Bonding 4 (Polarity)

The Covalent Model

Definition and Overview

  • The covalent model describes how atoms bond through the sharing of electron pairs, forming molecules.

Key Concepts

Bond and Molecular Polarity

  • Essential Question: What determines the covalent nature and properties of a substance?

Types of Covalent Bonds

  • Polar Covalent Bonds:

    • Defined as the unequal sharing of electron pairs between two atoms.

    • Occurs when one atom attracts the shared electrons more strongly than the other.

    • Electrons shift towards the atom with higher electronegativity (EN).

    • Electronegativity difference: 0.3<EN<1.7

    Non-Polar (Pure) Covalent Bonds:

    • Defined as the equal sharing of electron pairs between two atoms.

    • Occurs between atoms of the same element or those with similar electronegativities.

    • Electronegativity difference: EN<0.3

    • Electrons are equally distributed between the two atoms.

    • Example: Chlorine molecule (Cl2) has 3.2-3.2=EN=0 .

Bond Dipoles

  • Bond dipoles form due to the difference in electronegativity between bonded atoms, resulting in partial positive (∂+) and partial negative (∂−) charges on the molecule.

    • For H-Cl, where H (2.2) and Cl (3.2) results in riangle EN = 1.0 (polar covalent bond).

Non-Polar vs Polar Covalent Molecules

  • Non-Polar Covalent Molecules:

    • Electrons are distributed equally around the molecule.

    • No dipoles are present; hence, there are no areas of partial charge.

  • Polar Covalent Molecules:

    • Electrons are unequally distributed around the molecule.

    • Present dipoles representing partial charges (∂+, ∂−).

    • Example: Water (H2O) shows unequal sharing of electrons leading to a bent structure with significant polarity.

Electric Field Test

  • Polar Molecules: Align themselves in an electric field, demonstrating a net dipole moment.

    • Positive goes to negative

    • Negative goes to positive

  • Non-Polar Molecules: Remain randomly aligned without a net dipole moment in an electric field.

    • Don’t interact with the electric field

Deducing the Polarity of Molecules

Two Atom Molecules

  • A non-polar covalent bond (when EN{ \le }0.3 ) results in a non-polar covalent molecule with no dipoles.

  • A polar covalent bond (when 0.3<EN<1.7) lts in a polar covalent molecule with bond dipoles present.

Three Atom Molecules - Analysis

  • Example 1: For carbon dioxide (CO2), despite having polar covalent bonds, the molecule is equivalent to a non-polar molecule due to its linear shape that allows bond dipoles to cancel each other out.

  • Example 2: Water (H2O) is polar due to its bent shape, which does not allow bond dipoles to cancel, leading to an overall dipole moment.

  • Example 3: Molecules like ozone (O3) might have non-polar bonds, but due to asymmetry (lone electron pairs), they exhibit overall polarity.

The Role of Symmetry in Molecular Polarity

  1. Symmetrical Arrangement:

    • Bond dipoles cancel out, resulting in no net dipole moment, thus the molecule is non-polar.

  2. Non-Symmetrical Arrangement:

    • Result in a net dipole moment, hence the molecule is polar.

  • If both dipoles go up, then the net dipole goes up

    • Basically wherever the most dipoles are facing

Deducing the Polarity of Larger Molecules

Four Atom Molecules

  • Example: Ammonia (NH3) has polar bonds and an overall dipole moment due to its trigonal pyramidal geometry.

  • Example: Sulfur trioxide (SO3) is non-polar even though it consists of polar covalent bonds, due to symmetry.

Five Atom Molecules

  • Example: Methane (CH4) is non-polar as all bond dipoles are symmetrical and cancel each other out.

  • Example: Chlorofluoromethane (CH3F) has polar characteristics due to the presence of fluorine, leading to a net dipole moment.

IR Active Molecules

  • For a vibrational mode of a molecule to absorb infrared light, it must result in a change in the dipole moment of the molecule.

  • Characteristics of Bond Polarity and IR Absorption:

    • Greater polarity typically increases IR absorption efficiency.

    • Example: CO2 exhibits two stretching modes, symmetrical stretching (no IR activity) and asymmetrical stretching (IR active), which changes its dipole moment.

    • Deductions: A dipole moment forms with unequal electron distribution in a molecule.