Chapter 1 : Shapes of Molecules

Chapter 1: Structure Determines Properties

Shapes of Molecules

In order to view some of the structures for this section, you will need the web "plug-in", "CHIME". If you do not have it already installed, you should be able to download it for free from the linked Website. CHIME is a plug-in that allows you to visualise and manipulate molecules as 3D structures. Check here if you don't know how to use Chime.

When determining the shape of a molecule the premise is that groups of electrons surrounding an atom are repelled as far as possible from each other to minimize the overall energy of the molecule. Groups of electrons can refer to bonding or non-bonding valence electrons. The number and types of groups are determined from a Lewis diagram.

Not only is it important for a chemist to be able to visualize a structure, they must also be able to effectively (clearly!) transfer this knowledge onto a piece of paper!!!!!!

A molecule is visualized as if it were centered in the plane of the paper and, generally, so that the most number of atoms possible are in that plane. Unbroken lines are used to represent bonds in the plane of the paper, hashed wedges represent bonds going back out of the plane of paper and solid wedges represent bonds coming out of the plane of the paper.

There may be several different ways that you can represent a molecule on a piece of paper. You should be able to discriminate which of those drawings most clearly represent the structure of a molecule.

You should be able to determine the shape of a molecule by looking at its molecular formula.
In doing so you must be able to present clear drawings of your conclusions.

If you think you can do this already, try some questions.
Otherwise you will need to review VSEPR.

Molecular dipole moments

Once the 3-dimensional shape of a molecule is known, the molecular dipole can be determined by adding together the vectors of the individual bond dipoles.
Note that just because a molecule has polar bonds, it doesn't mean there is a molecular dipole. For example molecules like carbon dioxide and carbon tetrachloride have polar bonds but no net molecular dipole.

You should be able to indicate a molecule's polarity.