chapter 4 compounds and their bonds 4.1 octet rule and ions 1 chemistry: an introduction to general,...

Chapter 4Compounds and Their Bonds

4.1Octet Rule and Ions

Ionic and Covalent Bonds

Atoms that are not noble gases form octets• to become more stable • by losing, gaining, or sharing valence electrons• by forming ionic bonds or covalent bonds

Ionic bonds occur when atoms of one element losevalence electrons and the atoms of another elementgain valence electrons.Covalent bonds occur when nonmetal atoms shareelectrons to attain a noble gas arrangement.

The Octet Rule

An octet• is eight valence electrons• is associated with the stability of the noble gases

(except He)The octet rule states that elements gain or lose electronsand become ions to attain noble gas arrangement.

valence electrons

He 2 2

Ne 2, 8 8

Ar 2, 8, 8 8

Kr 2, 8, 14, 8 8

Ionic and Covalent Bonds4

Metals Form Positive Ions

Metals form positive ions• by a loss of their valence electrons• with the electron configuration of the nearest noble gas• that have fewer electrons than protons

Group 1A metals ion 1+

Formation of Magnesium Ion, Mg2+

Nonmetals Form Negative Ions

Nonmetals form negative ions:• by a gain of valence electrons• with the electron configuration of the nearest noble gas• with fewer protons than electrons

Group 5A nonmetals ion 3−

Formation of Chloride Ion, Cl−

Formulas and Names of Common Ions9

Learning Check

Write the formula and symbol of an ion with 16 protons and 18 electrons.

Solution

Write the formula and symbol of an ion with 16 protons and 18 electrons.

The element with 16 protons is sulfur, with the symbol S.

An ion of sulfur with 18 electrons gives sulfur a charge of 2−.

The sulfide ion is S2−.

Ionic Charges from Group Numbers

Group Charge

Group 1A 1+

Group 2A 2+ Positive Charges

Group 3A 3+

Group 5A 3−

Group 6A 2− Negative Charges

Group 7A 1−

Ionic Charges from Group Numbers13

Learning Check

Consider the elements calcium and chlorine.

A. Identify each as a metal or a nonmetal.

B. State the number of valence electrons for each.C. State the number of electrons that must be lost or gained for each to acquire an octet.D. Write the symbol, including its ionic charge, and name of each resulting ion.

Solution

Consider the elements calcium and chlorine.

A. Identify each as a metal or a nonmetal.

metal = calcium nonmetal = chlorine

B. State the number of valence electrons for each.

Calcium has two valence electrons.

Chlorine has seven valence electrons.

Solution

C. State the number of electrons that must be lost or gained for each to acquire an octet.

Calcium will lose 2 electrons to acquire an octet. Chlorine will gain 1 electron to acquire an octet.

D. Write the symbol, including its ionic charge, and name of each resulting ion. Ca2+ calcium

Cl− chloride

4.2Ionic Compounds

Properties of Ionic Compounds

Ionic Compounds• consist of positive and negative ions• have attractions called ionic bonds between positively

and negatively charged ions • have high melting points• are solid at room temperature

NaCl Is an Ionic Compound19

Formulas of Ionic Compounds

An ionic formula• consists of positively and negatively charged ions• is neutral• has charge balance

total positive charge = total negative charge

The symbol of the metal is written first, followed by the symbol of the nonmetal.

metal symbol, nonmetal symbol

Subscripts in Formulas21

Writing Ionic Formulas from Ion Charges

Subscripts in ionic compound represent the number of positive and negative ions that give an overall charge of zero.

total positive charge + total negative charge = 0

therefore,

total positive charge = total negative charge

Charge Balance for Na2S

In Na2S• 2 Na atoms lose their valence electrons to become

Na+ ions• an S atom gains 2 electrons to become an S2− ion• subscripts indicate number of each ions to give charge

balance total positive charge = total negative charge

Na+ Na+ + S2− = Na2S2(1+) = 2−

Write the symbol of the metal first followed by a subscriptof 2 and then the symbol of the nonmetal.

Charge Balance for Na2S24

Writing an Ionic Formula for Na3N

Charge balance is used to write the formula for sodium nitride, a compound containing Na+ and N3−.

3 Na+ + N3− = Na3N

3(+1) + 1(3−) = 0

Learning Check

Write the ionic formula of the compound formed with Ba2+ and Cl− ions.

Solution

Write the ionic formula of the compound formed with Ba2+ and Cl− ions.

Write the symbols of the ions:

Ba2+ Cl−

Balance the charges:

Ba2+ + 2 Cl− Cl− = BaCl2

2+ + 2(1−) = 0

Write the ionic formula using a subscript 2 for two chloride ions.

