1. CH 9: Ionic and Covalent Bonding
Renee Y. Becker
Valencia Community College
CHM 1045

2. Covalent bonds are formed by sharing at least one pair of electrons.
The attraction (nucleus/electrons) outweighs the repulsions (electron/electron & nucleus/nucleus)

3. Covalent Bonds
Every covalent bond has a characteristic length that leads to maximum stability. bond length

4. Strength of Covalent Bonds
Energy required to break a covalent bond in an isolated gaseous molecule is called the bond dissociation energy.
Same amount of energy released when the bond forms

5. Which of the following is correct? Energy is absorbed to form a bond
Example 1:
Which of the following is correct?
Energy is absorbed to form a bond
Energy is released when a bond is formed

6. Bond polarity is due to electronegativity differences between atoms.
Polar Covalent Bonds
Bond polarity is due to electronegativity differences between atoms.
Pauling Electronegativity: is expressed on a scale where F = 4.0

7. Pauling Electronegativities

9. Electron-Dot Structures
Using electron-dot (Lewis) structures, the valence electrons in an element are represented by dots.
Lewis symbols
Valence electrons are those electrons with the highest principal quantum number (n). .

11. Electron-Dot Structures
The electron-dot structures provide a simple, but useful, way of representing chemical reactions.
Ionic:
Covalent:

12. Electron-Dot Structures
Single Bonds:
Double Bonds:
Triple Bonds:

13. Drawing Lewis-Dot Structures
Rule 1: Count the total valence electrons.
Rule 2: Draw the structure using single bonds.
Rule 3: Distribute the remaining electron pairs around the peripheral atoms.
Rule 4: Put remaining pairs on central atom.
Rule 5: Share lone pairs between bonded atoms to create multiple bonds.

14. Drawing Lewis-Dot Structures
NH2F Amino Fluoride: In this molecule, nitrogen is the central atom.
Rule 1: Number of electrons = 5 + (2 x 1) + 7 = 14 = 7 pairs
Rule 2 Rule 3 Rule 4

15. Drawing Lewis-Dot Structures

16. Drawing Lewis-Dot Structures
Polyatomic molecules with central atoms below the second row ten:
In this compound there are 10 valence electrons on bromine; this is called an expanded octet. The extra pairs go into unfilled d orbitals.

17. Example 2: Drawing Lewis-Dot Structures
Draw electron-dot structures for:
C3H8 H2O2 CO2 N2H4
CH5N C2H4 C2H2 Cl2CO
H3S+ HCO3–

18. How is the double bond formed in O3?
Resonance Structures
How is the double bond formed in O3?
The correct answer is that both are correct, but neither is correct by itself.

19. Resonance Structures
When multiple structures can be drawn, the actual structure is an average of all possibilities.
The average is called a resonance hybrid. A straight double-headed arrow indicates resonance. O O O O O O

20. Resonance Structures
The nitrate ion, NO3–, has three equivalent oxygen atoms, and its electronic structure is a resonance hybrid of three electron-dot structures. Draw them.

21. Formal Charge: Determines the best resonance structure.
We determine formal charge and estimate the more accurate representation.

22. Example 3: Formal Charge
Calculate the formal charge and determine the most favorable of the following electron dot structures:
SO NO3– NCO– N2O O3 CO32–

23. What is the overall formal charge of the following structure?
Example 4:
What is the overall formal charge of the following structure? -2 -3 -1

24. Example 5: Ionic Radii of Ions
Compare ionic radii
Fe & Fe3+
Cl & Cl-