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Chapter 1: Structure Determines Properties |
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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.
2s2
2p6).
There are two ways in which this octet can be achieved, either
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.
Try some questions. If you are having
difficulty then you need to review
bond polarity.
