1. State University of New York at Brockport
Chemical Bonds
Mark P. Heitz
State University of New York at Brockport
© 2005, Prentice Hall, Inc.

2. Chemical Bonds: Electrostatics
Forces called chemical bonds hold atoms together in molecules and keep ions fixed in ionic solids
Chemical bonds are electrical forces that balance attraction and repulsion between electrically charged particles
EOS
Chapter 9: Chemical Bonds

3. Chapter 9: Chemical Bonds
Energy of Interaction
Scientists can determine the internuclear distances that correspond to the lowest energy states of molecules
EOS
– Quantum mechanical calculations can then be used to develop theoretical models
Chapter 9: Chemical Bonds

4. The Lewis Theory of Chemical Bonding: An Overview
Electrons, particularly valence electrons, play a fundamental role in chemical bonding
EOS
In losing, gaining, or sharing electrons to form chemical bonds, atoms tend to acquire the electron configurations of noble gases
Chapter 9: Chemical Bonds

5. Chemical Bonding: An Overview
When metals and nonmetals combine, valence electrons usually are transferred from the metal to the nonmetal atoms giving rise to ionic bonds
EOS
In combinations involving only nonmetals, one or more pairs of valence electrons are shared between the bonded atoms, producing covalent bonds
Chapter 9: Chemical Bonds

6. Chapter 9: Chemical Bonds
Lewis Symbols
Valence electrons are shown by dots around the element symbol
EOS
Use rules of electron configurations when forming dot structures …
e.g., electrons remain unpaired if possible
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7. Lewis Structures of Simple Molecules
A Lewis structure shows the bonded atoms with the electron configuration of a noble gas; that is, the atoms obey the octet rule
EOS
By double-counting the shared electrons in a Lewis structure, each atom appears to have a noble gas configuration
Chapter 9: Chemical Bonds

8. Chapter 9: Chemical Bonds
Lewis Structures
The shared pairs of electrons in a molecule are called bonding pairs
In common practice, the bonding pair is represented by a dash (—): e.g., H—Cl
EOS
The other electron pairs, which are not shared, are called nonbonding pairs, or lone pairs
Chapter 9: Chemical Bonds

9. Second Period Compounds
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Chapter 9: Chemical Bonds

10. Coordinate Covalent Bonds
In some cases, one atom provides both electrons of the shared pair to form a bond called a coordinate covalent bond H+
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Chapter 9: Chemical Bonds

11. Multiple Covalent Bonds
The covalent bond in which one pair of electrons is shared is called a single bond
e.g., H : Cl or H—Cl
Double bonds have two shared pairs of electrons
EOS
Triple bonds have three shared pairs of electrons
Chapter 9: Chemical Bonds

12. Chapter 9: Chemical Bonds
Electronegativity
Electronegativity (EN) is a measure of the ability of an atom to attract bonding electrons to itself
EOS
The greater the electronegativity of an atom in a molecule, the more strongly it attracts the electrons in a covalent bond
Chapter 9: Chemical Bonds

13. Pauling’s Electronegativities
Electronegativity Illustrated
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Chapter 9: Chemical Bonds

14. Electronegativity Difference and Bond Type
Two identical atoms have the same electronegativity and share a bonding electron pair equally. This is called a nonpolar covalent bond
Example: chlorine gas
EOS
All homonuclear diatomic molecules have nonpolar covalent bonds:
H2, N2, O2, F2, Cl2, Br2, I2
Chapter 9: Chemical Bonds

15. Electronegativity Difference and Bond Type
In covalent bonds between atoms with somewhat larger electronegativity differences, electron pairs are shared unequally. This is called a polar covalent bond
Example: hydrogen chloride gas, HCl
The electrons are drawn closer to the atom of higher electronegativity, Cl
EOS
Chapter 9: Chemical Bonds

16. Electronegativity Difference and Bond Type
With still larger differences in electronegativity, electrons may be completely transferred from metal to nonmetal atoms to form ionic bonds
Example: sodium chloride, NaCl
EOS
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17. Electronegativity Differences
EOS
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18. Electron Distributions and Covalent Bonds
EOS
Asymmetric
Distribution
Symmetric distribution
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19. Writing Lewis Structures
Hydrogen atoms are terminal atoms (bonded to only one other atom)
EOS
The central atom of a structure usually has the lowest electronegativity and the terminal atoms (except H) generally have higher electronegativities
Chapter 9: Chemical Bonds

20. Writing Lewis Structures
In oxoacids, hydrogen atoms are usually bonded to oxygen atoms
EOS
Molecules and polyatomic ions usually have compact, symmetrical structures
Chapter 9: Chemical Bonds

21. Writing Lewis Structures
Determine the total number of valence electrons
Write a plausible skeletal structure and connect the atoms by single dashes (covalent bonds)
Place pairs of electrons as lone pairs around the terminal atoms to give each terminal atom (except H) an octet
Assign any remaining electrons as lone pairs around the central atom
If necessary, move one or more lone pairs of electrons from a terminal atom to form a multiple bond to the central atom.
EOS
Chapter 9: Chemical Bonds

22. Resonance: Delocalized Bonding
Resonance theory states that whenever a molecule or ion can be represented by two or more plausible Lewis structures that differ only in the distribution of electrons, the true structure is a composite, or hybrid, of them
Resonance structures
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Chapter 9: Chemical Bonds

23. Molecules that Don’t Follow the Octet Rule
Molecules with an odd number of valence electrons have at least one of them unpaired and are called free radicals
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Chapter 9: Chemical Bonds

24. Molecules that Don’t Follow the Octet Rule
Some molecules have incomplete octets. These are usually compounds of Be, B, and Al, generally have some unusual bonding characteristics, and are often quite reactive
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25. Molecules that Don’t Follow the Octet Rule
Some compounds have expanded valence shells, which means that the central atom has more than eight electrons around it
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26. Molecules that Don’t Follow the Octet Rule
An expanded valence shell may also need to accommodate lone-pair electrons as well as bonding pairs
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27. Chapter 9: Chemical Bonds
Bond Lengths
Bond order indicates whether a covalent bond is single (b.o. = 1), double (b.o. = 2), or triple (b.o. = 3)
Bond length is the distance (in pm) between the nuclei of two atoms joined by a covalent bond
EOS
Bond length depends on the particular atoms in the bond and on the bond order
BondFormation
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28. Representative Bond Lengths
EOS
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29. Chapter 9: Chemical Bonds
Bond Energies (BE)
Bond-dissociation energy (D) is the quantity of energy required to break one mole of covalent bonds between atoms in a molecule in the gas phase
EOS
An average bond energy is the average of the bond-dissociation energies for a number of different molecules containing the particular bond
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30. Calculations Using BEs
The sum of the enthalpy changes for breaking the old bonds and forming the new bonds is the enthalpy change for the reaction
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31. Lewis Structure and Hydrocarbons
For saturated hydrocarbons the Lewis structure = molecular structure
Hydrocarbons with double or triple bonds between carbon atoms are called unsaturated hydrocarbons
alkenes
EOS
alkynes
Chapter 9: Chemical Bonds

32. Lewis Structure and Hydrocarbons
alkenes
EOS
alkynes
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33. Chapter 9: Chemical Bonds
Polymers
Polymers are compounds in which many identical molecules have been joined together
EOS
Nylon
Chapter 9: Chemical Bonds