Atomic Structure and Chemical Bonding

Atomic Structure and Chemical Bonding

The behavior of materials in civil engineering is fundamentally determined by the arrangement of atoms and the bonds holding them together.

The Periodic Table

The periodic table is a tabular arrangement of the chemical elements, ordered by their atomic number, electron configuration, and recurring chemical properties.

Periodic Trends

  • Atomic Radius: Decreases across a period (left to right) and increases down a group (top to bottom).
  • Ionization Energy: The energy required to remove an electron from a gaseous atom. Increases across a period, decreases down a group.
  • Electronegativity: The tendency of an atom to attract electrons. Increases diagonally from Fr (0.7) to F (4.0).
    • Determines bond polarity.

Chemical Bonding

Atoms combine to form compounds through chemical bonds to achieve a stable electron configuration (usually an octet).

Types of Chemical Bonds

  • Ionic Bonding: Complete transfer of electrons from a metal to a nonmetal. Held together by electrostatic attraction.
    • Examples: NaCl, CaO (Quicklime), CaCO3_3 (Limestone).
    • Properties: High melting points, brittle, conduct electricity when molten/dissolved.
  • Covalent Bonding: Sharing of electron pairs between nonmetals.
    • Examples: H2_2O, CO2_2, CH4_4, SiO2_2 (Silica/Sand).
    • Nonpolar Covalent: Equal sharing (similar electronegativity).
    • Polar Covalent: Unequal sharing (different electronegativity).
  • Metallic Bonding: "Sea of delocalized electrons" surrounding metal cations.
    • Examples: Fe, Al, Steel.
    • Properties: Conductivity, malleability, ductility.

Molecular Geometry (VSEPR Theory)

Valence Shell Electron Pair Repulsion (VSEPR) theory predicts the shape of individual molecules based on the repulsion between electron pairs in the valence shell.

| Electron Groups | Lone Pairs | Geometry | Bond Angle | Example | | :--- | :--- | :--- | :--- | :--- | | 2 | 0 | Linear | 180° | CO2_2 | | 3 | 0 | Trigonal Planar | 120° | BF3_3 | | 3 | 1 | Bent | <120° | SO2_2 | | 4 | 0 | Tetrahedral | 109.5° | CH4_4 | | 4 | 1 | Trigonal Pyramidal | <109.5° | NH3_3 | | 4 | 2 | Bent | <<109.5° | H2_2O |

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