Lewis Structures and
Molecular Geometry
Valence Electrons
Electrons fill orbitals that are on the
outside of the nucleus of the atom
The number of electrons in an orbital
varies depending on the atom, the
number of electrons, etc.
The electrons in the outside orbital are
called Valence Electrons
Valence Electrons
Draw a Bohr Diagram for the following atoms
H
Li
How many valence electrons in each atom?
Na
Valence Electrons
The number of valence electrons is the same for every atom in a group/family (column on the
periodic table)
We do not include the transition metals for this as they act very differently
Lewis Dot Diagrams
Valence electrons are often shown in Lewis Dot Diagrams
In these the nucleus is written in the middle with dots representing each valence
electron
Electrons MUST be drawn as single electrons before they can be paired up
Example
Draw a Lewis Dot Diagram for the following atoms
Fluorine
Sulphur
Lewis Dot Diagram Examples
Draw a Lewis Dot Diagram for the following atoms
Carbon
Beryllium
Arsenic
Krypton
Lewis Dot Diagrams for Molecules
As well as Lewis Dot Diagrams for atoms, we can draw them for molecules as well
There is a lot of background we need to learn first to know how to draw these
properly:
Fundamental Requirements:
All valence electrons of the atoms involved must be represented
& accounted for
Usually, all electrons are paired
Usually, each atom achieves a stable octet
Octet – 8 electrons around the atom
Sometimes multiple covalent bonds (double & triple) are
necessary. These are formed most readily by C, N, O, P, and S
atoms
Skeletal Structure:
 Refers to all the atoms involved arranged in the order in which
they are bonded to one another
 When more than 2 atoms are involved:
Central atom: bonded to 2 or more atoms
Terminal atom: bonded to only 1 atom
Rules regarding central & terminal
atoms:
 1. Hydrogen atoms are always terminal atoms
 2. Carbon atoms are always central atoms
 3. When carbon isn’t involved, the central atom
is determined by a characteristic known as
electronegativity
Electronegativity
To know how to come up with the correct structure you will need to know about
ELECTRONEGATIVITY
An atom’s ability to attract or compete for other electrons with other atoms to which it is
bonded
Electronegativity increases from left to right and bottom to top on the periodic table
Noble Gases are excluded from this rule
Lewis Dot Diagrams for Molecules
Steps
1. Count TOTAL valence electrons in the compound
2. Draw a Central Atom
Carbon atoms are always central atoms
Central atoms are those with the lowest electronegativity value compared to
other atoms
3. Connect the central atoms to the terminal atoms with a single bond
4. Count remaining electrons
Remember that a bond is 2 electrons
5. Add lone pairs to the terminal atoms first
6. Look for a way to complete the octet of the central atom (lone pairs or
double/triple bonds)
Common Bonding Patterns
When looking at a Lewis structure, look for common bonding patterns. This is a big clue if you got
the structure correct.
Examples
CF4
CO2
H2S
NH3