CHEMICAL BONDING Set 2

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CHEMICAL BONDINGSet 2

• Cocaine

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Credits

• Thank you to Mr. Neil Rapp who provided the bulk
  of this powerpoint on his website
  www.chemistrygeek.com
• Other information comes from Zumdahl, Steven, and
  Susan Zumdahl. Chemistry. Boston Houghton
  Mifflin, 2003.

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Partial Ionic Character of Covalent Bonds

• Recall that when atoms with different
  electronegativities react to form molecules, the
  electrons are not shared equally.
• How can we tell the difference between an ionic
  bond and a polar covalent bond?
• There are probably no totally ionic bonds between
  discrete pairs of atoms, so we measure a percent
  ionic character of a bond
• We use charts of ?EN vs. Ionic character to
  determine the ionic-ness of a bond (usually gt
  50 ionic)
• Polyatomic ions are held together by covalent
  bonds but are ions, so the definition of a truly
  ionic bond is ambiguous
• The Big Picture Any compound that conducts an
  electric current when melted or dissolved will be
  classified as ionic.

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Bond Polarities and Dipole Moments

• We now know that molecules are never really truly
  ionic
• Other options are polar covalent (uneven sharing
  of electrons) and nonpolar covalent (even sharing
  of electrons)
• Many compounds are polar covalent, such as water
  or HF
• When hydrogen fluoride is placed in an electric
  field, the molecule orients itself in a
  particular direction
• This shows that the molecule has partially
  positive and partially negative end
• Any molecule that has a center of positive charge
  and a center of negative charge is said to be
  dipolar, or to have a dipole moment

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Showing a Dipole

• To show the charge separation in a molecule, an
  arrow is usually placed above the bond with the
  arrowhead pointing toward the negative end, and a
  cross showing the positive end
• H F

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The presence of polar bonds does not always yield
a polar molecule! CO2 has polar bonds, but they
cancel each other out. Water has polar bonds that
do not cancel. More on polar/nonpolar molecules
later
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The Covalent Chemical Bond A Model

• Before we go any further into covalent bonding,
  it will be helpful to summarize some points made
  so far
• What is a chemical bond?
• Chemical bonds can be viewed as forces that cause
  a group of atoms to behave as a unit.
• Why do chemical bonds occur?
• There is no principle of nature that states that
  bonds are favored or disfavored. Bonds are
  neither inherently good nor inherently bad as
  far as nature is concerned bonds result from the
  tendency of a system to seek its lowest possible
  energy.

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Models An Overview

• Bonding is a model proposed to explain molecular
  stability
• _____________________________________
• Fundamental Properties of Models
• Models are human inventions, always based on an
  incomplete understanding of how nature works. A
  model does not equal reality.
• Models are often wrong. This property derives
  from the first property. Models are based on
  speculation and are always oversimplifications.
• Models tend to become more complicated as they
  age. As flaws are discovered in our models, we
  patch them and thus add more detail.

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Fundamental Properties of Models Contd

• 4) It is very important to understand the
  assumptions inherent in a particular model before
  you use it to interpret observations or to make
  predictions. Simple models usually involve very
  restrictive assumptions and can be expected to
  yield only qualitative information. Asking for a
  sophisticated explanation from a simple model is
  like expecting to get an accurate mass for a
  diamond using a bathroom scale.
• EXAMPLE The Aufbau principle accurately predicts
  the order for electron configurations for most
  elements, but chromium and copper are still
  exceptions! The complex interactions in these two
  elements are not accounted for in the simple
  electron configuration model however this does
  not mean we should discard the model that is
  useful for most other atoms!

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Fundamental Properties of Models ContdAgain

• 5) When a model is wrong, we often learn much
  more than when it is right. If a model makes a
  wrong prediction, it usually means we do not
  understand some fundamental characteristics of
  nature. We often learn by making mistakes. (Try
  to remember this when you get back your next
  chemistry test)

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Homework Questions

• 1) Some plant fertilizer compounds are (NH4)2SO4,
  Ca3(PO4)2, K2O, P2O5, and KCl. Which of these
  compounds contain both ionic and covalent bonds?
• 2) What is meant by a chemical bond? Why do atoms
  form bonds with each other? Why do some elements
  exist as molecules in nature instead of as free
  atoms?
• 3) Why are some bonds ionic and some covalent?