review periodic table arrangement and element classification in more detail.
• Practice distinguishing examples of homogeneous vs heterogeneous mixtures.
• Learn formula writing and naming for simple ionic and molecular compounds in upcoming lessons.
• Memorize common ion charges (e.g., Na+, Cl−) and basic examples of alloys.