1. Science: Physical Science
Bohr’s Model
Lewis Structure

2. Science: Physical Science
Niels Bohr
Gilbert N. Lewis
Born in Denmark
20th Century physicist (scientist)
Worked on the Manhattan Project during World War II. This is the project that produced the first atomic bomb.
Born in the USA
20th Century physicist (scientist)
Home-schooled until the age of 13 when he entered the preparatory school of the University of Nebraska.

3. Science: Physical Science
Bohr’s model drawing
looks very much like the solar system
Helps us understand electron configuration
Helps us understand why and how some elements will bond with other elements

4. Shells: Basics Let's cover some basics of atomic shells:
Science: Physical Science
Let's cover some basics of atomic shells:
The center of the atom is called the nucleus.
Electrons are found in areas called shells.
c. Shells are areas that surround the center of an atom. 
Shells: Basics

5. Important Things to Remember
Science: Physical Science
Each shell corresponds to a period on the Periodic Table.
You MUST load each shell before you move on to the following one.
The number of electrons on each shell can only be up to the number of elements found on the corresponding period. Example: The first shell can only have a maximum of 2 electrons because there are only 2 elements on the first period of the table (Hydrogen and Helium)
Bohr’s Model:
Step by Step
Directions
Important Things to Remember

6. Bohr’s Model: Step by Step Directions
Science: Physical Science
Locate the element on the Periodic Table and write the information about the element.
Draw the nucleus and write the number of protons and neutrons inside the nucleus.
Draw the correct amount of shells outside the nucleus based on the row number where the element is found.
Start loading electrons on each shell, beginning from the inside and moving to the outside.
Bohr’s Model:
Step by Step
Directions

7. Science: Physical Science
Bohr ‘s Model: Example 1
Element: Helium
Atomic #: 2
Atomic Mass: 4.003
Protons: 2
Neutrons = 4 – 2
(Atomic Mass –
Protons)
Electrons: 2
P= 2
N= 2

8. Science: Physical Science
Bohr ‘s Model: Example 2
Element: Neon
Atomic #: 10
Atomic Mass:
Protons: 10
Neutrons = 20 – 10
(Atomic Mass –
Protons)
Electrons: 10 (2 on the inner shell / 8 on the outer shell)
P= 10
N= 10

9. Science: Physical Science
Octet Rule (Lewis Rule of Eight)
When atoms combine to form molecules,
they generally each lose, gain, or share valence 
electrons until they attain or share eight.
Number of valence electrons is the same as number of the group where the element is found (don’t count transition metals). Example: Hydrogen = 1 valence electron, Calcium = 2 valence electrons, Carbon = 4 valence electrons.
Lewis Structure
Important things to remember

10. Science: Physical Science
For the example we will use HCN
Calculate the number of valence electrons that move around it. We have: = 10
Add a pair of bonding electrons between every two atoms H : C : N
Add the rest of the electrons to the outer atoms, until they meet the octet rule . Hydrogen is an EXCEPTION. It only needs 2 electrons around it.
H C N
Lewis Structure
Step by Step Directions
So far, Nitrogen satisfied the octet rule and we have placed the 10 valence electrons. However, Carbon only has 2 valence electrons. What’s next?

11. Science: Physical Science
Nitrogen will have to share electrons with Carbon. We can move 4 electrons.
H C N
We now have 8 electrons around each atom and a total of 10 valence electrons.
Next, we need to replace each pair of bonding electrons with a simple line to show the bond.
H C N
Lewis Structure
Step by Step Directions

12. Science: Physical Science
Carbon is in Group 4 (if we don’t count the transition metals).
Oxygen is in Group 6 (if we don’t count the transition metals).
Thus we have: C = 4 valence electrons; O = 6 valence electrons. Multiply 6 x 2 = 12 (remember there are 2 Oxygen atoms in Carbon dioxide. Total = 16.
Add a pair of bonding electrons between every 2 atoms.
O C O
We used 4 electrons out of 16. We need to start placing the other electrons to satisfy the octet rule.
Lewis Structure
Example using Carbon dioxide - CO2

13. Science: Physical Science
Place electrons around the outer atoms
O C O
7) The Oxygen met the octet rule but not the Carbon atom. Each Oxygen shares 2 electrons with the Carbon.
8) Replace the bonding electrons with simple
lines.
Lewis Structure
Example using Carbon dioxide - CO2 – Cont’
O C O
C O
C O
O C O
C O
O C O
C O
O C O
O C O
You are done!