Describe the intermolecular forces that are present in each...

Describe the intermolecular forces that are present in each of the following compounds. Which kind of force would have the greatest influence on the properties of each compound? (a) bromine pentafluoride, $\mathrm{BrF}_{5} ;$ (b) acetone, $\mathrm{C}_{3} \mathrm{H}_{6} \mathrm{O}$ (contains a central $\mathrm{C}=\mathrm{O}$ double bond); $(\mathrm{c})$ formaldehyde, $\mathrm{H}_{2} \mathrm{CO}$.

Key Concepts

Molecular Geometry and Polarity

Molecular geometry determines the spatial arrangement of bonds in a molecule, which in turn affects whether the individual bond polarities cancel out or add together to form a net dipole moment. A non-symmetrical molecular structure often results in a net polarity, allowing for dipole-dipole interactions in addition to the universal London dispersion forces. This concept is key to understanding how intermolecular forces influence the overall properties of a substance.

London Dispersion Forces

London dispersion forces are temporary, attractive forces that arise due to momentary uneven electron distribution in atoms and molecules. They occur in all substances and are particularly significant in nonpolar molecules but also contribute to the overall intermolecular attractions in polar compounds. Their strength increases with the size and polarizability of a molecule, influencing properties such as boiling and melting points.

Dipole-Dipole Interactions

Dipole-dipole interactions occur between molecules with permanent dipole moments, where the positive end of one molecule attracts the negative end of another. These attractions are stronger than London dispersion forces and play a major role in determining the physical properties of polar compounds. The presence of polar functional groups and the overall molecular geometry can enhance these interactions, affecting boiling points and solubility.

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