Ch 1 : Chemical Bonding

Chapter 1: Structure Determines Properties

Chemical Bonding

Simplistically, chemistry is about the interaction of molecules with other molecules.

But what is a molecule ?
But why do atoms form bonds ?
What makes bonding favourable ?

A molecule is a collection of atoms held together by bonds due to the interaction of the atomic electron clouds.

One of the principle driving forces that makes bonding favourable is an atoms desire to obtain a stable valence electron configuration (i.e. a valence energy level that contains a complete set of electrons)

For the atoms that are most important in organic chemistry (with the exception of H), this is an octet of electrons, which is similar to the valence electron configuration of the nearest noble gas (e.g. 2s²2p⁶).

There are two ways in which this octet can be achieved, either

transfer of electrons which results in ionic bonds, or,
by sharing electrons which result in covalent bonds.

Most bonds in organic molecules are covalent bonds but common exception are the bonds to metal atoms (where the metal atoms should be treated as ions).

You should be able to identify whether an ionic or covalent bond is present between atoms.

If you think you already can then try some questions.
If you are unsure, then you need to review the types of bonding in more detail.

It is important to note that the electron distribution within a covalent bond is not always symmetrical.
This is to be expected if the atoms are of different electronegativities.
If the electron distribution is not symmetrical, then the bond will be polar and a bond dipole exists.

C-C and C-H bonds in organic molecules are typically non-polar (e.g. in alkanes).

This is because they have similar electronegativities (2.5 and 2.1 respectively)

Bonds of C or H to atoms such as N and O are polar. These bonds are important in functional groups.

If a bond is covalent you should be able to identify whether it is polar or non-polar.

Now try some questions. If you are having difficulty then you need to review bond polarity.