Molecular Polarity

We use symmetry of charge distribution and bond dipoles to determine the polarity of molecules. If a molecule is polar, we need to use the relative strength of the bond dipoles to assign the poles of the molecule as "delta positive" and "delta negative".

Our expanded octet examples are a little more complicated, but apply the same rules:

Bond Polarity

We use electronegativity difference to determine if a bond is polar or not. Generally, in Year 13, if the atoms are different to each other, there will be an electronegativity difference, so we treat them as having a dipole (are polar).

Expanded Octet Shapes

In Level 3, we have met examples where the central atom of a molecule or ion has an extended octet. This means there must be some new shapes to learn...

Five Regions of Electron Density

Six Regions of Electron Density

VSEPR Theory & Shapes - Year 12 Recap

The VSEPR Theory explains that regions of electron density (bonding pairs and/or lone pairs or electrons) will repel each other. This gives molecules and ions their characteristic shapes.

In Year 12, we explored examples with 2, 3 or 4 regions of electron density. These videos and notes are a summary of those three examples:

Two Regions of Electron Density

Three Regions of Electron Density

Four Regions of Electron Density

Lewis Diagrams - Polyatomic Ions

We can even draw Lewis Diagrams for Ions. We just need to allow for the addition of electrons (anions) or removal of electrons (cations).

There are also a couple of special cases that we should be aware of - sulfate and phosphate. Have a go at drawing these, knowing they have expanded octets around the central atom. Then get your diagrams checked...

Lewis Dot Diagrams - Part Two

Some elements can break the Octet Rule by having more than eight electrons. We call this having an expanded octet. It can happen because electrons can be "promoted" into the 3d orbitals to accommodate bonding. There are a couple of "rules" around this:

1. The atom must already have electrons in both the 3s and 3p sub-levels.
2. The atoms must be the central atom (it would be anyway, as this increases its valency)

Lewis Dot Diagrams - Part One

This is a recap of Year 12 Chemistry.

We use Lewis Dot Diagrams to represent the bonding pairs and lone pairs of electrons in a molecule and an ion. We only focus on the valence electrons, as these are the ones that influence the shape of the ion/molecule, so affect their properties the most.

Simple Lewis Dot Diagrams
In a lot of molecules, the atoms automatically obey the Octet Rule once we include the lone pairs of electrons.

More Complex Lewis Dot Diagrams
These molecules (and ions) do not obey the Octet Rule once we assign the remaining electrons as lone pairs. Therefore, we need to shift pairs of electrons around to create double (or triple) bonds.