Chapter 4 Compounds and Their Bonds

4.6

Electronegativity and Bond Polarity

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Electronegativity

The electronegativity value

• indicates the attraction of an atom for shared electrons.

• increases from left to right going across a period on the periodic table.

• is high for the nonmetals, with fluorine as the highest.

• is low for the metals.

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Some Electronegativity Values for Group A Elements

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Low values

High values

` Electronegativity increases

` E

lectro

neg

ativity d

ecre

ase

s

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Nonpolar Covalent Bonds

A nonpolar covalent bond

• occurs between nonmetals.• is an equal or almost equal sharing of electrons.• has almost no electronegativity difference (0.0 to 0.4).

Examples: Electronegativity Atoms Difference Type of BondN-N 3.0 - 3.0 = 0.0 Nonpolar covalentCl-Br 3.0 - 2.8 = 0.2 Nonpolar covalentH-Si 2.1 - 1.8 = 0.3 Nonpolar covalent

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Polar Covalent Bonds

A polar covalent bond

• occurs between nonmetal atoms.• is an unequal sharing of electrons.• has a moderate electronegativity difference (0.5 to

1.7).

Examples: Electronegativity

Atoms Difference Type of BondO-Cl 3.5 - 3.0 = 0.5 Polar covalentCl-C 3.0 - 2.5 = 0.5 Polar covalentO-S 3.5 - 2.5 = 1.0 Polar covalent

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Comparing Nonpolar and Polar Covalent Bonds

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Ionic BondsAn ionic bond • occurs between metal and nonmetal ions.• is a result of electron transfer.• has a large electronegativity difference (1.8 or

more).

Examples: Electronegativity

Atoms Difference Type of BondCl-K 3.0 – 0.8 = 2.2 IonicN-Na 3.0 – 0.9 = 2.1 IonicS-Cs 2.5 – 0.7 = 1.8 Ionic

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Electronegativity and Bond Types

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Predicting Bond Types

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Examples

Use the electronegativity difference to identify the type of bond [nonpolar covalent (NP), polar covalent (P), or ionic (I)] between the following:

A. K-NB. N-OC. Cl-ClD. H-Cl

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Chapter 4 Compounds and Their Bonds

4.7Shapes and Polarity of

Molecules

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°

VSEPR

In the valence-shell electron-pair repulsion theory

(VSEPR), the electron groups around a central atom

• are arranged as far apart from each other as possible.

• have the least amount of repulsion of the negatively charged electrons.

• have a geometry around the central atom that determines molecular shape.

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Guide to Predicting Molecular Shape

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Four Electron Groups

In a molecule of CH4,

• there are 4 electron groups around C.

• repulsion is minimized by placing 4 electron groups at angles of 109°, which is a tetrahedral arrangement.

• the shape with four bonded atoms is tetrahedral.

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Three Bonding Atoms and One Lone Pair

In a molecule of NH3,

• 3 electron groups bond to H atoms, and the fourth one is a lone (nonbonding) pair.

• repulsion is minimized with 4 electron groups in a tetrahedral arrangement.

• with 3 bonded atoms, the shape is pyramidal.

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Two Bonding Atoms and Two Lone Pairs

In a molecule of H2O,

• 2 electron groups are bonded to H atoms and 2 are lone pairs (4 electron groups).

• 4 electron groups minimize repulsion in a tetrahedral arrangement.

• the shape with 2 bonded atoms is bent.

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Shapes with 4 Electron Groups

Electron Pairs

Bonded Atoms

Lone Pairs

Molecular Shape

Example

4 4 0 Tetrahedral CH4

4 3 1 Pyramidal NH3

4 2 2 Bent H2O

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Examples

State the number of electron groups, lone pairs, and use VSEPR theory to determine the shape of the following molecules or ions.

1) tetrahedral 2) pyramidal 3) bent

A. PF3

B. H2S

C. CCl4

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Study TipTo determine shape, 1. draw the electron-dot structure.2. count the electron pairs around the central atom.

3. count the bonded atoms to determine shape.

4 electron pairs and 4 bonded atoms = tetrahedral 4 electrons pairs and 3 bonded atoms = pyramidal

4 electron pairs and 2 bonded atoms = bent

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Polar Molecules

A polar molecule • contains polar bonds.

• has a separation of positive and negative charge called a dipole, indicated with + and -.

• has dipoles that do not cancel. + -

• •

H–Cl H—N—H dipole

H dipoles do

not cancel

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Nonpolar MoleculesA nonpolar molecule • contains nonpolar bonds.

Cl–Cl H–H

• or has a symmetrical arrangement of polar bonds.

O=C=O Cl

Cl–C–Cl

Cl

dipoles cancel

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Determining Molecular Polarity STEP 1: Write the electron-dot formula. STEP 2: Determine the shape. STEP 3: Determine if dipoles cancel or not.

Example: H2O

. .

H─O: H2O is polar

│ H

dipoles do not cancel

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ExamplesIdentify each of the following molecules as 1) polar or 2) nonpolar. Explain.

A. PBr3

B. HBr

C. Br2

D. SiBr4

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Chapter 4 Compounds and Their Bonds

4.8 Attractive Forces in Compounds

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°

Ionic Bonds

In ionic compounds, ionic bonds • are strong attractive forces.• hold positive and negative ions

together.

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Dipole-Dipole Attractions

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In covalent compounds, polar molecules •exert attractive forces called dipole-dipole attractions.

•form strong dipole attractions called hydrogen bonds between hydrogen atoms bonded to F, O, or N, and other very electronegative atoms.

Dispersion Forces

Dispersion forces are• weak attractions between nonpolar

molecules.• caused by temporary dipoles that develop

when electrons are not distributed equally.

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Attractive Forces

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Melting Points and Attractive Forces

• Ionic compounds require large amounts of energy to break apart ionic bonds. Thus, they have high melting points.

• Hydrogen bonds are the strongest type of dipole-dipole attractions. They require more energy to break than other dipole-dipole attractions.

• Dispersion forces are weak interactions and very little energy is needed to change state.

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Melting Points of Some Substances

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ExamplesIdentify the main type of attractive forces for each:1) ionic 2) dipole-dipole 3) hydrogen bonds 4) dispersion

A. NCl3

B. H2O

C. Br-BrD. KClE. NH3

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