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Chapter 10 Lecture

Basic ChemistryFourth Edition

Chapter 10 Properties of Solidsand Liquids10.3 Electronegativity and Polarity

Learning Goal Use electronegativityto determine the polarity of a bond ora molecule.

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Electronegativity and Polarity

We can learn more about the chemistry of compounds by understanding how electrons are shared in bonds.•Bonds formed by identical atoms share the bonding electrons equally.•Bonds formed between different atoms share the bonding electrons unequally.

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Electronegativity

Electronegativity• is the relative ability of atoms to attract shared

electrons • is higher for nonmetals; fluorine has the

highest with a value of 4.0 • is lower for metals; cesium and francium have

the lowest value of 0.7

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Electronegativity

We can use the periodic table to predict the relative electronegativity value for each element. Electronegativity•increases from left to right going across a period on the periodic table•decreases going down a group on theperiodic table

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Electronegativity

Figure 10.1 The electronegativity values of the representative elements in Group 1A (1) to Group 7A (17), which indicate the ability of atoms to attract shared electrons, increase going across a period from left to right and decrease going down a group.

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Learning Check

Using the periodic table, predict the order of increasing electronegativity for the elements O, K, and C.

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Solution

Using the periodic table, predict the order of increasing electronegativity for the elements O, K, and C. Answer: K, C, O•K, period 4, group 1A (1), has an electronegativity value of 0.8.•C, period 2, group 4A (14), has an electronegativity value of 2.5.•O, period 2, group 6A (16), has an electronegativity value of 3.5.

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Types of Covalent Bonds

Bonds can be described by the difference in the electronegativity of the bonding atoms. Two types of covalent bonds occur in molecules:•nonpolar covalent bonds; bonding electrons are shared equally•polar covalent bonds; bonding electrons are shared unequally

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Nonpolar Covalent Bonds

A nonpolar covalent bond between nonmetal atoms•consists of an equal (or almost equal) sharing of electrons•has a zero (or close to zero) electronegativity difference (0.0 to 0.4)

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Polar Covalent Bonds

A polar covalent bond between nonmetal atoms•consists of an unequal sharing of electrons•has an electronegativity difference of 0.5 to 1.7

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Polar and Nonpolar Covalent Bonds

Figure 10.2 In the nonpolar covalent bond of H2, electrons are shared equally. In the polar covalent bond of HCl, electrons are shared unequally.

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Dipoles and Bond Polarity

• Bonds become more polar as the difference in electronegativity increases.

• A polar covalent bond that has a separation of charges is called a dipole.

• The positive and negative ends are represented by the Greek letter delta, with a + or − charge.

• Arrows can also be used to represent dipoles.

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Variations in Bonding

Variations in bonding are continuous.•Bonds with an electronegativity difference of 0.0 – 0.4 are considered nonpolar.•Bonds with an electronegativity difference of 0.5 – 1.7 are considered polar covalent.•Bonds with an electronegativity difference greater than 1.8 are considered ionic.

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Electronegativity and Types of Bonds

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Predicting Bond Type

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Learning Check

Complete the following table for each of the bonds indicated.

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Solution

Complete the following table for each of the bonds indicated.

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Polarity of Molecules—Nonpolar

In a nonpolar molecule, all the bonds are nonpolar, or the polar bonds (dipoles) cancel each other out.

Molecules such as H2, Cl2 and CH4 are nonpolar because they contain only nonpolar bonds.

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Polarity of Molecules—Nonpolar

A nonpolar molecule also occurs when polar bonds (dipoles) cancel each other because of a symmetrical arrangement.

Molecules such as CO2 and CCl4 contain polar bonds with dipoles that cancel each other out.

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Polarity of Molecules—Polar

A polar molecule occurs when the dipoles from individual bonds do not cancel each other out.For molecules with two or more electron groups, the shape (such as bent or trigonal pyrimidal) determines whether or not the dipoles cancel.

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Polarity of Molecules—Polar

Examples of polar molecules include HCl, H2O, and NH3.•HCl is linear and contains a polar bond.

•H2O is bent and contains two polar bonds as well as two lone pairs on oxygen.

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Polarity of Molecules—Polar

• NH3 is trigonal pyrimidal, and contains three polar bonds and a lone pair on nitrogen.

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Guide to Determination of Polarity of a Molecule

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Learning Check

Determine if the molecule OF2 is polar or nonpolar.

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Solution

Determine if the molecule OF2 is polar or nonpolar.Step 1 Determine if the bonds are polar or nonpolar covalent.Oxygen has an electronegativity of 3.5, and fluorine has an electronegativity of 4.0. O—F bonds are polar covalent.

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Solution

Determine if the molecule OF2 is polar or nonpolar.Step 2 If the bonds are polar covalent, draw the electron-dot formula and determine if the dipoles cancel.Dipoles in O—F bonds do not cancel;the molecule is polar.