Electron Dot Diagrams

How to draw them

Drawing electon dot diagrams (or Lewis diagrams) are quite easy if you follow these steps/rules.

1. The nucleus is represented by the atomic symbol
2. Each electron is represented by a dot which are around the atomic symbol
3. There are four orbitals on each side of the nucleus. Each orbit can hold a maximum of 2 electrons
4. 8 electrons represent a closed shell, except for H, which needs 2 electrons to become stable
5. Determine the # of valence electrons

For example sodium is in group 1 therefore it has 1 valence electron

Here are some examples that include double/triple bonding

There are many types of electron diagrams, such as Bohr, Lewis or structural.

In a structural diagram, each bonded pair is represented as a line.
Here's an example:

Now, if we want to represent ionic bonds, we would look at the charge of the ion. For example, for NaCl:

You would put the negative ion in square brackets, and write the positive ion right beside it. So, you would write Na+ [Cl]- (but you would include dots around the four orbits to symbolize that the compound has satisfied the octet rule).

In Lewis structures, the number of bonds is shown, along with the lone paired electrons, which are electrons that do not participate in sharing.

But how do we determine which element is the central atom?
Here are a few rules to follow:
1. H and F are never in the middle
2. If there is a metal, the metal is always the central one
3. If a molecule contains only one atom of one element, and many of another, then the single atom is in the centre.
4. Atoms that need the most electrons to complete their valence shell is in the centre.

Let's try an example:
C2H2:

First, how many valence electrons do we have?

4x2 + 1x2 = 10

Now, we have two C's in the middle, with a bond in between them.

C - C

Then, we have two H's branching out from the C's.

H - C - C - H

That makes 6 electrons, so we have to distribute 4 more to the C's (since H's are already full)

If we distributed them evenly, we would have two lone electrons on each of the C's. (preferably one on each of the orbits)

Then, if we took one of the electrons and bonded it with an electron from the other C, we would have a double bond between the C's and one lone electron remaining on each C.

Now, if we take those ones and bond them together, we will successfully get our full shell, with a triple bond between the Carbons.




Here's some practice:
http://misterguch.brinkster.net/PRA017.pdf
http://www.arps.org/users/hs/thompsom/honors/chap_07/Lewis_Structure_Worksheet_1.pdf