11

Bond Energy ActivityBond Energy Activity

• Using the table of bond energies (table 1), Using the table of bond energies (table 1), calculate the heat of reaction (calculate the heat of reaction (H) for the for the formation of the diatomic molecules in the formation of the diatomic molecules in the (table 2)(table 2)• We will divide up the table amongst groups.We will divide up the table amongst groups.• Show calculation work in notebooks.Show calculation work in notebooks.

22

ElectronegativityElectronegativity

• As the bond energy shows, As the bond energy shows, Pauling found that HCl has a Pauling found that HCl has a greater bond energy (431 kJ) greater bond energy (431 kJ) than the average of the Hthan the average of the H22 and and ClCl22 bonds (339.5 kJ) bonds (339.5 kJ)

• He attributed this extra He attributed this extra strength to partial ionic strength to partial ionic character, which lowered the character, which lowered the chemical energy of the chemical energy of the molecule.molecule.

33

ElectronegativityElectronegativity

• Using these bond strength concepts, Pauling Using these bond strength concepts, Pauling assigned arbitrary values of assigned arbitrary values of “electronegativity”“electronegativity”• Electronegativity: the ability of an atom to Electronegativity: the ability of an atom to

attract electrons in a covalent bond to itself.attract electrons in a covalent bond to itself.

• What do theseWhat do these trends remind trends remind you of ?you of ?

44

Trends in ElectronegativityTrends in Electronegativity

55

Trends in ElectronegativityTrends in Electronegativity

• Now try page 2 of activityNow try page 2 of activity

66

Electronegativity and Bond Polarity

Electronegativity and Bond Polarity

• In molecules, differences in In molecules, differences in electronegativity allows us to describe the electronegativity allows us to describe the “sharing” of electrons between atoms.“sharing” of electrons between atoms.• Does the average location of the shared Does the average location of the shared

electron pair have to be half-way between the electron pair have to be half-way between the atoms?atoms?

77

Polarity of Covalent BondsPolarity of Covalent Bonds

• Unequal sharing of electrons in a Unequal sharing of electrons in a bond leads to the development of bond leads to the development of partial charges separated from one partial charges separated from one another - this phenomenon is called another - this phenomenon is called polarity.polarity.

• The greater the charge separation, The greater the charge separation, the more like an ionic bond the the more like an ionic bond the covalent bond becomes. We speak covalent bond becomes. We speak of the relative ionic and covalent of the relative ionic and covalent character of the bond.character of the bond.

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Bonds: Ionic, Polar Covalent, Non-Polar Covalent

Bonds: Ionic, Polar Covalent, Non-Polar Covalent

• Bonds can be Bonds can be found with a found with a range of range of polarities, polarities, from from completely completely ionic to ionic to completely completely covalent.covalent.

• When will a bond be polar?When will a bond be polar?

Figure 7.10Figure 7.10

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Electronegativity and Bond Type

Electronegativity and Bond Type

• Differences in electronegativity also allow Differences in electronegativity also allow us to approximate whether atoms will us to approximate whether atoms will transfer or share electrons when forming a transfer or share electrons when forming a bond.bond. Electronegativity

difference ()Bond type

0 – 0.4 Covalent

0.4 – 1.7 Polar covalent

> 1.7 Ionic

1010

Practice: Polarity and Electronegativity

Practice: Polarity and Electronegativity

• What is the relative polarity of the bonds in What is the relative polarity of the bonds in the following sets?the following sets?FF22, HF, HF

FCl, ClFCl, Cl22

OF, MgOOF, MgOOH, CH, HH, HFOH, CH, HH, HF

ElectronegativitiesElectronegativitiesFF 4.04.0OO 3.53.5ClCl 3.03.0CC 2.52.5HH 2.12.1BB 2.02.0MgMg 1.21.2