Bond and Molecular Polarity

Jul 8, 2025

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Overview

This lecture explains the concepts of bond polarity and molecular polarity, using electronegativity trends and examples to help identify the overall polarity of molecules.

Types of Covalent Bonds

  • Non-polar covalent bonds share electrons evenly between identical non-metals.
  • Example: H₂ has a non-polar covalent bond with evenly distributed electron density.
  • Polar covalent bonds share electrons unevenly between different non-metals.
  • Example: HF is polar because fluorine attracts more electron density than hydrogen.

Electronegativity

  • Electronegativity (EN) is an atom's ability to attract shared electrons.
  • Trend: EN increases from left to right and bottom to top on the periodic table.
  • Fluorine is the most electronegative element (excluding noble gases).
  • Comparing atoms: closer proximity to fluorine means higher EN.

Showing Bond Polarity

  • Partial charges (δ+ for less EN atom, δ– for more EN atom) indicate bond polarity.
  • Example: In HF, H is δ+ and F is δ–; in CO, C is δ+ and O is δ–.
  • Full charges occur in ionic bonds (metal + non-metal), not covalent bonds.
  • Polarity arrows: positive sign on less EN atom, arrow points to more EN atom.

Identifying Bond Types

  • Non-polar covalent: identical non-metals (e.g., H₂, Cl₂).
  • Polar covalent: different non-metals (e.g., HBr, HCl).
  • Ionic: metal with non-metal (e.g., NaI, MgO).

Determining Molecular Polarity

  1. Draw the molecule's correct shape (Lewis structure and geometry).
  2. Draw polarity arrows for all polar bonds.
  3. Add the vectors (arrows); if they cancel, the molecule is non-polar; if not, it's polar.

Examples of Molecular Polarity

  • CO₂: Linear, symmetric arrows cancel; non-polar molecule.
  • H₂O: Bent, arrows do not cancel; molecule is polar (net dipole).
  • NH₃: Trigonal pyramidal, arrows do not cancel; molecule is polar.
  • BF₃: Trigonal planar, symmetric arrows cancel; non-polar.
  • CH₂O: Net dipole points up; polar molecule.
  • CH₄: Tetrahedral, symmetric; non-polar molecule.
  • CH₃F: Tetrahedral, net dipole present; polar molecule.
  • HCN: Linear, net dipole; polar molecule.
  • CS₂: Linear, arrows cancel; non-polar molecule.

Key Terms & Definitions

  • Covalent Bond — bond formed by sharing electrons between atoms.
  • Non-Polar Covalent Bond — equal sharing of electrons between identical non-metals.
  • Polar Covalent Bond — unequal sharing of electrons between different non-metals.
  • Electronegativity (EN) — atom’s tendency to attract shared electrons.
  • Partial Charge (δ+, δ–) — small charge on atoms in a polar bond.
  • Ionic Bond — bond formed by transfer of electrons from metal to non-metal.
  • Polarity Arrow — notation indicating direction of electron density in a bond.
  • Net Dipole — overall direction of polarity in a molecule.

Action Items / Next Steps

  • Memorize the periodic trend of electronegativity.
  • Practice drawing Lewis and shape structures for molecules.
  • Practice identifying polar vs. non-polar molecules using arrows and partial charges.

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