1. Section 5.2— Drawing Molecules
Objectives:
use the periodic table to identify the number of valence electrons in an atom
draw Lewis Dot structures for ionic and covalent compounds
introduce the idea of isomers

2. Drawing Molecules on Paper
Lewis Structures (or Dot Structures) are one way we draw compounds on paper
Since paper is 2-D and compounds aren’t, it’s not a perfect way to represent how atoms bond…but it’s a good way to begin to visualize compounds

3. Drawing Ionic Compounds

4. How do we determine the number of valence electrons in an atom?
The number found with the “A” in the group number IS the number of valence electrons. 1 2 3 4 5 6 7 8

5. Remember: Put electrons on each side of the element’s symbol.
Put one in each spot before doubling up!

6. Draw the Lewis Structure for an oxygen atom
Practice: Oxygen
Oxygen is in group VIA or 6A.
There are 6 valence electrons.
Example:
Draw the Lewis Structure for an oxygen atom O

7. To Draw Ionic Compounds
Draw each atom
Transfer electrons from metal atoms to non-metal atoms, keeping track of their new charge

8. Draw the Lewis Structure for KCl
Practice: KCl
Potassium has 1 electron
Chlorine has 7 electrons
Example:
Draw the Lewis Structure for KCl K Cl

9. Draw the Lewis Structure for KCl
Example:
Draw the Lewis Structure for KCl +1 -1 K Cl

10. Add more atoms if needed
Example:
Draw the Lewis Structure the ionic compound of Barium fluoride

11. Draw the Lewis Structure the ionic compound of Barium fluoride
Barium has 2 electrons
Fluorine has 7 electrons
Example:
Draw the Lewis Structure the ionic compound of Barium fluoride Ba F
The fluorine is full, but the Barium isn’t!

12. Draw the Lewis Structure the ionic compound of Barium fluoride
Another fluorine atom is needed!
Example:
Draw the Lewis Structure the ionic compound of Barium fluoride Ba F F

13. Draw the Lewis Structure the ionic compound of Barium fluoride
Now all have full valence shells and the charges are balanced, just as when you learned to write in BaF2 in Chapter 2!
Example:
Draw the Lewis Structure the ionic compound of Barium fluoride -1 +2 Ba F -1 F

14. A note about Ionic Dot Structures
The atoms are not sharing the electrons—make sure you clearly draw the atoms separate!

15. Practice Problems
Draw Lewis Dot Structures for compounds that form between the following atoms. What are the formulas for the compounds?
Al and Cl
Ba and O
Na and P

16. Drawing Covalent Compounds

17. Arrange the Atoms
Atoms found in the middle of covalent compounds are the atoms with the LEAST electron affinity.
Hydrogen & Halogens (F, Cl, Br, I) can only bond with one other atom—they can’t go in the middle of a molecules
Always put them around the outside
In general, write out the atoms in the same order as they appear in the chemical formula

18. Next:
Use the periodic table to decide how many electrons are around each atom
Write the electrons around each atom

19. Draw the Lewis Structure for CH4
Practice: CH4
Carbon is in the middle – the H’s are around it.
Carbon has 4 electrons
Each hydrogen has 1
Example:
Draw the Lewis Structure for CH4 H H C H H

20. 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—it can only hold 2)
Each pair of electrons shared by two atoms forms a bond
A bond can also be represented by a single dash H H C H H

21. Sometimes
Sometimes, you don’t end up having a full valence shells for all atoms in the compound.

22. Draw the Lewis Structure for CH2O
Practice: CH2O
C has to go in the middle – the O and 2 H’s are around it.
C has 4 electrons, oxygen has 6 and each H has 1.
Example:
Draw the Lewis Structure for CH2O H H C O

23. Draw the Lewis Structure for CH2O
Note: the carbon and oxygen only have 7 each
However, 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

24. Draw the Lewis Structure for CH2O
Now the carbon and oxygen both have
8 valence electrons! (And the H’s have 2!) H
Example:
Draw the Lewis Structure for CH2O H C O

25. Double Bonds
Double bonds are when 2 pairs of electrons are shared between the same two atoms
Double Bond

26. Lone Pairs
Lone pairs are a pair of electrons not shared—only one atom “counts” them
Lone pair

27. And when a double bond isn’t enough…
Sometimes forming a double bond still isn’t enough to have all the valence shells full

28. Draw the Lewis Structure for C2H2
Practice: C2H2
Both C’s go in the middle and 1 H goes on each side.
The C’s have 4 electrons each; the H’s have 1.
Example:
Draw the Lewis Structure for C2H2 H C C H

29. Draw the Lewis Structure for C2H2
Each carbon atom only has 7 electrons…not a full set of 8!
But they each have an un-paired electron left!
Example:
Draw the Lewis Structure for C2H2 H C C H

30. Draw the Lewis Structure for C2H2
Each carbon atom only has 7 electrons…not a full set of 8!
But they each have an un-paired electron left!
Example:
Draw the Lewis Structure for C2H2 H C C H
Now they each have 8 electrons!

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

32. Properties of multiple bonds
Single Bond
Double Bond
Triple Bond
Shorter bonds (atoms closer together)
Stronger bonds (takes more energy to break)

33. Practice Problems
Draw Lewis dot structures for:
H2O O2
HCN

34. Special Notes

35. Polyatomic Ions
They are a group of atoms bonded together that have an overall charge
Polyatomic ions have a covalent bond within themselves…
But an ionic bond with other ions

36. Isomers
For many compounds, there is more than one correct way to draw a Lewis Dot Structure H C
Chemical Formula: C3H4
Contains 2 sets of double bonds between carbons
Both structures have full valence shells! H C
Chemical Formula: C3H4
Contains 1 triple bond and 1 single bond between carbons

37. H C H C Both are “correct”
The chemical formula alone does not give you enough information to differentiate between the two structures H C
Chemical Formula: C3H4 H C

38. Isomers
Isomers: Structures with the same chemical formula but different chemical structure
Atoms can be bonded differently (multiple versus single bonds) or in a different order) but have the same overall chemical formula in isomeric structures.