1. Chemistry 4: Drawing Atoms
Science 9
Chemistry 4: Drawing Atoms

2. Objectives By the end of the lesson you should be able to:
Determine the difference between an atom and an ion
Draw Bohr Model Diagrams for the first 20 elements on the Periodic Table
Use atomic structure to explain why elements behave differently

3. Atoms VS Ions
Atoms are elements as you see them on the Periodic Table: with equal amounts of p+ and e-
EX: sodium atom has 11 p+ and 11 e-
Ions are elements that have gained/lost electrons and now have a charge
EX: sodium ion has 1+ charge because it has lost 1 e-

4. Why Ions….? Atoms are unhappy without a full outer shell, so……
Atoms gain/lose electrons to get a full outer shell
When atoms gain/lose electrons they turn into ions
Atoms give or gain the electrons from other atoms….this is called BONDING 

5. Practice! Anything with a charge is an ion, all the others are atoms
State if the following are Atoms or Ions
Na K H+ Cl- Ca
O2- Ne Be2+ Al N3-
Anything with a charge is an ion, all the others are atoms

6. Ions and Electrons
To determine the number of electrons in an ion you need to subtract the charge to the atomic number
EX: Calcium has an atomic number of 20 and a 2+ charge so, its ion has = 18 electrons
EX: Fluorine has an atomic number of 9 and a 1- charge so, its ion has 9 – (-1) = 10 electrons (remember subtracting a negative is like adding!)
Complete the worksheet “The Number Game with Atoms and Ions”

7. Bohr Model Diagrams
Niels Bohr described how the electrons are arranged in the shells around a nucleus in an atom
His theory has 3 ideas:
Electrons move around the nucleus in shells
Each shell is a certain distance away from the nucleus and can hold a definite number of electrons
After the shell closest to the nucleus is full, electrons start filling the next shell

8. How to draw an atom:
Draw a circle and put the symbol and number of protons and neutrons inside of it
Add the appropriate number of shells (this is determined by the period number)
EX: Magnesium is in the 3rd period so it will have 3 shells Mg
12p+
12no
There are now 3 shells in total

9. Filling Electron Shell Rules
Maximum of 2 electrons in the first shell
Maximum of 8 electrons in the next 2 shells
Maximum of 18 electrons in the 4th shell and all shells after that (we won’t be making these!!)
Electrons occur in pairs
There must be 4 single electrons before the electrons will occur in pairs

10. Filling Shells EX: Magnesium has 12 electrons (from atomic number)
So, 2 electrons in the first shell, 8 in the second and 2 in the last
Watch how the electrons are added (up, down, side to side) Mg
12p+
12no

11. Valence Electrons
This is a fancy way of saying “outer shell electrons”
If you are ever asked for the number of valance electrons, they are just asking how many electrons are found in the last, outermost, shell only
EX: in the previous slide, the magnesium atom has 2 valence electrons

12. Your Turn!
Complete the worksheet “Drawing Bohr Model Diagrams”

13. Element Behaviour
GOLDEN RULE: all atoms want a full outer shell (it makes them more stable) – they will gain/lose electrons to achieve this!
Thus, elements that have the same number of valence electrons will behave the same way as they need to gain/lose the same number of electrons!

14. THINK ABOUT IT! Why do sodium and cesium behave the same way?
Why do oxygen and sulphur behave the same way?
Why are the noble gases un-reactive (inert)?
Why is this statement true?
“All atoms are trying to become like their nearest Noble Gas”

15. Summary All atoms want full outer electron shells
All metals lose electrons
All non-metals gain electrons
Complete worksheets “Analyzing Bohr Model Diagrams” and “The Periodic Table and Atomic Theory”