CHEM 130 Lecture 12: Lecture 12 - Chapter 3 cont.
3 Oct 2018
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Lecture 12: Chapter 3 cont.
• Partial Ionic Character of Covalent Bonds
o Formula for Percent Ionic Character of a Bond
• No bond between discrete pairs of ions is 100% ionic
• Percent ionic character of a bond:
▪ (Measured dipole moment of X-Y/Calculated dipole moment of X+Y-) x 100%
• Relationship Between Ionic Character of a Covalent Bond and Electronegativity Difference of Bonded
Atoms
o Practical definition of ionic compounds: Any compound that conducts an electric current when
melted
• Fundamental properties of Models:
o Human interventions based on incomplete understanding of nature
o Oversimplifications, often wrong
o Become more complicated as they age
o Rely on underlying assumptions which limit the scope of conclusions we can make
o Wrong models often teach us more than correct models
• The covalent bond is a model
o Bonding is a model for explaining molecular stability
o Provides method for dividing up the energy released when a molecule is formed
• Covalent bond energies and chemical reactions
o Bond energy: the energy required to break a bond
o We use "average bond energy" because of molecular environment sensitivity
• Types of bonds:
o Single bond:
• Sharing one pair of electrons
o Double bond:
• Sharing 2 pairs of electrons
o Triple bond:
• Sharing 3 pairs of electrons
o Bond length decreases as number of shared electron pairs increases
• Bond energy increases with # of shared electron pairs
• Bond energies:
o How to calculate bond energies:
• Add energy to break bonds (+)
• Release energy to make bonds (-)
• Overall deltaE (change in energy)
• DeltaE = Sum(n x Dbonds broken) - Sum(n x Dbonds formed)
▪ = energy required - energy released
• The localized electron bonding model
o Localized electron (LE) model
• A molecule is composed of atoms bound together by sharing pairs of electrons using the
valence atomic orbitals of the bound atoms
• Electrons are localized to one of the atoms or the space in between
▪ Electron pairs localized on specific atom: lone pairs
▪ Electron pairs localized to space between: bonding pairs
o Parts of LE Model - we will explore one by ne
