CHEM 101 Review

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CHEM 101 Review
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Covalent Bonding

• Three basic principles of bonding
• bonds form when electrons are exchanged or shared
• only valence (outer) electrons are used
• in this process each atom tries to achieve a
  noble gas electron configuration
• this is also called valence bond theory
• Covalent Bonds
• formed from non-metallic elements
• formed by sharing valence electrons
• Example H H ---gt H2 or HH or H-H

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Atomic Orbital Overlap
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Learning Concepts

• Concepts are a single word or phrase that
  represents a complex idea or it is a
  definition.
• A definition contains the essential
  characteristics of a concept.
• Variable characteristics not all examples of a
  concept have these characteristics.
• Examples and non-examples clarify concepts.

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Concept of Covalent Bonds

• Definition The attractive forces holding two
  atoms together resulting from the sharing of a
  pair of electrons.
• Essential Characteristics
• attractive force holding atoms together
• formed by sharing valence electrons
• Variable Characteristics
• formed primarily between non-metallic elements
• Can be single, double or triple bonds
• Sharing of electrons may not be equal (polar
  covalent)

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Examples and Nonexamples of Covalent Bonds
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How to Learn Concepts

• Read and learn its formal definition by writing
  it in your own words
• Increase your sensitivity to the exact meaning of
  chemical words
• Determine its essential and variable
  characteristics
• Look for many examples and nonexamples
• Use outlines or diagrams to understand its
  characteristics and its relationship to other
  concepts

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Common Difficulties in Learning Concepts

• Many concepts are taught in one class
• Concepts are not always presented in detail and
  without explicit links to related concepts
• Concepts are taught simultaneously with other
  types of content
• Concepts are extended in later class periods

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Testing Concepts

• Multiple Choice identify a concept word, a
  definition, essential characteristics or examples
• Short Answer supply a definition for a concept
  and/or give an example
• Matching link a concept to an example,
  discriminate between related concepts
• T/F identify correct examples or characteristics

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Examples of Concept Qs

• Multiple Choice Which of the following has
  covalent bonds?
• NaCl (B) PCl3 (C) Mg
• T/F The compound NaCl contains covalent bonds.
  (regardless of whether you answer is T or F,
  explain why).

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Lewis Symbols

• The number of valence electrons for an atom can
  be obtained by using the second or only digit of
  elements Group number.
• Electron-dot symbols (Lewis symbols) are used to
  represent the valence electrons around an atom.
• One dot for each valence electron is placed
  around the element symbol at the four compass
  points.
• All elements in a Group have the same
  electron-dot structures. They are isoelectronic.

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Lewis Symbols
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Lewis Structures of Compounds

• Example Cl2 gt Ne3s23p5 Ne3s23p5
• Only the unpaired electrons in the unfilled 3p
  orbitals creates a bond.
• The electrons not used in bonding are called
  lone pairs.

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Lewis Structures of Compounds

• Example O2 gt He2s22p4 He2s22p4
• Shared pairs between the unfilled 2p orbitals
  creates two bonds.
• More than one bond between two atoms is possible
  if this completes an octet.Multiple bonds are
  common between period two elements, particularly
  C, N and O.

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Drawing Lewis Structures

• 1. Count the total number of valence electrons in
  the molecule. To write the electron-dot structure
  of ions you must add or subtract electrons from
  the total, corresponding to the charge on the
  ion. Electrons are added to anion and subtracted
  from cations.
• 2. Identify the center atom using one of the
  following rules
• a. The element of which there is the fewest in
  the formula may be the center atom.
• b. The element closest to the center of its
  period may be the center atom since it can form
  the largest number of bonds.
• c. Elements of the same group but with a
  higher atomic number tend to be center atoms.
• 3. Write the element symbol of the center atom
  and place the outer atom element symbols around
  it. If the molecule has a chain or ring, write
  the element symbols for all the chain or ring
  atoms in a linear or circular pattern. Chains
  and rings are the most common for compounds
  containing C and H in the same formula or for
  compounds with H and more than one B, Si, N, O, P
  or S.
• 4. Draw single bonds from the center atom(s) to
  each of the outer atoms. Subtract the number of
  electrons used to create the bonds (2 per bond)
  from the total. Note that because its valence
  (number of bonds) is 1, hydrogen is always an
  outer atom.
• 5. If there are any electrons left, give the
  outer atoms enough electron pairs to fill their
  octets. Subtract the electrons used in this step
  from the total. Note that hydrogen requires only
  two electrons to complete its noble gas shell and
  never gets an octet.
• 6. If there are still electrons left, give the
  center atom the remaining electrons to help
  complete its octet.
• 7. If the center atom still does not have an
  octet at this point, create multiple bonds
  between it and the outer atom(s) by sharing one
  of the lone pairs on the outer atoms with the
  atom at the center.

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What are Chemical Rules?

• A rule is a generalization that summarizes
  chemical behavior.
• Examples scientific laws, chemical equations,
  math formulas, determination of significant
  figures, all nitrates are soluble, Lewis
  structures, etc.
• Rules free you from learning many related facts
  but must be applied under specific circumstances.

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To Learn A Rule

• Learn its underlying facts and concepts.
• Try to restate it in your own words.
• Learn to identify when a rule is applied.
• Identify what the rule accomplishes.
• Identify exceptions to the rule.

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Other Strategies for Learning Rules

• Some rules can only be memorized.
• Link rules to concepts to create a frame of
  reference for applying the rule.
• Draw diagrams or flowcharts to summarize rules.
• Develop your own chemical rules when faced with
  tables or graphs.