Learning Check

Select the correct formula for each of the following ionic compounds.

A. Na+ and O2−

1) NaO 2) Na2O 3) NaO2

B. Al3+ and Cl−

1) AlCl3 2) AlCl 3) Al3Cl

C. Mg2+ and N3−

1) MgN 2) Mg2N3 3) Mg3N2

Solution

Select the correct formula for each of the following ionic compounds.

A. Na+ and O2− 2) Na2O Check:

2Na+ + O2− = 2(1+) + 1(2−) = 0

B. Al3+ and Cl− 1) AlCl3 Check: Al3+ + 3Cl− = (3+) + 3(1−) = 0

C. Mg2+ and N3− 3) Mg3N2 Check: 3Mg2+ + 2N3− = 2(3+) + 2(3−) = 0

4.3Naming and Writing Ionic Formulas

Names of Some Ionic Compounds31

Naming Ionic Compounds Containing Two Elements

Metals that form a single ion include the following:

Metal(s) Ion ChargeGroup 1A 1+Group 2A 2+Al 3+Zn and Cd 2+Ag 1+

Naming Ionic Compounds33

Naming the Ionic Compound K2O

Name the ionic compound K2O.

Step 1 Identify the cation and anion. The cation, K+, is from Group 1A, and the anion,O2−, is from Group 6A.

Step 2 Name the cation by its element name.The cation, K+, is potassium.

Naming the Ionic Compound K2O

Name the ionic compound K2O.

Step 3 Name the anion by using the first syllable of itselement name followed by ide. The anion, O2−, is oxide.

Step 4 Write the name of the cation first and the name of the

anion second. K2O is named potassium oxide.

Learning Check

Write the names of the following compounds.

A. CaO ___________

B. Al2O3 ___________

C. MgCl2___________

Solution

Step 1 Identify the cation and anion.

A. CaO: The cation, Ca2+, is from Group 2A, and the anion, O2−, is from Group 6A.

B. Al2O3: The cation, Al3+, is from Group 3A, and

the anion, O2−, is from Group 6A.

C. MgCl2: The cation, Mg2+, is from Group 2A, and the anion, Cl−, is from Group 7A.

Solution

Write the names of the following compounds:

Step 2 Name the cation by its element name.

A. CaO The cation, Ca2+, is calcium.

B. Al2O3 The cation, Al3+, is aluminum.

C. MgCl2 The cation, Mg2+, is magnesium.

Solution

Step 3 Name the anion by using the first syllable of its element name followed by ide.

A. CaO The anion, O2−, is oxide.

B. Al2O3 The anion, O2−, is oxide.

C. MgCl2 The anion, Cl−, is chloride.

Solution

Write the names of the following compounds:

Step 4 Write the name of the cation first and the name of the anion second.

A. The name of CaO is calcium oxide.

B. The name of Al2O3 is aluminum oxide.

C. The name of MgCl2 is magnesium

chloride.

Learning Check

Write the formulas and names for compounds of the following ions:

Br− S2− N3−

Solution

Write the formulas and names for compounds of the following ions:

Br− S2− N3−

NaBr Na2S Na3N Na+ sodium bromide sodium sulfide sodium nitride

AlBr3 Al2S3 AlN

Al3+ aluminum aluminum aluminum

bromide sulfide nitride

Metals That Form More Than One Cation

Transition metals except for Zn2+, Cd2+, and Ag+ form two or more positive ions (cations).

We use a Roman numeral to identify ionic charge.

Cu2+ copper(II) Pb2+ lead(II)Cu+ copper(I) Pb4+ lead(IV)Fe2+ iron(II) Cr3+ chromium(III)Fe3+ iron(III) Cr6+ chromium(VI)

Metals with Variable Charge44

Determination of Variable Charge

Use the charge on the anion and charge balance to calculate charge on the metal ion.

Mn charge + 2 F− charge = 0? + 2 (1−) = 02+ + 2− = 0Mn2+ + 2 F− = 0

Metals Form Positive Ions; Nonmetals Form Negative Ions46

Naming Compounds with Variable Charge Metals47

Naming FeCl2

Name the ionic compound FeCl2.

Step 1 Determine the charge of the cation. Because there are two Cl− anions, the charge onthe Fe ion must be 2+.

Step 2 Name the cation by its element name and use aRoman numeral in parenthesis for the charge. The charge on Fe is represented by a Romannumeral, iron(II).

Naming FeCl2

Name the ionic compound FeCl2.

Step 3 Name the anion by using the first syllable of itselement name followed by ide. Chlorine becomes chloride.

