Introductory Chemistry, 2nd Edition Nivaldo Tro

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Introductory Chemistry, 2nd EditionNivaldo Tro
Chapter 10 Chemical Bonding
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Bonding Theories

• Chemical bonding describes way atoms attach to
  make compounds
• Understanding of bonding allows chemists to
• predict shapes of molecules and properties of
  substances
• design and build molecules with particular sets
  of chemical and physical properties

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Lewis Symbols of Atoms

• Also known as electron dot symbols
• Symbol of element represents nucleus and inner
  electrons
• Valence electrons are dots in imaginary 4 sides
  around symbol
• put one electron on each side first, then pair
• Elements in same group have same number of
  valence electrons therefore their Lewis dot
  symbols will look alike

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Lewis Bonding Theory

• Atoms bond because it results in a more stable
  electron configuration
• Atoms bond together by
• transferring electrons (ionic bond) or
• sharing electrons (covalent bond) so that all
  atoms obtain an outer shell with 8 electrons
  Octet Rule
• there are exceptions to this rule the key to
  remember is to try to get a valence electron
  configuration like a noble gas
• Which noble gas is an exception?

He, why?
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Lewis Symbols of Ions

• Cations have Lewis symbols without valence
  electrons
• Lost in the cation formation
• Anions have Lewis symbols with 8 valence
  electrons
• Electrons gained in the formation of the anion

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What determines which type of bond is formed?

• Electronegativity attraction for electrons
• ionic bond two atoms have a large difference in
  electronegativity.
• covalent bond atoms have similar
  electronegativities

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Ionic bond very different electronegativities
Metals give up electrons easily become
Non-metals like to gain electrons become -
Metal() and non-metal(-) ionic bond will be
formed (not covalent).
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Covalent bond similar electronegativities
2 non-metals covalent bond will be formed (not
ionic).
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Ionic Bonds

• Metal transfers electron(s) to nonmetal
• Metal loses electrons to form cation
• Nonmetal gains electrons to form anion
• Ionic bond results from to - attraction
• larger charge stronger attraction
• smaller ion stronger attraction
• Lewis Theory allows us to predict the correct
  formulas of ionic compounds

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Example ionic bond
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Example ionic bond
Na
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Charges of ions

• If an atom Its charge is now
• loses an e- 1
• gains an e- -1
• loses 2 e- 2
• gains 2 e- -2

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Draw Lewis structure for compound formed between
Mg and O.
-2
2
x
Mg
O
x
magnesium oxide
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Draw Lewis structures for the compounds formed by

• sodium sulfur
• calcium bromine

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Name these!
sodium sulfur Na2S calcium
bromine CaBr2
sodium sulfide
calcium bromide
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Covalent Bonds sharing of electrons

• Type of bond for two nonmetal atoms
• Attraction for electrons (electronegativity) is
  similar for both atoms involved

H2
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Covalent Bonds

• Formed between two nonmetals
• Atoms bonded together to form molecules
• strong attraction
• Sharing pairs of electrons to attain octets
• Molecules generally weakly attracted to each
  other
• observed physical properties of molecular
  substance due to these attractions

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Cl2
Cl - Cl
Shared electrons are always in pairs. single bond
one shared pair of electrons
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Single Covalent Bonds

• Two atoms share one pair of electrons
• 2 electrons
• One atom may have more than one single bond

H
H
O



H
H

O

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Write the Lewis structure for CH4
How many single bonds does it have?
4
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Double Covalent Bond

• Two atoms sharing two pairs of electrons
• 4 electrons
• Shorter and stronger than single bond

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Triple Covalent Bond

• Two atoms sharing 3 pairs of electrons
• 6 electrons
• Shorter and stronger than single or double bond

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Ionic or Covalent? and Name?

• NaCl
• MgBr
• PH3
• NO
• KNO3
• Mg(OH)2
• NH4Br
• H2O

ionic sodium chloride
ionic magnesium bromide
covalent phosphorus trihydride
covalent nitrogen monoxide
ionic potassium nitrate
ionic magnesium hydroxide
ionic ammonium bromide
covalent dihydrogen monoxide
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Ionic or Covalent? Write the formula

• ammonium sulfide
• dihydrogen dioxide
• potassium sulfate
• copper II nitrate

ionic (NH4)2S
covalent H2O2
ionic K2SO4
ionic Cu(NO3)2
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Exceptions to the Octet Rule

• H Li, lose one electron to form cation
• Li now has electron configuration like He
• H can also share or gain one electron to have
  configuration like He
• Be shares 2 electrons to form two single bonds
• B shares 3 electrons to form three single bonds

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Exceptions to the Octet Rule

• Expanded octets for elements in Period 3 or below
  
• using empty valence d orbitals
• Some molecules have odd numbers of electrons
• NO

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Molecular Geometry

• Molecules are 3-dimensional objects
• Describe shape of a molecule with terms that
  relate to geometric figures
• These geometric figures have characteristic
  corners that indicate positions of surrounding
  atoms with central atom in center of the figure
• The geometric figures also have characteristic
  angles that we call bond angles

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Some Geometric Figures

• Linear
• 2 atoms on opposite sides of central atom
• 180 bond angles
• Trigonal Planar
• 3 atoms form a triangle around the central atom
• Planar, 120 bond angles
• Tetrahedral
• 4 surrounding atoms form a tetrahedron around
  central atom
• 109.5 bond angles

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Predicting Molecular Shape VSEPR
Valence Shell Electron Pair Repulsion Theory
the shape of a molecule can be predicted by
assuming that the electron pairs repel each other.
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Tetrahedral Distribution of Four Electron Pairs
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Central Atom with 4 electron pairs bonded to 4
atoms in corners
CH4 molecule is tetrahedral
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What would be the shape of these molecules?
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• pyramidal
• bent

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When there are only two atoms, the only
molecular shape possible is
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Linear Shapes

• Linear
• 2 areas of electrons around the central atom,
  both bonding
• Or two atom molecule
• 180 Bond Angles

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Bond Polarity

• Unequal sharing of electrons between unlike atoms
• one atom pulls electrons in the bond closer to
  its side
• one end of the bond has larger electron density
  than the other
• The end with the larger electron density gets a
  partial negative charge and the end that is
  electron deficient gets a partial positive charge

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Electronegativity

• Attraction an atom has for bonding electrons in
  covalent bond
• Increases across period (left to right)
• Decreases down group (top to bottom)
• Larger difference in electronegativity, more
  polar the bond
• negative end toward more electronegative atom

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Electronegativity
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Electronegativity
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Electronegativity Bond Polarity

• Nonpolar covalent difference in
  electronegativity between bonded atoms is 0 to
  0.3
• Polar covalent difference in electronegativity
  between bonded atoms is 0.4 to 1.9
• Ionic difference in electronegativity between
  bonded atoms larger than or equal to 2.0

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Bond Polarity
3.0-3.0 0.0
4.0-2.1 1.9
3.0-0.9 2.1
covalent
ionic
non polar
polar
0
0.4
2.0
4.0
Electronegativity Difference
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Polarity of Molecules

• For a molecule to be polar it must
• have polar bonds
• electronegativity difference of 0.4 1.9
• bond dipole moments - measured
• have an unsymmetrical shape
• Polarity affects the intermolecular forces of
  attraction

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Polarity of Molecules
polar bonds, but nonpolar molecule because pulls
cancel
polar bonds, and unsymmetrical shape causes
molecule to be polar
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Polar or Nonpolar Molecule?
CH2Cl2 m 2.0 D
CCl4 m 0.0 D