1. Chapter 8 “Covalent Bonding”

2. Section 8.1 Molecular Compounds
OBJECTIVES:
Distinguish between the melting points and boiling points of molecular compounds and ionic compounds.
Describe the information provided by a molecular formula.

3. Bonds Covalent bonds Forces that hold groups of atoms together
and make them function as a unit:
Ionic bonds
transfer of electrons (gained or lost)
Resulting particle is called an “ionic compound”
Covalent bonds
sharing of electrons.
The resulting particle is called a “molecule” or “molecular compound”

4. Molecules Many elements found in nature are in the form of molecules:
a neutral group of atoms joined together by covalent bonds.
For example, air contains oxygen molecules, consisting of two oxygen atoms joined covalently
Called a “diatomic molecule”

5. How do diatomic molecules form?
You would expect the nuclei to repel each other, since they both have a positive charge, and like charges repel. - + + -
But, remember:
H has only one electron in energy level 1.
Energy level 1 is full when it has 2 electrons.
H would be more stable if it had 2 electrons.

6. How do diatomic molecules form?
So both nuclei are attracted to both electrons + +
They share the electrons;
this is called a “covalent bond”;
It involves only NONMETALS!
Instead of… + +

7. Covalent bonds The Problem:
Because covalent bonds happen between Nonmetals, & because Nonmetals hold on to their valence electrons…
They can’t give away electrons to bond.
But they still want a noble gas configuration.
The Solution:
They share valence electrons
By sharing, both atoms get to count the electrons toward a noble gas configuration.

8. Example of Covalent bonding
Each Fluorine atom has seven valence electrons F F

9. Example of Covalent bonding
By sharing electrons, both Fluorine atoms can have eight valence electrons F F

10. Molecular Compounds
Compounds that are bonded covalently (like water and carbon dioxide) are called molecular compounds
Remember from the previous chapter, compounds that were bonded ionically are called Ionic Compounds

11. Molecular Compounds vs Ionic Compounds
tend to have relatively lower melting and boiling points than ionic compounds.
Thus, tend to be gases or liquids at room temperature
Have molecular formulas:
Shows the actual # of atoms of each element a molecule contains
Ionic compounds:
tend to have relatively higher melting and boiling points than ionic compounds.
Thus, tend to be solids at room temperature
We use formula units to show the lowest ratio of elements in an ionic compound
The actual # of atoms is a multiple of this ratio

12. Molecular Formulas Let’s look at an example:
The formula for water is written as H2O
The subscript “2” beneath hydrogen means there are 2 atoms of hydrogen;
if there is only one atom, the subscript 1 is omitted
Molecular formulas do not tell any information about the structure or arrangement of the various atoms.

13. Polyatomic Ions
We also use molecular formulas to represent Polyatomic Ions, Polyatomic Ions are:
Tightly bound group of atoms that has a positive or a negative charge and behaves as a unit
Most polyatomic cations and anions contain covalent bonds
Table E – Selected List of Polyatomic Ions
Ionic compounds containing polyatomic ions include both ionic and covalent bonding
The ammonium ion can be shown as an example:
NH4+

14. - Page 215
3. The ball and stick model is the BEST, because it shows a 3-dimensional arrangement.
These are some of the different ways to represent ammonia:
1. The molecular formula shows how many atoms of each element are present
2. The structural formula ALSO shows the arrangement of these atoms!

15. Section 8.2 The Nature of Covalent Bonding
OBJECTIVES:
Describe how electrons are shared to form covalent bonds, and identify exceptions to the octet rule.
Demonstrate how electron dot structures represent shared electrons.
Describe how atoms form double or triple covalent bonds.
Distinguish between a covalent bond and a coordinate covalent bond, and describe how the strength of a covalent bond is related to its bond dissociation energy.
Describe how oxygen atoms are bonded in ozone.

16. A Single Covalent Bond is...
A sharing of two valence electrons; it can happen between:
two nonmetals, or
A nonmetal and hydrogen
Hydrogen can:
behave as a metal & donate e-
behave as a nonmetal & accept e-
(it behaves as a nonmetal most of the time)
When H forms a bond, it is considered an ionic bond if the electronegativity difference b/w H and the other atom is >1.7 – we’ll do more of this later in the chapter

17. Let’s look at Water Covalent Bond involving Hydrogen:
Each hydrogen has 1 valence electron
Each hydrogen wants 1 more
The oxygen has 6 valence electrons
The oxygen wants 2 more
They share to make each other complete H O