Step 4 Write the name of the cation first and the name of the

anion second.The ionic compound is named iron(II) chloride.

Learning Check

Name the following ionic compounds containing metals that form two kinds of positive ions:

A. Fe2O3

B. SnCl2

Solution

Step 1 Determine the charge of the cation. A. Fe2O3 Because there are three O2– anions and

two Fe ions, the charge on the Fe ion must be 2+.

B. SnCl2 Because there are two Cl− anions, the charge on the Sn ion must be 2+.

Solution

Step 2 Name the cation by its element name and use a Roman numeral in parenthesis for the charge.

A. Fe2O3 The charge on Fe is represented by a Roman numeral: iron(III)

B. SnCl2 The charge on Sn is represented by a Roman numeral: tin(II)

Solution

Step 3 Name the anion by using the first syllable of itselement name followed by ide.

A. Fe2O3 Oxygen becomes oxide.

B. SnCl2 Chlorine becomes chloride.

Solution

Step 4 Write the name of the cation first and the name ofthe anion second. A. Fe2O3 is iron(III) oxide.

B. SnCl2 is tin(II) chloride.

Writing Formulas from the Name of an Ionic Compound55

Formula for Chromium(III) Bromide

Step 1 Identify the cation and anion. The Roman numeral(III) indicates that the charge on the chromium ionis 3+, Cr3+.

Step 2 Balance the charges.

Cr3+ Br−

1(3+) + 3(1−) = 0Step 3 Write formula, cation first, using subscripts from the

charge balance. CrBr3

Learning Check

Write chemical formulas for the following compounds:

A. Nickel(II) sulfide

B. Zinc chloride

C. Iron(III) oxide

Solution

Write chemical formulas for the following compounds.Step 1 Identify the cation and anion.

A. Nickel(II) sulfideThe Roman numeral (II) indicates that thecharge on the nickel ion is 2+, Ni2+.

B. Zinc chloride Zinc is one of the transition metals with a fixedcharge of 2+, Zn2+.

C. Iron(III) oxideThe Roman numeral (III) indicates that thecharge on the iron ion is 3+, Fe3+.

Solution

Write chemical formulas for the following compounds.

Step 2 Balance the charges. A. Nickel(II) sulfide

Ni2+ S2−

1(2+) + 1(2−) = 0

B. Zinc chloride Zn2+ Cl−

1(2+) + 2(1−) = 0

Solution

Write chemical formulas for the following compounds.

Step 2 Balance the charges. C. Iron(III) oxide

Fe3+ O2−

2(3+) + 3(2−) = 0

Solution

Write chemical formulas for the following compounds:

Step 3 Write formula, cation first using subscripts from

the charge balance.

A. Nickel(II) sulfide NiS

B. Zinc chloride ZnCl2

C. Iron(III) oxide Fe2O3

4.4Polyatomic Ions

Polyatomic Ions

Polyatomic Ions• are a group of atoms with an overall charge• often consist of a nonmetal such as phosphorus, sulfur, carbon, or nitrogen and oxygen• usually have a 1−, 2−, or 3− charge

Names of Polyatomic Ions

1. Names of most common polyatomic ions end in ate.SO4

2− sulfate

PO43− phosphate

NO3− nitrate

2. When a related ion has one less oxygen, its name

ends in ite.SO3

2− sulfite

PO33− phosphite

NO2− nitrite

3. Exceptions to these rules are: CN− cyanideOH− hydroxideNH4

+ ammonium

4. Add an H+ and reduce negative charge.CO3

2− + H+ = HCO3−

carbonate + H+ = bicarbonate orhydrogen carbonate

SO42− + H+ = HSO4

sulfate + H+ = bisulfate or hydrogen sulfate

5. Halogens form 4 polyatomic ions with oxygen each have a 1 charge.

ClO4− perchlorate

ClO3− chlorate

ClO2− chlorite

ClO− hypochlorite

Sodium chlorite is used in the processing and bleaching of pulp from wood fibers and recycled cardboard.

Compounds Containing Polyatomic Ions

When writing formulas for ionic compounds containing polyatomic ions, we use the same rules of charge balance as those for simple ionic compounds.

Consider the formula for magnesium nitrate:

Mg2+ + NO3−

NO3−

(2+) + 2(1−) = 0

Mg2+ + 2 NO3− = Mg(NO3)2

Learning Check

Select the correct formula for each.

A. aluminum nitrate

1) AlNO3 2) Al(NO)3 3) Am(NO3)3

B. copper(II) nitrate

1) CuNO3 2) Cu(NO3)2 3) Cu2(NO3)

C. iron(III) hydroxide

1) FeOH 2) Fe3OH 3) Fe(OH)3

D. tin(IV) hydroxide

1) Sn(OH)4 2) Sn(OH)2 3) Sn4(OH)

Solution

Select the correct formula for each.

