Covalent Bonds

Covalent Bonds

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Definition

  • Covalent Bonding: Occurs when two nonmetal atoms share valence electrons to obtain the electron configuration of a noble gas, forming a molecule.

    • A molecule is formed when two or more atoms bond covalently.

    • Examples of diatomic molecules include H₂, O₂, N₂, F₂, Cl₂, Br₂, I₂.

Types of Covalent Bonds

  • Single Bond: One pair of electrons is shared between atoms.

    • Example: H₂ (H-H)

  • Double Bond: Two pairs of valence electrons are shared.

    • Example: O₂ (O=O)

  • Triple Bond: Three pairs of valence electrons are shared.

    • Example: N₂ (N≡N)

Examples of Covalent Compounds

  • Water (H₂O): H-O-H represents a molecule formed by oxygen and hydrogen.

  • Carbon Dioxide (CO₂): O=C=O shows a double bond between carbon and oxygen.

  • Nitrogen (N₂): N≡N indicates a triple bond between nitrogen atoms.

Lewis Dot Structures

Purpose

  • Lewis Dot Structures illustrate the bonding relationships between atoms and lone pairs of electrons in a molecule.

Steps to Write Lewis Dot Structures

  1. Count the Total Number of Valence Electrons in the molecule and divide by 2 for pairs.

  2. Determine the Central Atom:

    • Hydrogen (H) is never a central atom.

    • Carbon (C) is always a central atom.

    • For one atom of an element, that atom is the central atom.

    • The least electronegative atom is typically the central atom.

  3. Place Electron Pairs: Connect each peripheral atom to the central atom with a pair of electrons (ligands).

  4. Check for Octets: Each atom should have eight electrons (octet rule), except hydrogen (needs 2) and boron (can be stable with 6). Use multiple bonds if necessary.

Bond Types in Lewis Structures

  • Ligand: Shared pair or bond represented as a line.

  • Single Bonds: Represented by one line.

  • Double Bonds: Represented by two lines.

  • Triple Bonds: Represented by three lines.

Electronegativity

  • Electronegativity values help determine bond types by measuring how strongly atoms attract electrons when forming bonds.

  • Closer electronegativity values between nonmetals indicate stronger covalent bonds.

Sigma and Pi Bonds

  • Sigma Bonds: The first bond formed between two atoms; single bonds are sigma bonds.

  • Pi Bonds: The second bond in a double bond and the second and third bonds in a triple bond.

Summary of Characteristics

  • Molecule Types: Diatomic (same or different elements), formed via sharing electrons.

  • Bonding Preferences: Central atom rules to apply for determining structure; octet rule serves as a guide for electron sharing, with specific exceptions noted.

Exceptions to the Octet Rule

  • Molecules may have an odd number of valence electrons (e.g., NO₂) or fewer than 8 (e.g., BH₃). Some may exhibit expanded octets (e.g., PCl₅, SF₆) where central atom can hold more than 8 electrons.