4. Lone
Pairs and Bond Pairs of Electrons.
If
we examine the structure of methane (CH4), we can see that there
are four pairs of electrons involved in covalent bonds. We say
there are four bond pairs of electrons around carbon.
If we now
examine the structure of water (H2O), we can see that there
are two bond pairs of electrons around oxygen, but there
are also two pairs of electrons around oxygen that are not involved
in covalent bonds. These are known as lone pairs of
electrons.
In water
there are two lone pairs of electrons, and two bond pairs
of electrons.
Checkpoint
4.
Using
the dot and cross diagrams you have already drawn in this section,
count up the bond pairs and lone pairs around the central atom
in the molecule. Check your answers below.
a. H2
Click
here to see the answer
b. BeCl2
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here to see the answer
c. BF3
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here to see the answer
d. PF5
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here to see the answer
e. SF6
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here to see the answer
f. NH3
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here to see the answer
g. H2S
(you will need to draw a dot and cross diagram for this one)
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here to see the answer
5. Dative Covalent Bonds.
A
covalent bond contains a pair of electron, with each atom contributing
one electron to the bond. In a dative covalent bond, both of
the shared electrons in the bond have come from one atom. Dative
covalent bonds are also known as coordinate bonds. This type
of bonding can occur between molecules with a pair of electrons
to donate i.e. molecules with a lone pair, and molecules that
have room for a pair of electrons i.e. those with an incomplete
outer shell.
The
ammonium ion (NH4+) is an example of a compound containing
a dative covalent bond. It is formed when ammonia and a hydrogen
ion react together.
Notice
how all the atoms now have full outer shells of electrons
- remember that there are only two electrons in the first
shell.
The
dative covalent bond is shown using an arrow, with the arrow
pointing to the atom accepting the electrons.
One important
thing to remember is that there is now way to tell apart
a covalent and dative covalent bond - both contain a shared
pair of electrons and there is no way to tell where these
electrons came from!
Checkpoint 5.
Draw
dot and cross diagrams for the following compounds that all
contain a dative covalent bond. It will help if you draw dot
and cross diagrams for the starting materials first! All atoms
should have full outer shells of electrons.
a. BF3NH3 This compound is formed when BF3 and NH3 react together.
Click
here to see the answer
b. H3O+ This compound is formed when H2O and H+ react together.
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here to see the answer
c. CO This is very difficult - this molecule is called carbon monoxide.
It has a multiple bond as well as a dative covalent bond.
Click
here to see the answer
