1. SNC2D – Academic Science
Models of the Atom
SNC2D – Academic Science

2. Learning Goals I will learn to…
Draw Lewis dot (electron dot) diagrams to represent neutral atoms and ions.
Use the periodic table to predict the ionic charge of an element and draw the corresponding Lewis dot diagram.

3. Recall: Bohr-Rutherford Diagram
When representing a neutral atom, we can use a Bohr-Rutherford Diagram to show the atomic structure of an element.
The atomic number indicates the number of protons and electrons.
Each electron shell has a different capacity of electrons that it can hold.
The outermost shell is also known as the valence shell.
To find how many neutrons there are in the nucleus of an atom:
# Neutrons=Mass Number −# Protons
Protons & neutrons have similar mass, while the electrons have 1800th the mass of a proton. Hence, the mass of the electrons is usually omitted from the calculation.
1st Shell: 2
2nd Shell: 8
3rd Shell: 8
4th Shell: 2

4. A New Model: Lewis Dot Diagrams
Another model to represent an atom is the Lewis Dot Diagram (Electron Dot Diagram) which shows the bonding between atoms of a molecule and the lone pairs of electrons that may exist in the molecule. It is a simpler model that includes the symbol for the element with the number of valence shell electrons (VSE) in that atom or ion.
The element symbol is written and only the VSE are placed around it.
The nucleus and inner orbits are not included.
Always show electrons above, below, or to the sides of the symbol
Start from the top and move clockwise to arrange the electrons
Write electrons singly first, before “pairing” them up

5. Examples of Lewis Dot Diagrams
Calcium
Fluorine
Sulphur
Aluminum
Argon
Sodium

6. Determining the Number of VSE
The number of valence shell electrons is equal to the group number an element is in:
Note: For group numbers with double digits, ignore the “1”; the second number tells you the number of valence shell electrons.
Group Number
Number of VSE 1 2 13 3 14 4 15 5 16 6 17 7 18 8

7. What are Ions?
An ion is an atom that has an unequal number of protons and electrons, giving it an overall charge.
A positively charged ion (cation) is formed when a metal atom loses its valence shell electrons.
A negatively charged ion (anion) is formed when a non-metal atom gains electrons to fill its valence shell.

8. How Do I know How many Electrons Are Lost or Gained?
Noble gases are in group 18.
They have 8 electrons in their valence shell.
A filled shell makes atoms stable and unreactive (also known as a stable octet).
This is the reason why noble gases are unreactive and do not form compounds.
Ions form by losing or gaining electrons to achieve a filled valence shell.
Examples
Sodium Na (Cation) Chlorine Cl (Anion)

9. How do I know how many electrons are lost or gained?
Group Number
Number of VSE
Number of Electrons Gained/Lost 1
Lose 1 e- 2
Lose 2 e- 13 3
Lose 3 e- 14 4
Lose/Gain 4 e- 15 5
Gain 3 e- 16 6
Gain 2 e- 17 7
Gain 1 e- 18 8
Stable Octet (None)

10. Determining Ion Charges
Since an ion has an uneven number of positive and negative charges, this must be shown.
Metal ions have a positive charge and the value is the same as the number of electrons lost from the valence shell.
Ex.: Na+1
The sodium atom loses its 1 VSE to become an ion and have a filled shell
Now, it has 11 protons and only 10 electrons
Therefore it has a charge of +1
The math:

11. Determining Ion Charges
Non-metal ions have a negative charge and the value is the same as the number of electrons gained to complete the valence shell.
Ex.: O-2
The oxygen atom gains 2 VSE to become an ion and have a filled valence shell
Now it has 8 protons and 10 electrons
Therefore it has a charge of -2 charge
The math:

12. Lewis Dot Diagrams of Ions
For positively charged ions (metal ions):
Write the chemical symbol
Draw square brackets around the symbol
Write the ionic charge in the top right corner outside of the brackets
DO NOT draw any valence electrons since they have all been lost
[Na]+1

13. Lewis Dot Diagrams of Ions
For negatively charged ions (non-metal ions):
Write the chemical symbol
Place the 8 VSE around the symbols in pairs
Draw square brackets around the symbol and electrons
Write the ionic charge in the top right corner outside of the brackets
[ O ]-2