chapter 4 compounds and their bonds 4.5 covalent compounds 1 copyright © 2009 by pearson education,...

Chapter 4 Chapter 4 Compounds and Their BondsCompounds and Their Bonds

4.5Covalent Compounds

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Copyright © 2009 by Pearson Education, Inc.

Covalent BondsCovalent Bonds

Covalent bonds form

• when atoms share electrons to complete octets.

• between two nonmetal atoms.

• between nonmetal atoms from Groups 4A (14), 5A (15), 6A (16), and 7A (17).

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Hydrogen MoleculeHydrogen Molecule

A hydrogen molecule

• is stable with 2 electrons (helium).

• has a shared pair of electrons.

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Forming Octets in MoleculesForming Octets in Molecules

In a fluorine, F2,, molecule, each F atom

• shares 1 electron.• attains an octet.

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Carbon Forms 4 Covalent Carbon Forms 4 Covalent BondsBonds

In a CH4 (methane) molecule,

• 1 C atom shares electrons with 4 H atoms to attain an octet.

• each H atom shares 1 electron to become stable, like helium.

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Multiple BondsMultiple Bonds

In a nitrogen molecule, N2,

• each N atom shares 3 electrons.• each N attains an octet.• the bond is a multiple bond called a triple

bond.• the name is the same as the element.

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Naming Covalent CompoundsNaming Covalent Compounds

In the names of covalent compounds, prefixes are used to indicate the number of atoms (subscript) of each element. (mono is usually omitted)

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Naming Covalent CompoundsNaming Covalent Compounds

What is the name of SO3?

1. The first nonmetal is S sulfur.

2. The second nonmetal is O, named oxide.

3. The subscript 3 of O is shown as the prefix tri.

SO3 -> sulfur trioxide

The subscript 1 (for S) or mono is understood.

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Naming Covalent CompoundsNaming Covalent Compounds

Name P4S3.

1. The first nonmetal, P, is phosphorus.

2. The second nonmetal, S, is sulfide.

3. The subscript 4 of P is shown as tetra.

The subscript 3 of O is shown as tri.

P4S3 -> tetraphosphorus trisulfide

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Formulas and Names of Some Formulas and Names of Some Covalent CompoundsCovalent Compounds

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ExamplesExamples

Select the correct name for each compound.

A. SiCl4 1) silicon chloride

2) tetrasilicon chloride3) silicon tetrachloride

B. P2O5 1) phosphorus oxide2) phosphorus pentoxide3) diphosphorus pentoxide

C. Cl2O7 1) dichlorine heptoxide2) dichlorine oxide3) chlorine heptoxide

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ExamplesExamples

Write the name of each covalent compound.

CO _____________________

CO2 _____________________

PCl3 _____________________

CCl4 _____________________

N2O _____________________

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Example: Writing Formulas Example: Writing Formulas for Covalent Compoundsfor Covalent Compounds

Write the formula for carbon disulfide.

STEP 1: Elements are C and S

STEP 2: No prefix for carbon means 1 C

Prefix di = 2

Formula: CS2

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ExamplesExamples

Write the correct formula for each of the following:

A. phosphorus pentachloride

B. dinitrogen trioxide

C. sulfur hexafluoride

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ExamplesExamples

Identify each compound as ionic or covalent, and give its correct name.

A. SO3

B. BaCl2

C. (NH4)3PO3

D. Cu2CO3

E. N2O4

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ExamplesExamples

Determine if each is ionic (I) or covalent (C ), and write the formula.

A. calcium nitrate

B. boron trifluoride

C. aluminum carbonate

D. dinitrogen tetroxide

E. copper(I) phosphate

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ExamplesExamplesIdentify each compound as ionic or covalent and give

its correct name.

A. Ca3(PO4)2

B. FeBr3

C. SCl2

D. Cl2O

E. N2

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ElectronegativityElectronegativity

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 Some Electronegativity Values for Group A ElementsGroup 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 BondsNonpolar 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 BondsPolar Covalent Bonds

A polar covalent bond

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

to 1.8).

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 Comparing Nonpolar and Polar Covalent BondsCovalent Bonds

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Ionic BondsIonic Bonds

An 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 TypesElectronegativity and Bond Types

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ExamplesExamples

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

A. K-N

B. N-O

C. Cl-Cl

D. Si-Cl

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4.74.7 Shapes and Polarity ofShapes and Polarity of

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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Steps to determine the Steps to determine the shapeshape

To 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.

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Four Electron GroupsFour 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 Three Bonding Atoms and One Lone PairLone 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 Two Bonding Atoms and Two Lone PairsPairs

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 GroupsShapes 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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ExamplesExamples

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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Polar MoleculesPolar 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 MoleculesNonpolar Molecules

A 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 PolarityDetermining Molecular Polarity

STEP 1: Write the electron-dot formula. STEP 2: Determine the shape. STEP 3: Determine if dipoles cancel or

not.

Example: H2O

H2O is polar

dipoles do not cancel

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H O

H

ExamplesExamples

Identify 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 Chapter 4 Compounds and Their BondsCompounds and Their Bonds

4.8 Attractive Forces in Compounds

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4.8 Attractive Forces in 4.8 Attractive Forces in CompoundsCompounds

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

together.

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Dipole-Dipole AttractionsDipole-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.

Dipole-dipole Hydrogen bonds

Dispersion ForcesDispersion 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 ForcesAttractive Forces

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Melting Points and Attractive Melting Points and Attractive ForcesForces

• 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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solid

(strong forces of interaction)

melting point

similar forces of interaction,so the phase change is not too helpful is seeing how strong they are

liquid

(strong forcesof interaction)

boiling point

drastically dif ferent forces of interaction, so the phase changeis very helpful for seeing how strong they are

gas

(no forcesofattraction)

ExamplesExamples

Identify 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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