1. 9.5-9.6 Lewis Structures and Covalent Bonding

2. H O Lewis Diagrams Provide models for covalent bonding
Shows bonded atoms sharing their valence electrons
Also depict single atoms and their valence electrons H O

3. Single Covalent Bonds
Atoms share valence electrons to obtain an octet (or duet in case of Hydrogen/Helium) and become stable
In single covalent bonds, only one pair of electrons is shared
This pair of shared electrons is called a bonding pair
Pair that belongs only to one atom is known as the lone pair

4. Single Bonds H O
One Valence Electron
Six Valence Electrons

5. Single Bonds
Duet
Duet
Octet H H O H

6. Representing Bonding Pairs
Bonding pairs are usually shown as a line H O _

7. Halogens
Lewis structures show why halogens are diatomic Cl –

8. O =O Double Covalent Bonds
Two atoms can share two electron pairs to achieve an octet O =O
Double bonds are shorter and stronger than single bonds

9. N Triple Covalent Bonds Atoms can also share three electron pairs
Triple bonds are even shorter and stronger than double bonds

10. Covalent Bonding: Models and Reality
Lewis model predicts properties of molecular compounds
Accounts for why certain combinations of atoms form molecules while others do not
Also shows why covalent bonds are highly directional

11. Directional vs. Non Directional
Covalently bonded atoms prefer specific orientations in space relative to one another
As a result, covalently bonded molecules have definite shape

12. Directional vs. Non Directional Continued
Ionic bonds have no electron sharing and the number of anions surrounding a cation is limited
Ionic bonds are limited by a number of factors:
Charges of ions, sizes and efficiency of the lattice packing

13. Attraction Between Covalently Bonded Atoms
Attraction between atoms is due to the sharing of electron pairs
Each bond links one pair of atoms UNLIKE ionic bonds
Fundamental units of covalent bonds are molecules

14. Intermolecular Forces vs. Intramolecular Forces
Covalently bonded molecules have weaker intermolecular forces
Covalently bonded molecules have stronger intramolecular forces

15. Melting/Boiling Points of Molecular Compounds
Molecular compounds tend to have lower melting and boiling points than ionic compounds
Why?
Molecules remain intact when the melt/boil, only INTERACTIONS between separate molecules are overcome and since molecular compounds have weaker intermolecular forces, they tend to boil/melt at lower temperatures.

16. Electronegativity and Bond Polarity
There are limitations to the Lewis theory
Shared electrons always seem to be shared equally H F

17. (shown without valence electrons)F ẟ+ ẟ- H F _
Electron Density is greater on the fluorine atom

18. H F Polar Bonds The hydrogen-fluorine bond is said to be polar
This means it has a positive (hydrogen) and negative (fluorine) pole

19. Polar Covalent Bonds
Intermediate between pure covalent bonds and ionic bonds
Most common covalent bonds between dissimilar atoms are polar covalent

20. Practice Problem
Explain the distribution of charge in the following molecule - + N H + +

21. Definitions
Homonuclear Molecules- Molecules composed of one type of element
Heteronuclear Molecules- Molecules composed of more than one type of element

22. Electronegativity
Quantified by American Chemist Linus Pauling
He compared bond energy of a heteronuclear diatomic molecule with the bond energies of its homonuclear counterpart

23. Electronegativity Continued
H₂ and F₂ bond energies =436 kJ/mol
Pauling reasoned that if HF bond was purely covalent, the bond energy of HF would be an average of the bond energies
256 kJ/mol
Bond energy turned out to be 565 kJ/mol
Pauling suggested that the additional bond energy was do to the ionic character of the bond

24. Electronegativity Trends
Electronegativity generally increases across a period and up a column
Fluorine is the most electronegative element
Facium is the least electronegative element
Electronegativity is inversely related to atomic size
The larger the atom the less ability it has to attract to electrons to itself

25. Bond Polarity, Dipole Moment and Percent Ionic Character
Degree of polarity in a chemical bond depends on the electronegativity difference (∆EN) between the two bonding atoms
The greater the difference the more polar the bond

26. Electronegativity Differences
The Effect of Electronegativity Difference on Bond Type
Electronegativity Difference Bond Type Example
Small (0-0.4) Pure Covalent Cl₂
Intermediate ( ) Polar Covalent HCl
Large (2.0+) Ionic NaCl

27. Practice Problem
Determine whether the bond formed between each pair of atoms is covalent, polar covalent or ionic
Electronegativities
Sr= F=4.0
N= Cl= 3.0
O=3.5
Sr and F
N and Cl
N and O
Ionic
Pure Covalent
Polar Covalent

28. Continuum of Bond Types

29. Dipole Moment
Polarity of a bond is quantified by the size of its dipole moment
Dipole moment- the mathematical product of the separation of the ends of a dipole and the magnitude of the charges
Dipole Moment (µ) occurs anytime there is a seperation of a positive and negative charge

30. Dipole Moment Continued
The magnitude of a dipole moment is created by separating two particles of equal but opposite charges of magnitude (q) by a distance ( r)
µ=qr
Dipole moment of a completely ionic bond can be understood by calculating the dipole moment that results from separating a proton and electron (q=1.6 X 10^-19 C) by a distance of r=130 pm you end up with 6.2 D

31. Dipole Moment Continued
Debye (D) is the unit commonly used for reporting dipole moments
1D= 3.34 X 10 ^ -30 C*m
The smaller the magnitude of the charge separation and the smaller the distance between the charges the smaller the dipole moment

32. Dipole Moments of Different Molecules
Dipole Moments of Several Molecules in the Gas Phase
Molecule ∆EN Dipole Moment
Cl₂
CIF HF
LiF

33. Percent Ionic Character
Comparing the actual dipole moment of a bond to what the dipole moment would be if the electron were completely transferred from one atom to the other allows a sense of the degree to which the electron is transferred
This quantity is called percent ionic character

34. Percent Ionic Character Continued
Measured dipole moment of bond
Dipole moment if electron were completely transferred
X 100%
A bond in which an electron is completely transferred from one atom to another would have a 100% ionic character
ALTHOUGH even the most ionic bonds do not reach this ideal

35. Practice Problem
In the gas phase, silver chloride (AgCl) has a dipole moment of What percent ionic character is closest to that of silver chloride?
10%
30%
50%
90%

36. In general, bonds greater than 50% ionic character are referred to as ionic bonds

37. Thanks!