Lewis Dot Structures
Lewis Dots
Lewis Dot structures describe the covalent
bonding in molecules
They describe how the valence electrons
(outermost s + p electrons) are arranged
around atoms and molecules
Because they describe molecules they
almost always refer to non-metals.
They also can be used to describe
molecular shapes
Around atoms
• When describing simple atoms (not bonded),
you just lay out the electrons on the four sides of
an atomic symbol.
• One lays them out in order. Using Neon as an
example (8 valence electrons)
first two on the right
Next three on the
other three sides
Last three
filling in the
pairs
Ne
The Logic of Dots for Atoms
The Dots
Ignore the kernel electrons
Electron Configuration
first two on the right
(these are the s electrons)
Next three on the
other three sides
Last three
filling in the
pairs
2p
Ne
2s
1s
Lewis Dots around other atoms
• More Examples:
C
(carbon) 4 valence electrons
I
(iodine) 7 valence electrons
S
(sulfur) 6 valence electrons
Forming Molecules
• Atoms will covalently bond (share
electrons) so that they will end up with 8
valence electrons.
• The shared pair counts for both atoms and
makes up the bond
• Here is flourine bonding (7 valence each)
Bonding of 2 Fluorine atoms
to make F2
The bond is shown with a dashed line
F
F
The shared pair of electrons
IS
Shared,bond.
each atom
the covalent
Now
8 electrons
Each flourine atom
hashas
7 electrons
Rules
• Each atom has a number of valence
electrons (= present)
• Each atom needs 8
– Except hydrogen =2
– Boron = 6
– Phosphorus, arsenic and up MAY use 8 or 10
– Sulfur, selenium and up MAY use 8,10 or 12
– Chlorine, bromine and up MAT use 8,10 or 12
Calculating bonds
• First, the number of bonds must be
calculated.
• Add up all valence electrons present (=P)
• Add all needed electrons ( = N)
– Assume all atoms need 8 (except H and B)
unless there aren’t enough bonds
• Bonds present = (N-P)/2
Calculating Bonds: Example
• H2O
• H present = 1(each) 2 (total)
– Need = 2 (each) (4 total)
• O present = 6
– Need = 8
• Bonds =(N-P)/2 = (12-8)/2 = 4/2 = 2 Bonds
Calculating Bonds: Example 2
• CO2
• O present = 6(each) 12 (total)
– Need = 8 (each) (16 total)
• C present = 4
– Need = 8
• Bonds =(N-P)/2 = (24-16)/2
= 8/2 = 4 Bonds
Making a Lewis Dot Diagram: H2O
•
Locate the central atom.
•
Place the outer atoms ( and
hydrogen) surrounding the central
atom.
•
Place a single bond between each
atom and the central atom.
•
Place extra bonds between the
central atom and surrounding
atoms. (no extra bonds here)
•
Fill in the open spaces on all the
atoms the unbonded pairs of dots.
H O H
Making a Lewis Dot Diagram: CO2
•
Locate the central atom.
•
Place the outer atoms (oxygen)
surrounding the central atom.
•
Place a single bond between each
atom and the central atom.
•
Place extra bonds between the
central atom and surrounding
atoms. (two extra bonds here)
•
Fill in the open spaces on all the
atoms the unbonded pairs of dots.
O C O
Coordinate Bonding
• The usual definition of
covalent bonding
involves a “shared
pair” of electons.
• They do NOT need to
come equally from
both atoms
• Some of the bonds
may be made of
electrons that come
from just one atom.
Carbon Monoxide
(C in black, O in red)
(Note that the top bond come only
From the Oxygen’s electrons)
C O