Covalent Bonding

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Covalent Bonding Molecular Geometry

• Mrs. Daniels
• .2 Chemistry
• November 2006

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Rules for Lewis Dot Structures

• 1. Count and add up total number of valence
  electrons
• 2. Choose your central atom (usually the first
  element in the formulabut never hydrogen) and
  arrange remaining atoms around the central atom
• 3. Draw two electrons between each atom
  (representing the electron pair that they are
  sharing)
• 4. Place remaining electrons around the atoms
  satisfying their outer shells (start with outer
  atomsin case you run out!)

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Rules for Lewis Dot Structures

• What if you run out of electrons?
• Can you have a double bond or a triple bond?
• Can atoms share more than one pair?
• How many pairs are they sharing in a double bond?
• 2
• How many pairs are they sharing in a triple bond?
• 3

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Lewis Dot with Multiple Bonds

• Determine how many of each atom you will need to
  form a molecule. Then draw the Lewis Dot
  structures for each of the following
• H Cl --gt
• P Cl --gt
• H S --gt
• Cl Cl --gt

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Lewis Dot with Multiple Bonds

• Determine how many of each atom you will need to
  form a molecule. Then draw the Lewis Dot
  structures for each of the following
• H Cl --gt HCl
• P Cl --gt PCl3
• H S --gt H2S
• Cl Cl --gt Cl2
• Which of the above have polar covalent bonds?
  What is unique about Cl2?

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Diatomic Molecules

• First of all, weve been using the term
  molecule
• What is a molecule?
• A covalently bonded compound
• A diatomic molecule is one that is made up of two
  of the same atoms
• Since they are the same atom and have equal
  electronegativities, they are 100 non polar (and
  are the only molecules who are)

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Back to Lewis Dot Structures

• For all of those math minded people, lets look
  at an easy way to determine the number of bonds
  that will be formed
• Determine the of electrons needed to satisfy
  each atoms outer shell (most want 8)
• Then add up the of electrons available
• Subtract the two and the answer will tell you how
  many electrons must be SHARED N - A
  Shared (S)

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• Ex. Carbon Dioxide CO2
• Needed Each atom wants 8, so 8 x 3
  24
• Available Carbon has 4 each oxygen has 6, so
  4 6(2) 16
• 24-16 8 electrons must be shared
• Put Carbon in the center of your board
• Draw an oxygen on either side and draw the C
  sharing half of the 8 with each O
• OCO Then, fill in the rest of your
• available dots

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• Now try SO3
• N 32
• A 24
• Shared 8 electrons
• What will be your central atom?
• Now draw 3 oxygens around it and place your
  shared electrons
• What type of bonds do you have?

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Resonance

• Does it matter which Sulfur to oxygen bond you
  drew as a double bond?
• No, it could have been on any of them
• These are called resonance structures

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Polyatomic ion revisit

• How well do you know your polyatomic ions?
• Now, youre going to draw Lewis dot structures of
  them
• How does this differ from what youve been doing?
  
• The only difference is in your number of
  available electrons (add in those that youve
  gained or subtract those you lost)

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Covalent Bonding

• According to electronegativity, what determines
  the type of bond that will form?
• 2 ionic
• 1.7 or less covalent
• 1.7 - 2 two nonmetals covalent
• metal and nonmetal ionic

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Covalent Bonding

• What is the difference between ionic and covalent
  bonds?
• Ionic bonds involve an actual transfer of
  electrons from one atom to another, thus creating
  charged particles or ions
• Covalent bonds do NOT involve a transferinstead
  atoms SHARE electrons

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Covalent Bonding

• Have you ever shared something with someone?
• Was it perfectly equal sharing?
• Sometimes atoms dont share equally
• One atom will have possession of the electron
  more of the time than the other
• This situation is called bond polarity

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Polar Covalent

• If the difference in electronegativites is 0-0.5
  then the bond is nonpolar covalent
• In other words, they share pretty equally
• If the difference in electronegativities is
  0.5-1.69 then the bond is polar
• The more electronegative atom has a stronger pull
  and will have the electron more often than the
  less electronegative atom

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Covalent Bonds - Polar or Not?

