Electronegativity and Molecular Polarity

Electronegativity Trends

  • Definition: Electronegativity is the ability of atoms in a molecule to attract electrons to themselves.
  • Periodicity:
    • Electronegativity increases from left to right across a row of the periodic table.
    • Electronegativity increases from the bottom to the top of a column in the periodic table.

Polar Covalent Bonds

  • Key Concept: The greater the difference in electronegativity between two bonded atoms, the more polar the bond.
  • Electronegativity Values (for hydrogen and halogens):
    • H: 2.1
    • F: 4.0
    • Cl: 3.0
    • Br: 2.8
    • I: 2.5

Relative Bond Polarity

  • Electronegativity Difference and Bond Classification:
    • If the difference in electronegativity > 2.0:
    • The bond is considered ionic.
    • If the difference is between 0.5 and 2.0:
    • The bond is considered polar covalent.
    • If the difference < 0.5:
    • The bond is considered non-polar covalent.

Types of Bonds

  • General Classification of Bonds Based on Elements:
    • Ionic Bond:
    • Occurs between a metal and a non-metal where electrons are assumed to be completely transferred.
    • Polar Covalent Bond:
    • Occurs between two different non-metals where electrons are shared unequally.
    • Non-Polar Covalent Bond:
    • Occurs between the same elements where electrons are assumed to be shared equally.

Special Case: C-H Bond

  • In organic chemistry and biochemistry, the bond between carbon and hydrogen (C-H) is often considered to exhibit equal sharing of electrons.

Electronegativity Skills

  • Understanding of key concepts is essential for various question types, including multiple-choice and short answer.
  • Importance of the electronegativity chart:
    • Ability to use it to determine whether a bond is ionic, polar covalent, or non-polar covalent and to understand the implications of these classifications.
  • Key questions include:
    • If a bond is polar, which atom has the partial negative charge?

Examples of Bond Classification

  • Determine the bond type and partial charges for:
    • Na – Br
    • F – O
    • C – C
    • Cl – I

Molecular Polarity

  • Next topic: Determining whether an entire molecule is polar or non-polar.
    • Must account for the entire molecular geometry.

Understanding Polar Molecules

  • Definition: “Separation of charge” within a molecule.
  • Influence of Charge Distribution:
    • Asymmetric distribution of shared electrons results in a partial charge, indicating polarity.
    • Symmetric distribution leads to a non-polar molecule.

Importance of Molecular Polarity

  • The polarity of a molecule influences various physical properties such as:
    • Boiling and melting points.
    • Solubility in water or organic solvents.
    • Chemical reactivity.

Determining Molecular Polarity

  • Questions to consider:
    • Does the molecule behave as if it possesses a charge?
    • Asymmetry in a molecule implies it is polar; symmetry implies it is non-polar.

Molecular Shape and Polar Bonds

  • A molecule may have polar bonds, yet still be non-polar overall depending on its shape.
  • Key Consideration: Molecular geometry must be analyzed.

Types of Molecular Geometry

  • Fully Symmetric Shapes:
    • Linear
    • Trigonal planar
    • Tetrahedral
  • Asymmetric Shapes (always polar):
    • Bent
    • Trigonal pyramidal
  • Others (like linear, trigonal planar, or tetrahedral) may or may not be polar based on the surrounding atoms.

Identifying Polar and Non-Polar Molecules

  • Identification Procedure:
    1. Draw the Lewis structure.
    2. Determine the molecular geometry.
    3. If the molecule is bent or trigonal pyramidal, classify it as polar.
    4. For other geometries, check if surrounding atoms are identical:
    • If yes, label as non-polar.
    • If asymmetric, label as polar.

Assessment of Molecular Polarity

  • Evaluate the following molecules for polarity:
    • Br2
    • HCN
    • CH2O
    • CCl4
    • CHCl3
    • NH3
    • BF3
    • CO2
    • HCN

Additional Examples of Polar vs Non-Polar

  • Analyze the polarity of the following compounds:
    • (a) CS2
    • (b) SeS2
    • (c) CCl2F2
    • (d) PCl3 (where P is the central atom)
    • (e) ClNO (where N is the central atom)