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Polar Bonds and Molecules. Electronegativity. Why does ice float?. Polar Bonds. When involved in a bond, atoms of some elements attract the shared electrons to a greater extent than atoms of other elements – This property is called Electronegativity (EN)

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polar bonds and molecules

Polar Bonds and Molecules

Electronegativity

polar bonds
Polar Bonds
  • When involved in a bond, atoms of some elements attract the shared electrons to a greater extent than atoms of other elements – This property is called Electronegativity (EN)
  • The following chart is used to determine the electronegativities of each atom
slide5
In general, electronegativity increases from left to right and from the bottom up
  • As atomic radius increases, electronegativity decreases.
difference in electronegativity en
Difference in Electronegativity = ΔEN
  • Based on the difference in electronegativities of atoms we can predict the type of bond that will form
    • Formula:
        • ∆EN = |ENA – ENB |
        • Chart:
examples
Examples
  • Potassium Fluoride, KF
    • ∆EN = ENF – ENK = 3.98 – 0.82 = 3.16
    • IONIC BOND
  • Oxygen, O2
    • ∆EN = ENO – ENO = 3.44 – 3.44 = 0
    • NON-POLAR COVALENT
  • Carbon Tetrachloride, CCl4

(look at the ∆EN for one of the C-Cl bonds)

    • ∆EN = ENCl – ENC = 3.0 – 2.5 = 0.5
    • POLAR COVALENT
slide8
With respect to polar covalent bonds, the differences in electronegativity tell us about the sharing of electrons
  • Example: Carbon Tetrachloride (CCl4)
    • Cl has EN = 3.0
    • C has EN = 2.5
    • From this, we say that chlorine has stronger attraction for electrons than carbon
    • Thus, electrons will spend more time around the Cl than C
slide9
This results in a slight separation of positive and negative charges which we call “partial charges” and represent them as δ+ orδ-
  • Example: CCl4
    • Chlorine with greater EN will have greater attraction of e- and thus will have partial negative charge δ-
    • Carbon with lower EN will have less attraction of e- and thus will have partial positive charge δ+
    • Shown as

δ+C-Clδ-

slide10
Equal sharing of electrons

ΔEN = 0

Unequal sharing of electrons

ΔEN > 0

slide12
When the bond is separated into partial positive and negative charges we call this bond a polar bond
  • We represent dipole bonds with a vector arrow that points to the more electronegative atom
  • Example CCl4

δ+C-Cl δ-

examples1
Examples
  • Remember to
    • Determine the bond type (by finding ∆EN)
    • Assign the partial charges
    • Place the dipole moment
  • Carbon and Oxygen

δ+C-O δ-

  • Carbon and Fluorine

δ+C-F δ-

polar molecules
Polar Molecules
  • We use our information on polar bonds to predict whether molecules will be polar or non-polar
  • We also must know our VSEPR shapes in order to do this!!
water h 2 0
Water H20
  • Determine bond type
    • ∆EN = ENO – ENH = 3.44 – 2.20 = 1.24
    • Thus is POLAR COVALENT
  • Determine partial charges
    • O has higher EN and H has lower EN
    • Our partial charges are:
  • If we include the dipoles

Bent shape according to VSEPR

back to the question why does ice float
Back to the question:Why does ice float?
  • Density of water=1g/mL Density of ice = 0.92g/mL
vsepr theory
VSEPR Theory
  • Valence Shell Electron Pair Repulsion Theory
  • This theory predicts the shapes of molecules based on the number of areas of electron density around the central atom
  • Electron density can be a lone pair or a bonding pair of electrons
  • The areas of electron density want to be as far apart as possible and as such form predictable molecular shapes
slide21
This is where VSEPR is important! -- You must know the shape of the molecule in order to determine it’s polarity
  • Water has two partially positive ends and one partially negative end
  • The two dipole arrows point in the same direction. If we add these together we can see the molecule will have an overall net dipole
  • Because the dipoles do not cancel each other a net dipole is produced and we say that the molecule is POLAR
carbon dioxide co 2
Carbon Dioxide CO2
  • Determine bond type
    • ∆EN = ENO – ENC = 3.44 – 2.55 = 0.89
    • Thus is POLAR COVALENT
  • Determine partial charges
    • O has greater EN than C
    • Our partial charges are:
  • If we include the dipoles

