10.3: Molecular Polarity

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Introduction

Review sections:
8.7 Bond Polarity and Electronegativity
8.8 Bond and Molecular Polarity:

Key Concepts

You should know the following terms and concepts from general chemistry 1. Please review that material

electronegativity (\(\chi\))
- periodic trends
- possible values
- Use in predicting bond types
Dipole Moment (\(\mu\))
- Debye (concept)
3 Fundamental VSEPR Geometries
- linear
- trigonal planar
- tetrahedral
2 Secondary VSEPR Geometries
- Trigonal Bipyrimidal (linear along axis,trigonal planar along equatorial plan)
- Octahedral (linear along three Cartesian axes)
Ability to identify if a molecule is polar and predict direction (not magnitude) of dipole
- Rudimentary Vector Addition

Electronegativity

The electronegativity of an atom is used to indicate if bonding electrons are equally, or uneqully shared, when that atom is bonded to another atom. The atom with the higher electronegativity pulls the electron cloud more than the atom with the lower electronegativity. The values of electronegativity range from 0.7 to 4.0, it is unitless, and although it is assigned to an atom, it is not a property of an atom like ionization potential or electron affinity. The following YouTube gives a good description of electronegativity, and please review section 8.7: Bond Polarity and Electronegativity. You need to know the periodic trends of increasing (or decreasing) electronegativity.

3:32 min YouTube by professor Dave going over the basics of electronegativity (https://youtu.be/PoQjsnQmxok).

Dipole Moment

A dipole moment (\(\mu\)) occurs when the center of positive charge on a molecule does not align with the center of negative charge, and this results in a region of partial positive and partial negative charge, ie., a dipole. HCl has a dipole moment.

\[ \begin{matrix}
_{\delta ^{+}}& & _{\delta ^{-}}\\
H\; \; &-& Cl
\end{matrix} \]

Molecules with a dipole moment not only align with external electric fields, but interact with each other through dipole-dipole interactions (where the opposite charged dipoles attract and the like charged ones repel).

Molecular Shape and Polarity

A polar bond in a diatomic molecule results in a polar molecule. If there are more than one bond in a molecule, all the bonds contribute to the molecules dipole moment, and if they are symmetrically oriented, they cancel out. So a symmetric molecule like carbon dioxide, which has polar bonds, is nonpolar. Review section 8.8: Bond and Molecular Polarity. The following YouTube gives a quick and decent coverage of symmetry and molecular polarity.

Professionally developed YouTube video uploaded by Wendell Thomas.

Note, at the end of the above youtube they use polarity to predict which of two isomers (molecules with the same formula) has the higher boiling point, which is the kind of problem you will have on your first exam.

Summary of Symmetric Structures

A=central atom, B,C,D = atoms of type B,C or D, E=Lone pair electrons

sp hybrid
- Linear
  - AB₂
sp² hybrid
- Trigonal planar
  - AB₃
sp³ hybrid
- tetrahedral
  - AB₄
sp³d hybrid
- trigonal bipyramidal
  - AB₅
  - AB₂C₃ (B in axial positions, C in equatorial plane)
- linear
  - AB₂E₃ (B in axial positions, E - lone pairs in equatorial plane)
sp³d² hybrid
- Octahedral
  - AB₆
  - AB₄C₂ (B along X & Y axes, C along Z axis)
  - AB₂C₂D₂ (B along X, C along Y and D along Z axes)
- Square Planar
  - AB₄E₂ (B along X & Y axes, E - lone pairs along Z axis)