18. H O H Water Hydrogen is happy with 2 e- in energy level 1
Oxygen still needs one more e-
A second hydrogen attaches
Every atom has full energy levels
Note the two “unshared” pairs of electrons H O H

19. Multiple Bonds
Sometimes atoms share more than one pair of valence electrons.
A double bond is when atoms share two pairs of electrons (4 total)
A triple bond is when atoms share three pairs of electrons (6 total)
Table 8.1, p Know these 7 elements as diatomic:
H2 O2 N2 Cl Br2 I F2

20. Carbon dioxide – An Example of Multiple Bonding
CO2 - Carbon is central atom ( more metallic )
Carbon Has 4 valence electrons
Wants 4 more
Oxygen Has 6 valence electrons
Wants 2 more C O

21. C O Carbon dioxide If Carbon & Oxygen share 2 electrons:
Oxygen has 7 valence e-, (wants 1 more e-)
Carbon has 5 valence e-, (wants 3 more e-)
So we’re not done… C O

22. O C O Carbon dioxide Add another Oxygen, now:
each Oxygen has 7 valence e-, wants 1 more
Carbon has 6 valence e-, wants 2 more
But CO2 has only 1 Carbon and 2 Oxygen, how does this happen? How does CO2 exist as a stable molecule? O C O

23. Carbon dioxide
In order to achieve a “stable” number of valence electrons, they share more electrons
When two atoms share 4 e- instead of 2e-, we call it a double bond.
Here CO2 has two double bonds O O C

24. O C O Carbon dioxide Each atom can count all the electrons in the bond
Carbon has 8 valence electrons
8 valence electrons O C O

25. O C O Carbon dioxide Each atom can count all the electrons in the bond
The Left hand Oxygen has 8 valence electrons
8 valence electrons O C O

26. O C O Carbon dioxide Each atom can count all the electrons in the bond
The Right hand Oxygen has 8 valence electrons
8 valence electrons O C O

27. How do I know what bonds will form?
We’ll look at two examples that will illustrate how to determine the number and type of bonds that will form

28. Example - NH3, (ammonia) Step 1:
# valence e- molecule WANTS 8 2 14
# valence e- molecule HAS 5 1 8
Atom
Nitrogen
Hydrogen
SUM: H

29. Example - NH3, (ammonia) Step 2: Calculate # Bonds
WANTS – HAS = # of bonds 2
14 – 8 = =
3 bonds H

30. Example - NH3, (ammonia) Step 3: Draw It
Start with the “central atom,” the atom that is furthest from what it wants
Here Nitrogen wants 8 e-, needs 3 more (most)
Hydrogen atoms will bond to the central atom H
3 bonds…as predicted,
and
Everyone has what they want! N H H

31. Another Example: HCN - Hydrogen Cyanide
This example involves a molecule with multiple bonds

32. Example: HCN - Hydrogen Cyanide Step 1:
# valence e- molecule WANTS 8 2 18
# valence e- molecule HAS 5 4 1 10
Atom
Nitrogen
Carbon
Hydrogen
SUM: C H

33. HCN - Hydrogen Cyanide Step 2: Calculate # of Bonds
WANTS – HAS = # of bonds 2
18 – 10 = = C
4 bonds H

34. HCN - Hydrogen Cyanide Step 3: Draw It
Start with the “central atom”
Carbon is the central atom; it needs 4 more e-
Nitrogen forms a bond with Carbon
Hydrogen forms a bond with Carbon
What’s wrong with this picture?
1. Only Hydrogen
has the # of e- it wants
2. We predicted 4 bonds
– here there are only 2 H C N

35. H C N HCN - Hydrogen Cyanide We need 2 more bonds
H is full, so bonds must go between C & N
Add one bond, C & N still don’t have 8 e- H C N

36. What type of bond is between C & N?
HCN - Hydrogen Cyanide
We need 2 more bonds
H is full, so bonds must go between C & N
Add one bond, C & N still don’t have 8 e-
Add another bond
Now everyone has the # of e- they want
What type of bond is between C & N?
A triple bond H C N

37. Another way to draw bonds
Use a line to indicate a bond
Each line represents 2 valence e-
This is called a structural formula H N H H C N H

38. H O H C H H C H H CH2O CH4 H | H - C - H | H O || H-C-H
Molecular Formula
Molecular Formula
CH2O
CH4
Lewis Dot Diagram
Lewis Dot Diagram H O H C H H C H H
Structural Formula
Structural Formula
H | H - C - H | H
O || H-C-H

39. End of Chapter 8.1 & 8.2