Linear shape according to VSEPR

slide23
The dipoles created in this molecule are pointing in opposite directions and thus will cancel each other
  • This molecule has no net dipole and therefore is said to be NON-POLAR
hydrogen cyanide hcn
Hydrogen Cyanide HCN
  • Determine bond type
    • ∆EN = ENN – ENC = 3.04 – 2.55 = 0.49
    • Thus is slightly POLAR COVALENT
    • ∆EN = ENC – ENH = 2.55 – 2.20 = 0.35
    • Is also slightly POLAR COVALENT
  • Determine partial charges
    • N has greater EN than C – N will have δ-
    • C has greater EN than H – C will have δ-
slide25
When we assign the dipoles
  • We see that they are both pointing the same direction
  • Thus they will not cancel, but will result in an overall net dipole
  • This molecule is said to be POLAR
note the difference
Note the Difference!
  • When we had a linear molecule with the same atoms attached to the central atom the molecule was non-polar ex. CO2
  • When we had a linear molecule with two different atoms attached to the central atom, the molecule was polar Ex. HCN
  • It is very important to look at the electronegativities associated with the atoms and not just the VSEPR shape
sulfur trioxide so 3
Sulfur Trioxide SO3
  • Determine bond type
    • ∆EN = ENO – ENS = 3.44 – 2.58 = 0.86
    • Thus is POLAR COVALENT
  • Determine partial charges
    • O has greater EN than S
    • Our partial charges are:

Trigonal Planar shape according to VSEPR

slide28
When we assign dipole arrows
  • All the dipoles are pulling away from the central atom
  • You may think that because there are three dipoles they will not cancel and will result in a polar molecule
  • This is not correct however!!
slide29
Look at the horizontal and vertical components of the vectors (red and green arrows)
  • The red arrows will cancel
  • The green arrows can add together
  • This green arrow will cancel with the blue vector created by the top O
  • Therefore all dipole vectors will cancel in this molecule creating no net dipole and therefore the molecule is NON-POLAR
slide30
Similar to our linear molecule, difference will occur when the atoms attached to the central atom are different
  • We must be sure to look at the electronegativities of each atom when comparing the dipole vectors
  • Ex. CCl2O
  • O has higher EN than Cl and will therefore have a greater dipole
  • The two dipoles from Cl will add together but they will still be less than that of O
  • Overall net dipole will result and thus molecule is POLAR
ammonia nh 3
Ammonia NH3
  • Determine bond type
    • ∆EN = ENN – ENH = 304 – 2.20 = 0.84
    • Thus is POLAR COVALENT
  • Determine partial charges
    • N has greater EN than H
    • Our partial charges are:

Pyramidal shape according to VSEPR

slide32
Assign dipole vectors
  • The three vectors will add together to create an overall net dipole
  • This will result in a POLAR molecule
carbon tetrachloride ccl 4
Carbon Tetrachloride CCl4
  • Determine bond type
    • ∆EN = ENCl – ENC = 3.16 – 2.55 = 0.61
    • Thus is POLAR COVALENT
  • Determine partial charges
    • Cl has greater EN than C
    • Our partial charges are:

Tetrahedral shape according to VSEPR

slide34
When we assign dipoles
  • We can see that all the dipoles are of the same magnitude because the EN differences are all the same
  • There are equal amounts of dipoles in opposite directions and thus they will all cancel
  • This results in no net dipole and therefore the molecule is NON-POLAR
chloroform chcl 3
Chloroform CHCl3
  • Determine bond type
    • ∆EN = ENCl – ENC = 3.16 – 2.55 = 0.61
    • Thus is POLAR COVALENT
    • ∆EN = ENC – ENH = 2.55 – 2.20 = 0.35
    • Thus is slightly POLAR COVALENT
  • Determine partial charges
    • Cl has greater EN than C
    • C has greater EN than H
    • Our partial charges are:

Tetrahedral shape according to VSEPR

slide36
Assign dipoles (blue arrows)
  • We can see that the dipoles to Cl will all add up to create the larger green dipole vector
  • This is opposite to the dipole vector created by H-C but does not have the same magnitude
  • Thus, it will not cancel and result in a net dipole
  • This molecule is POLAR
summary of polarity of molecules
Summary of Polarity of Molecules
  • Linear:
    • When the two atoms attached to central atom are the same the dipoles will cancel, leaving no net dipole, and the molecule will be Non-Polar
    • When the two atoms are different the dipoles will not cancel, resulting in a net dipole, and the molecule will be Polar
slide38
Bent:
    • The dipoles created from this molecule will not cancel creating a net dipole and the molecule will be Polar
  • Pyramidal:
    • The dipoles created from this molecule will not cancel creating a net dipole and the molecule will be Polar
summary of polarity of molecules1
Summary of Polarity of Molecules
  • Trigonal Planar:
    • When the three atoms attached to central atom are the same the dipoles will cancel, leaving no net dipole, and the molecule will be Non-Polar
    • When the three atoms are different the dipoles will not cancel, resulting in a net dipole, and the molecule will be Polar
summary of polarity of molecules2
Summary of Polarity of Molecules
  • Tetrahedral:
    • When the four atoms attached to the central atom are the same, the dipoles will cancel, leaving no net dipole, and the molecule will be Non-Polar
    • When the four atoms are different, the dipoles will not cancel, resulting in a net dipole, and the molecule will be Polar
with your group
With your group,
  • Read through the tutorial on pg 106-107 and answer question 1 on pg 107

Homework

  • Read pg 102-108Questions pg 108 # 1, 2, 5
examples to try
Examples to Try
  • Determine whether the following molecules will be polar or non-polar
    • SI2
    • CH3F
    • AsI3
    • H2O2
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