• Use the table of electronegativities to determine
  whether the following bonds are polar or
  non-polar covalent
• O with O
• P with H
• Se with F
• N with O
• C with H
• S with O

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Money Activity

• Okay, youve done it once with ionic bondingnow
  lets do it with covalent.
• This time when you bond, youll have to share
  your valence electrons. In order to demonstrate
  sharing, you must both have your hands on the
  electrons at the same time.

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Molecular Geometries

• VSEPR - stands for Valence Shell Electron Pair
  Repulsion
• What charge is an electron?
• So what do negative charges do with each other?
• This model just puts that into practicethe
  electron pairs are going to repel each other and
  be as far apart as possible

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• http//www.shef.ac.uk/chemistry/vsepr

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Molecular Geometries

• Recall that when you draw Lewis Dot structures,
  the first atom written is usually the central
  atom
• We refer to this as A
• The atoms attached to the central atom directly
  are referred to as X
• Any other electrons that are unshared around the
  central atom are called E

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Molecular Geometries

• Add these to your geometric shape table
• A2 and AX2 are linear
• AX3 are trigonal planar
• AX2E are bent
• AX4 are tetrahedral
• AX3E are trigonal pyramidal
• AX2E2 are bent

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Lets Try a Few

• NH3 -
• A one central atom (Nitrogen)
• X three atoms attached to central atom
• E one lone pair of electrons (unshared)
• So, its AXE designation would be AX3E

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• H2O -
• Oxygen is the central atom (A)
• Two hydrogens are attached to central atom (X2)
• Two unshared (lone) pairs (E2)
• So, AX2E2

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• Try CCl4
• There are no electrons unshared on the central
  atom
• AX4
• Try NCl3
• AX3E

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Polar and Nonpolar molecules

• We already know how to determine whether or not a
  bond is polar or non-polar
• Recallwhat is polar?
• Unequal sharing of electrons
• If one atom has a stronger pull on the electrons,
  they will have a partial negative charge (the
  other atom will have a partial positive charge)

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Polar and Nonpolar molecules

• Draw the Lewis dot structure for BCl3
• Is the B - Cl bond polar or nonpolar?
• The electronegativity of B is 2.01
• The electronegativity of Cl is 3.00
• The difference is 0.99 and is therefore polar
• Is the molecule polar?

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Polar and Nonpolar molecules

• What is the molecular geometry of BCl3?
• Draw the Lewis dot structure
• Trigonal planar is the molecular geometry
• Draw arrows showing the pull of electrons and the
  partial charges?
• Do they cancel each other out?
• Yes, so the molecule is nonpolar (even though it
  has polar bonds within it)

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Polar and Nonpolar molecules

• Lets look at another example
• Ex. In NH3, the N - H bond is polar
• The electronegativity of N is 3.07
• The electronegativity of H is 2.20
• The difference is 0.87, which is greater than 0.5
  and is therefore polar
• So, is the molecule polar or nonpolar?

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Polar and Nonpolar molecules

• Lets think about the molecular geometry
• Draw the Lewis dot structure for NH3 on your
  white board
• There is a lone pair of electrons off the N
• Draw an arrow showing the partial charges in each
  of the polar bonds
• Is there equal pulling in opposite directions?
• Do the charges cancel each other out?

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Polar and Nonpolar molecules

• No, there is an overall partial negative charge
  at the top and an overall partial positive charge
  at the bottom
• Therefore, the molecule is polar

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Polar and Nonpolar Molecules

• So
• Can you have a nonpolar molecule that has polar
  bonds in it?
• YES
• But can you have a polar molecule that has only
  nonpolar bonds?
• NO
• There is NO pulling of electrons if they are non
  polar

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• Lets go back and review atomic structure
• Where are the protons?
• Where are the neutrons?
• Where are the electrons?
• Do we know at any given moment EXACTLY where the
  electrons are in an atom?
• NO, we know where the highest probability is for
  them to be found (these are the orbitals s, p, d,
  and f)
• What are the electrons doing in these orbitals?