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Testing Rules

• Questions which ask you to apply rules are the
  most common.
• What is the Lewis structure of N2?
• What Is the IUPAC name of (CH3)2-CH-OH?
• Problems Applying Rules
• Questions that test exceptions
• What is the Lewis structure of PCl5?
• Choosing among related rules
• What Is the name of (CH3)2-CH-OH

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Learning Check

• Draw the Lewis Structures of the following
  molecules
• A. CO3-2
• B. PCl3
• C. SiH4

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3-D Shapes of Molecules

• Determined by the positions of the valence
  electrons around the central atom
• The 3-D shape is a result of repulsions of bonded
  pairs and lone pairs of electrons
• Example H2O or H-O-H is bent and NOT linear
• You must use VSEPR theory (valence-shell-electron-
  pair repulsion) to determine 3-D shape of
  molecules

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3-D Shapes of Molecules
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VSEPR Electron Pair Geometry

• Valence Shell Electrons wish to minimize the
  Electron Pair Repulsions between them.
• The repulsions between independent regions of
  electron density around an atom produce
  electron pair geometries.
• Electron pair geometries include linear(2),
  trigonal planar(3), tetrahedral(4).
• The electron pair geometries determine the
  approximate bond angles for a structure.

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VSEPR Electron Pair Geometry
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Learning Facts

• Facts are true statements names, properties,
  measured values, etc.
• Example water is a colorless liquid that freezes
  at 0.0 C.
• Facts are often presented as tables of data
• Facts can only be memorized!

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Memorization for Learning

• You have a short and long term memory.
• Information is first placed in the short term
  memory where it is quickly lost.
• Memorization for learning is actually a
  transfer from short to long term memory.
• In both types of memory the links fade over time
  and must be reinforced.

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Memorization Techniques

• Use as many senses as possible.
• Look for patterns that organize the facts.
• Group items into chunks that fit into your short
  term memory.
• Practice items in random order with a friend.
• Spread the memorization task over time.

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Testing Facts

• Straight Recall
• 1. What is the molecular geometry of water?
• Recognition of Facts of Factual Errors
• 2. Which is the molecular geometry of water?
• A) trigonal B) linear C) angular
• Use of Memorized Information in Problems
• 3. Determine the molecular geometry of water.

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VSEPR Molecular Geometry

• A single bond, a multiple bond and a lone pair
  each count as one independent region of electron
  density.
• Electron pair geometries are further broken down
  into molecular geometry subclasses.
• The names for these subclasses is determined by
  the 3-D arrangement of atoms.

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VSEPR Molecular Geometry
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CHEM 101 Review
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Using the VSEPR Model

• 1. Draw the Lewis structure.
• 2. Count the number of each type of electron
  region around the center atom. Note that
• a. Each single, double or triple bond counts
  as only one region (a bonding region).
• b. Each lone pair counts as one region, an
  unshared region
• 3. Use the table provided to determine the
  molecular geometry.
• What is the electron pair and molecular geometry
  of water?

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Table of Molecular Geometry
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Learning Check

• Determine the electron pair and molecular
  geometry of each of the following molecules
• A. CO3-2
• 1) trigonal planar 2) pyramidal 3)
  angular
• B. PCl3
• 1) trigonal planar 2) pyramidal 3) angular
  
• C. SiH4
• 1) tetrahedral 2) pyramidal 3) angular

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Solving Problems

• Descriptions of different problem types for the
  assigned exercises will be provided on Learning
  Objective sheets for each chapter.
• Combines related facts, concepts and rules.
• Classify generic problem strategies presented as
  examples in the text, use them to classify others
  and practice them.
• What is the name of the problem?
• What mathematical formula is used?
• What type of answer is needed?
• What is the rule or concept linked to it?

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Solving Generic Problems

• Identifying each discrete step in the solution.
• Determining which are the "decision" and
  "operation" steps
• Operation steps are steps in which you follow
  explicit directions
• Decision steps require that you make a choice.
• Be sure you can do the operation steps correctly.
• Determine the rules and/or concepts required to
  make the correct choice at the decision steps.
• Think about the answer you came up with-does it
  make sense?

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Examples of Generic Problems

• What is the electron pair and molecular geometry
  of water?
• Are the bonds in water polar and what type of
  secondary forces are present?
• Draw the different structural formulas of
  pentane.
• What are the possible products from a reaction of
  n-butane and chlorine, Cl2?

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Polar Covalent Bonds

• Occurs when electrons are not shared equally.
• Identified by comparing the electronegativity
  of the two atoms.
• The atom with the higher electronegativity
  attracts the electron pair more strongly and gets
  a ?- charge.
• Fluorine is the most electronegative element.
• Even with polar covalent bonds a molecule like
  CO2 is nonpolar due to the molecules 3D
  structure the two polar bonds are 180 apart.

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Polar versus Nonpolar
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How to Determine Molecular Polarity

• Determine whether each bond within the molecule
  is polar or nonpolar. A bond is polar if the
  electronegativity difference is larger than 0.5.
• Examine a diagram of the structure to determine
  if all the polar bonds and all the lone pairs are
  symmetrically placed around the center atom and
  balance each other exactly.
• If they are totally symmetric, the molecule is
  non-polar. If they are not totally symmetric, the
  molecule is polar

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Polar versus Nonpolar
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Polar versus Nonpolar
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Learning Check

• Determine whether the bonds in these molecules
  are polar or non polar and if the molecule has an
  overall dipole
• A. CO3-2
• B. PCl3
• C. SiH4