Molecular Substances

LEWIS DOT DIAGRAMS

Lewis Dot Diagrams show us the bonding electrons in atoms and molecules. We use our knowledge of the Periodic Table to work out how many bonding (valence) electrons an atoms has.

Once we work out how to draw Lewis Diagrams for atoms, we try to put them together to represent the bonding in molecules.

Find the total number of valence electrons
Assign the central atom - the one with the lowest electronegativity (this is usually the one with the highest valency)
Draw a skeleton structure using single bonds.
Allocate all remaining valence electrons - this should make each atom obey the Octet Rule

If this does not work, we will need double (or even triple) bonds...

SHAPES OF MOLECULES

Once we have the Lewis Diagram for a molecule, we can infer its shape.

Regions of Electron Density: We talk about "regions of electron density when explaining shapes. These are:

Lone pairs of electrons around the central atom
Bonds between the central atom and other atoms.

There are some shapes and bond angles to remember. We used toothpicks to represent regions of electron density, and wine gums to represent bonded atoms.

There are two factors that affect shape:

Regions of electron density (lone pairs and/or bonds) around the central atom
Number of atoms bound to the central atom

You do need to remember the:

Names of the shapes
Bond angles

Bent/Angular is also called V-Shaped.

ELECTRONEGATIVITY AND BOND POLARITY

Electronegativity is a measure of relative attraction to bonding electrons. The higher the value, the more tightly an atom will "hold onto" electrons in a bond with other atoms.

Electronegativity difference is used to infer the type of bond:

a large electronegativity difference (above 1.6) usually leads to an ionic bond
between 0.5 and 1.7 the bond is polar covalent
less than 0.5 will be a non-polar (pure) covalent bond

MOLECULE POLARITY

We can explain a lot of properties about molecules once we understand if they are polar or not. How do we work out if they are polar?

Polar molecules have a molecular dipole due to asymmetric charge distribution. One side of the molecule is slightly negative and the other side is slightly positive. This is usually due to:

asymmetric shape (due to lone pairs of electrons around the central atom) and/or
bond dipoles that do not cancel out

Polar molecules have higher melting/boiling points, and can dissolve ionic substances.

When there are more than two atoms in the molecule, the following flow chart works well:

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