1. Section 5.2—Drawing Molecules

2. Drawing Molecules on Paper
Lewis Structures (or Dot Structures) are one way we draw molecules on paper
A flaw of the Lewis method is that it is 2-D and molecules are 3D. However, Lewis Dots are essential to finding out the correct 3-D shape!

3. Drawing Ionic Compounds

4. Step One: How many valence electrons are on each atom?
The main groups of the periodic table each have 1 more valence electron than the group before it. 1 2 3 4 5 6 7 8

5. Step 2: Place the valence electrons around an atom
When atoms bond, they bond with their valence orbitals. Those are the outmost s or s and p. So, they have 4 orbitals available (1 “s” and 3 “p”s). There are 4 places to put electrons. Imagine the element symbol has a box around it. Each side of the box, represents an orbital.
Put one on each side before doubling up!
Example:
Draw the Lewis Structure for a sulfur atom

6. Step 2: Place the valence electrons around an atom
When atoms bond, they bond with their valence orbitals. Those are the outmost s or s and p. So, they have 4 orbitals available (1 “s” and 3 “p”s). There are 4 places to put electrons. Imagine the element symbol has a box around it. Each side of the box, represents an orbital.
Put one on each side before doubling up!
Sulfur is in the 6th main group.
There are 6 valence electrons.
Example:
Draw the Lewis Structure for a sulfur atom S

7. Step 3: Transfer electrons to form the ionic bond
Transfer electrons from metal atoms to non-metal atoms, keeping track of their new charge
Example:
Draw the Lewis Structure for KCl

8. Step 3: Transfer electrons to form the ionic bond
Transfer electrons from metal atoms to non-metal atoms, keeping track of their new charge
Example:
Draw the Lewis Structure for KCl K Cl
Potassium has 1 electron
Chlorine has 7 electrons

9. Step 3: Transfer electrons to form the ionic bond
Transfer electrons from metal atoms to non-metal atoms, keeping track of their new charge
Example:
Draw the Lewis Structure for KCl +1 -1 K Cl
Potassium has 1 electron
Chlorine has 7 electrons

10. Step 4: Add more atoms if needed
If the transfer from one atom to another doesn’t result in full outer shells, add more atoms
Example:
Draw the Lewis Structure for the ionic compound of Calcium Chloride

11. Step 4: Add more atoms if needed
If the transfer from one atom to another doesn’t result in full outer shells, add more atoms
Example:
Draw the Lewis Structure for the ionic compound of Calcium Chloride Ca Cl
Calcium has 2 electron
Chloride has 7 electrons
The chlorine is full, but the calcium isn’t!

12. 4: Add more atoms if needed
If the transfer from one atom to another doesn’t result in full outer shells, add more atoms
Example:
Draw the Lewis Structure the ionic compound of Calcium Chloride Ca Cl Cl
Calcium has 2 electron
Chlorine has 7 electrons
Add another chlorine atom

13. 4: Add more atoms if needed
If the transfer from one atom to another doesn’t result in full outer shells, add more atoms -1 +2
Example:
Draw the Lewis Structure the ionic compound of Calcium Chloride Ca Cl -1 Cl
calcium has 2 electron
chloride has 7 electrons
Now all have full valence shells and the charges are balanced, just as when you learned to write in Chapter 2—CaCl2!

14. A note about Ionic Dot Structures
The atoms are not sharing the electrons—make sure you clearly draw the atoms separate! To complete the Ionic Dot Structure, we place brackets around each ion with a charge. And we organize the ions so the similar charges are not near each other!

16. Drawing Covalent Compounds

17. Tips for arranging atoms
Hydrogen & Halogens (F, Cl, Br, I) can only bond with one other atom—they can’t go in the middle of a molecule
Always put them around the outside
In general, write out the atoms in the same order as they appear in the chemical formula

18. Steps for the Covalent Lewis Dot
Use the periodic table to decide how many electrons are around each atom
Write the electrons around each atom
Example:
Draw the Lewis Structure for CH4

19. Repeat first two steps from before
Use the periodic table to decide how many electrons are around each atom
Write the electrons around each atom H
Carbon has 4 electrons
Each hydrogen has 1
Example:
Draw the Lewis Structure for CH4 H C H H
Remember, “H” can’t go in the middle…put them around the Carbon!

20. Step 3: Count electrons around each atom
Any electron that is being shared (between two atoms) gets to be counted by both atoms!
All atoms are full with 8 valence electrons (except H—can only hold 2) H
Example:
Draw the Lewis Structure for CH4
Carbon has 8 H C H
Each Hydrogen has 2 H
All have full valence shells—drawing is correct!

21. Bonding Pair or Shared Pair
Pair of electrons shared by two atoms…they form the “bond”. They are shared! H C
Bonding pair

22. What if they’re not all full after that?
Sometimes, the first 3 steps don’t leave you with full valence shells for all atoms
Example:
Draw the Lewis Structure for CH2O

23. What if they’re not all full after that?
Sometimes, the first 3 steps don’t leave you with full valence shells for all atoms H
Example:
Draw the Lewis Structure for CH2O H C O
Remember that hydrogen atoms can’t go in the middle!

24. What if they’re not all full after that?
Sometimes, the first 3 steps don’t leave you with full valence shells for all atoms
The two hydrogen atoms are full
But the carbon and oxygen only have 7 each! H
Example:
Draw the Lewis Structure for CH2O H C O

25. What if they’re not all full after that?
Sometimes, the first 3 steps don’t leave you with full valence shells for all atoms
But they each have a single, unshared electron.
They could share those with each other! H
Example:
Draw the Lewis Structure for CH2O H C O

26. What if they’re not all full after that?
Sometimes, the first 3 steps don’t leave you with full valence shells for all atoms
Now the carbon and oxygen both have a full valence! H
Example:
Draw the Lewis Structure for CH2O H C O
This is called a double bond!!

27. Double Bonds & Lone Pairs (Unshared Pairs or Nonbonding Pairs)
Double bonds are when 2 pairs of electrons are shared between the same two atoms
Lone pairs are a pair of electrons not shared—only one atom “counts” them H C O
Lone pair
Double Bond

28. And when a double bond isn’t enough…
Sometimes forming a double bond still isn’t enough to have all the valence shells full
Example:
Draw the Lewis Structure for C2H2

29. And when a double bond isn’t enough…
Sometimes forming a double bond still isn’t enough to have all the valence shells full
Example:
Draw the Lewis Structure for C2H2 H C C H
Remember that hydrogen atoms can’t go in the middle!

30. And when a double bond isn’t enough…
Sometimes forming a double bond still isn’t enough to have all the valence shells full
Each carbon atom only has 7 electrons…not full
Example:
Draw the Lewis Structure for C2H2 H C C H

31. And when a double bond isn’t enough…
Sometimes forming a double bond still isn’t enough to have all the valence shells full
But they each have an un-paired electron left!
Example:
Draw the Lewis Structure for C2H2 H C C H

32. And when a double bond isn’t enough…
Sometimes forming a double bond still isn’t enough to have all the valence shells full
Now they each have 8 electrons!
Example:
Draw the Lewis Structure for C2H2 H C C H
This is a Triple Bond!

33. Triple Bonds
A Triple Bond occurs when two atoms share 3 pairs of electrons H C
Triple Bond

34. Properties of multiple bonds
Single Bond
Double Bond
Triple Bond
The more bonds shared the
Shorter the bond’s length
(atoms are closer together)
The More bonds shared the Stronger the bonds (takes more energy to break)
C-C Requires 348 kJ/mole to break
C=C Requires 614 kJ/mole to break
C≡C Requires 839 kJ/mole to break