03.01 Elements, Compounds, and Mixtures

a. Elements

Definition:

• A pure substance that cannot be broken down into simpler substances by chemical methods.

Characteristics:

• Unique Properties: Each element has its own distinct properties (e.g., gold is shiny and malleable, while sulfur is yellow and brittle).
• Number of Elements:
  • Over 100 known elements
  • 94 occur naturally (e.g., Hydrogen, Oxygen, Carbon)
• Classification:
  • Metals: Conduct electricity, are shiny, malleable (e.g., Iron, Copper)
  • Non-Metals: Poor conductors, diverse properties (e.g., Oxygen, Nitrogen)
• Atomic Number: All atoms of an element have the same number of protons.
  • Example: All carbon atoms have 6 protons.

Examples of Elements:

• Hydrogen (H)
• Carbon (C)
• Gold (Au)
• Oxygen (O₂)

b. Compounds

Definition:

• A substance formed when two or more elements chemically combine in fixed proportions.

Characteristics:

• Unique Properties: Compounds have properties different from their constituent elements.
  • Example: Sodium (a reactive metal) + Chlorine (a poisonous gas) → Sodium Chloride (table salt, edible)
• Chemical Formula: Indicates the elements in a compound and their ratios.
  • Examples:
    • Water: H₂O (2 Hydrogen atoms + 1 Oxygen atom)
    • Carbon Dioxide: CO₂ (1 Carbon atom + 2 Oxygen atoms)

Types of Compounds:

1. Molecular (Covalent) Compounds:
   • Formation: Atoms share electrons.
   • Examples:
     • Water (H₂O)
     • Ammonia (NH₃)
     • Methane (CH₄)
2. Ionic Compounds:
   • Formation: Transfer of electrons creates ions, which attract each other.
   • Examples:
     • Sodium Chloride (NaCl)
     • Calcium Carbonate (CaCO₃)

c. Mixtures

Definition:

• A combination of two or more substances (elements or compounds) that are not chemically bonded.

Characteristics:

• Variable Composition: The ratio of components can change.
  • Example: Air can have varying amounts of nitrogen, oxygen, and other gases.
• Properties: Each component retains its individual properties.
  • Example: In a salad, lettuce remains crunchy, and tomatoes remain juicy.

Types of Mixtures:

1. Solutions (Homogeneous Mixtures):
   • Definition: Uniform composition throughout.
   • Examples:
     • Saltwater (Salt dissolved in water)
     • Vinegar
2. Heterogeneous Mixtures:
   • Definition: Non-uniform composition; components can be seen separately.
   • Examples:
     • Sand and Water
     • Air (mixture of gases)
     • Alloys like Brass (Copper + Zinc)

Separation Methods:

• Physical Techniques:
  • Filtration: Separates solids from liquids (e.g., tea leaves from tea).
  • Magnetism: Uses magnetic properties to separate materials (e.g., iron filings from sand).
  • Distillation: Separates based on boiling points (e.g., separating alcohol from water).

d. Comparison: Compounds vs. Mixtures

Additional Examples for Better Understanding

• Elements:
  • Metals:
    • Iron (Fe): Used in construction and manufacturing.
    • Copper (Cu): Used in electrical wiring.
  • Non-Metals:
    • Nitrogen (N₂): Makes up about 78% of Earth’s atmosphere.
    • Sulfur (S): Used in fertilizers and chemicals.
• Compounds:
  • Water (H₂O): Essential for life, used in various chemical reactions.
  • Carbon Dioxide (CO₂): Produced by respiration and combustion, used by plants in photosynthesis.
  • Sodium Chloride (NaCl): Common table salt used in cooking and preserving food.
• Mixtures:
  • Homogeneous:
    • Air: A mixture of nitrogen, oxygen, argon, and other gases.
    • Brass: An alloy of copper and zinc.
  • Heterogeneous:
    • Trail Mix: A mixture of nuts, dried fruits, and seeds.
    • Oil and Water: Do not mix uniformly.

5. Key Terminology

• Element: A substance made of only one type of atom (e.g., Oxygen).
• Compound: A substance formed from two or more elements in fixed ratios (e.g., Water – H₂O).
• Mixture: A combination of two or more substances not chemically bonded (e.g., Air).
• Covalent Bonding: Sharing of electron pairs between atoms (e.g., H₂O).
• Diatomic Molecule: A molecule composed of two atoms (e.g., O₂, N₂).
• Chemical Formula: A representation of a compound using symbols and numerical ratios (e.g., CO₂).
• Dot-and-Cross Diagram: A diagram showing electron sharing in covalent bonds.
• Intermolecular Forces: Forces between molecules, such as hydrogen bonds or dipole-dipole interactions.
• Intramolecular Forces: Forces within a molecule, primarily covalent bonds.

Examples

Question 1:

Covalent bonding involves electrons being shared between the atoms bonded together. Methane is made up of covalently bonded molecules. Which diagram represents the bonding in methane?

Options:

• Diagram A: Carbon atom at the center with four hydrogen atoms each sharing a pair of electrons.

A)

B)

• Diagram B: Sodium and chlorine ions arranged in a lattice structure.

Answer:

• Methane (CH₄): Carbon shares electrons with four hydrogen atoms through single covalent bonds.

Correct Selection: Diagram A

Explanation:

Methane has a central carbon atom bonded to four hydrogen atoms by sharing electrons, forming a tetrahedral shape. Diagram B represents an ionic compound (like NaCl), not a covalent molecule.