User:Benjah-bmm27/Polarity

A place to organise information in order to improve the article Chemical polarity.

Proposals

• Change article name from Chemical polarity to Molecular polarity.
• Rewrite introduction. The first sentence should be the definition of a polar molecule, not about intermolecular forces due to electric dipoles.
• Explain the roles of molecular symmetry and electronegativity in determining the electric dipole moment of a molecule.
• Explain that one way of thinking about a molecule's electric dipole moment is to consider it a superposition of bond dipole moments.
• Note pathological examples, such as carbon monoxide (oxygen more electronegative than carbon, but more negative end of molecule is the carbon end).
• Introduce undergraduate-level symmetry arguments, but towards the end of the article, so as not to frighten off casual readers.

Introduction

• A molecule is polar if it has a permanent electric dipole moment.
• The molecule's symmetry and the electronegativities of its constituent atoms determine its polarity (i.e. the direction and magnitude of its permanent electric dipole moment).
• For example, tetrahedral molecules of the the type AB₄ (with A as the central atom) cannot be polar, no matter what the electronegativities of A and B are. The four bond dipole moments cancel each other out.

Detail

Bond polarity and molecular polarity are different

Title says it all.

Molecules with polar bonds can be non-polar

Classic examples are CF₄ and CO₂. The symmetry of these molecules causes the individual bond dipoles to cancel each other out. While carbon in CF₄ is significantly more positive than in CH₄, the partition of charge is not along any one line.

Even alkanes can be polar

Some supposedly non-polar molecules actually have a small but real dipole moment, such as propane (μ = 0.084 ± 0.001 D), presumably because the C-H and/or C-C bonds are slightly polarised. C and H do not have exactly the same electronegativity (2.55 vs. 2.20, according to Electronegativity, so C is δ− and H is δ+), which in any case depends on the atom's environment within the molecule. Electronegativity is not entirely fixed and transferable, although it usually seems to be approximately so.

Consequences of polarity

Polar molecules experience dipole-dipole interactions which attract them to other nearby polar molecules.

This primarily electrostatic effect is a form of chemical bonding that is weaker than normal covalent and ionic bonds, but stronger than van der Waals forces.

How strong is it compared to hydrogen bonding?

Defintions

Atkins

“ A polar molecule is one with a permanent electric dipole moment (HCl, O₃ and NH₃ are examples). ” — Atkins' Physical Chemistry, 7^th Edition, p. 461

Oxford

“ polar ...Physics & Chemistry having electrical or magnetic polarity.
•(of a liquid, especially a solvent) consisting of molecules with a dipole moment. •(of a solid) ionic. ” — Oxford Dictionary of English, p. 1360

Symmetry

Atkins, p. 462: ...only molecules belonging to the groups C_n, C_nv, and C_s may have a permanent electric dipole moment. For C_n and C_nv, that dipole moment must lie along the symmetry axis. Thus ozone, O₃, which is angular and belongs to the group C_2v, may be polar (and is), but carbon dioxide, CO₂, which is linear and belongs to the group D_∞h, is not.