A Lewis Dot Structure is a diagram that shows the
bonding between atoms within a molecule and the lone pairs that may exist in
the molecule. They simply show the configuration of a molecule in a 2-D plane.
To understand Lewis Structures you first have to understand the Lewis Theory.
The
Lewis Theory is based on Valence Electrons-the electrons in the outermost
shell, Ionic Bonds-bond formed between nonmetals and metals, and Covalent
Bonds-bonds formed between metals and metals. There is also something called
the Octet Rule which states that in chemical bonding electrons are
transferred/shared so that each atom may reach a more stable configuration i.e.
the noble gas configuration which contains eight valence electrons.
A
Lewis Symbol for an element is composed of a chemical symbol surrounded by
dots. These dots represent the valence electrons. The image below shows both
Carbon with 4 valence electrons and Oxygen with 6 valence electrons.
When drawing Lewis Structures it can get
complicated, but there is an easy 7 step way to follow.
1.
Step 1: Find the Total Number of Valence
Electrons
a.
Add the total number of valence electrons
in the molecule. for example H2CO would have 12
b.
H-1, H-1, C-4, O-6=12
2.
Step 2: Find the Number of Electrons
Needed to Make the Atoms “Happy”
a.
This means to makes the outer shell of
the atom full
b.
H-1, H-1, C-4, O-2=8
i.
Hydrogen is odd in the sense that it on
wants one to complete its outer shell.
c.
In our example of H2CO, for each Hydrogen
atom there will need to be a Carbon so that takes care each valence electron
for the two Hydrogen atoms and then two of the four valence electrons for the
Carbon atom.
d.
Oxygen will then bond with Carbon, but in
this case you have to remember that Carbon sometimes creates a doubles bond
with Oxygen. In this case it will create a double bond because carbon still
needs 8 valence to complete the Octet Rule.
3.
Step 3: Determine the Number of Bonds in
the Molecule
a.
Step 2 tells how many electrons are
needed and Step 1 is how many electrons you have. Subtracting the number in
Step 2 from the number in Step 1 gives you the number of electrons needed to
complete the octets.
4.
The Lewis Structure Should then look like
the image below
Lewis Structures take practice and some getting used
to, but once you get the hang of them and understand that every element really
just wants to be like the noble gases (except hydrogen, he’s the odd child)
with eight valence electrons it makes things a little easier.
Lewis
structures are used to further understand the configuration of an element such
as Carbon’s configuration is 1s^2 2s^2 2p^2 and looks like the image below.
They are also used to get a better understanding for 3-D structures because in
Lewis Structures lone pairs are demonstrated which could alter the 3-D shape or
angle of a molecule.
Do you think you could draw the Lewis Dot Structure
for H2O?
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