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• Are the electrons in a molecule doing anything
  differently?
• No, they may be SHARED between two atoms in a
  molecule in a covalent bond, but they are still
  buzzing about
• Is it possible for a majority of the electrons in
  a molecule to be found on the same side of the
  molecule?
• Yes, what would that do (temporarily) to that
  side of the molecule?

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Intermolecular Forces

• Part of the molecule can be partially negative
  for a short time while the electrons are on that
  side
• The other side would then be partially positive
• Even though they are weak charges, what will
  opposite charges do?
• Can a partial positive of one molecule be
  attracted to a partial negative on another?

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Intermolecular Forces

• When this occurs, it is called induced dipole
  (a.k.a. London dispersion forces)
• Dipole means that there are two poles ( and -)
• Where else have we seen dipoles?
• In polar covalent bonds between two atoms
• When the pole of one molecule is attracted to
  the - pole of another molecule, the attraction or
  force is called a dipole-dipole

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Intermolecular Forces

• Hydrogen bonding is the third type of
  intermolecular force (really it is a very strong
  version of a dipole-dipole)
• When an atom of hydrogen bonds to fluorine,
  oxygen, or nitrogen (FON) there is a large
  difference in electronegativity
• The electrons spend more time around the
  electronegative atom and less time around the
  less electronegative atom

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Hydrogen Bonding

• The atom who has possession of the electron the
  majority of the time takes on a PARTIAL negative
  charge
• (remember it does not own the electron, so it is
  not an ion)
• The hydrogen then will take on a PARTIAL positive
  charge
• The bond between the two is a POLAR COVALENT
  BONDnot a hydrogen bond

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Hydrogen Bonding

• If these molecules are in a solution and can come
  into contact with other molecules like
  themselves, the partial positive begin to attract
  the partial negatives and form a bond
• A hydrogen bond
• This is what happens in water
• Lets draw it out

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Hydrogen Bonding


-
-


As you can see, a water molecule can be joined
with up to 4 neighbors via hydrogen bonding


-
hydrogen bond

-


-

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Intermolecular Forces

• Types
• Induced dipole
• Dipole-dipole
• Hydrogen bonds
• So, why do we call these INTERmolecular forces?
• These are forces or attractions BETWEEN molecules
  that pull them closer together
• In order to overcome these attractions, more
  energy must be added

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Binary Compounds

• Many of the examples that weve just used were
  binary compounds
• Made up of only 2 elements (no matter what the
  ratio)
• Ternary compounds are made up of 3 different
  elements

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Molecular Compounds

• Not all binary compounds are ionic
• Many are molecular (made up of 2 non-metallic
  elements)
• Prefixes are often used to name molecular
  compounds because non-metals can bond in various
  ratios
• For example, what ratio does Carbon bond to
  Oxygen?
• CO or CO2

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Molecular Compounds

• It is important to learn the prefixes used in
  naming binary molecular compounds
• Mono- (1) Hexa- (6)
• Di- (2) Hepta- (7) or septa
• Tri- (3) Octa- (8)
• Tetra- (4) Nona- (9)
• Penta- (5) Deca- (10)

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Molecular Compounds

• Try naming some of the following
• N2O PCl3 SF6
• Dinitrogen monoxide phosphorus trichloride
  sulfur hexafluoride
• Unless there are multiples of the first element,
  just call the first element by its name with no
  prefix
• Always name molecular compounds in the order
  given in the formula

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Acids

• Acids always have the hydrogen that makes it an
  acid out in front of its formula
• Look at the hydrogen(s) as acid, then look at
  what is left to determine the name of this acid
• For example, what is the polyatomic ion NO3
  -called?
• The suffix ic replaces the usual ending of the
  polyatomic ion or any anion and then acid is
  added to the end of the name
• HNO3 would be called what?