A. aluminum nitrate 3) Al(NO3)3

B. copper(II) nitrate 2) Cu(NO3)2

C. iron(III) hydroxide 3) Fe(OH)3

D. tin(IV) hydroxide 1) Sn(OH)4

Guide to Naming Ionic Compounds with Polyatomic Ions70

Naming Compounds with Polyatomic Ions

Name the following ionic compounds:Step 1 Step 2 Step 3 Step 4

cation/anion Name Name Name cation anion compound

A.Ca(NO3)2 Ca2+ NO3− calcium nitrate calcium

nitrate

B. FePO4 Fe3+ PO43− iron(III) phosphate iron(III)

phosphate

Learning Check

Name each of the following compounds containing polyatomic ions.

A. MgSO3

B. MgSO4

C. Ca(ClO3)2

Solution

cation/anion Name Name Namecation anion compound

A. MgSO3 Mg2+ SO32− magnesium sulfite magnesium

sulfite

B. MgSO4 Mg2+ SO42− magnesium sulfate magnesium

sulfate

C. Ca(ClO3)2 Ca2+ ClO3− calcium chlorate calcium

chlorate

Learning Check

Name each of the following compounds:

A. Fe2(SO4)3

B. Ba3(PO3)2

C. PbCO3

Solution

cation/anion Name Name Name cation anion compound

A. Fe2(SO4)3 Fe3+ SO42− iron(III) sulfate iron(III)

sulfate

B. Ba3(PO3)2 Ba2+ PO33− barium phosphite barium

phosphite

C. PbCO3 Pb2+ CO32− lead(II) carbonate lead(II)

carbonate

4.5Covalent Compounds and Their

Covalent Bonds

Covalent compounds form

• when atoms share electrons to complete octets

• between nonmetal atoms from Groups 4A, 5A, 6A, and 7A

Formation of a Hydrogen Molecule

A hydrogen molecule, H2,

• forms as the atoms move closer and the nucleus of one atom attracts the electron of the other

• is stable with 2 electrons (helium)• has a shared pair of electrons

Formation of a Hydrogen Molecule – H2

A covalent bond forms as H atoms move close together to share electrons.

Formation of Octets in Molecules – F2

In a fluorine, F2,, molecule, each F atom shares one electron to attain an octet.

Writing Electron-Dot Formulas

To draw the electron-dot formula for methane, CH4, • we start with the electron-dot symbols for carbon and hydrogen

• we need four hydrogen atoms to complete carbon’s octet

Double and Triple Covalent Bonds

A double bond:• occurs when atoms share two pairs of electrons• forms when there are not enough electrons to complete

octets

A triple bond:• occurs when atoms share three pairs of electrons• forms when there are not enough electrons to complete

octets

Guide to Drawing Electron-Dot Formulas

Electron-Dot Formulas for Some Covalent Compounds – CO2

Step 1 Determine the arrangement of atoms. O C O

Step 2 Determine the total number of valence electrons.

C 2(O) = CO2

4e− + 2(6−) = 16e−

Electron-Dot Formulas for Some Covalent Compounds – CO2

Step 3 Attach each bonded atom to the central atom

with a pair of electrons.

Step 4 Place the remaining electrons using single or

multiple bonds to complete octets.

Triple Bond – N2

In a nitrogen molecule, N2,

• each N atom shares three electrons to gain an octet• the multiple bond formed is called a triple bond• the name is the same as the element

Names and Formulas of Covalent Compounds

When naming covalent compounds:• the first nonmetal in the formula is named by its element

name• the second nonmetal is named by using the first syllable

of its name followed by ide• if a subscript is used in the formula a prefix is used in

front of its name• when vowels o and o or a and o appear together, the first

vowel is omitted

Guide to Naming Covalent Compounds88

Prefixes Used in Naming Covalent Compounds89

Naming Covalent Compounds – NCl3

Name the covalent compound NCl3.

Step 1 Name the first nonmetal by its element name.

The first nonmetal (N) is nitrogen.

Step 2 Name the second element by using the first syllable of its name followed by ide.

The second nonmetal (Cl) is chloride.

Naming Covalent Compounds – NCl3

Step 3 Add prefixes to indicate number of atoms.

Because there is one nitrogen atom, no prefix

is needed.

The subscript three for the Cl atoms is shown as the

prefix tri.

NCl3, nitrogen trichloride

Some Common Covalent Compounds92

Learning Check

Select the correct name for each compound.

A. SiCl4 1) silicon chloride

2) tetrasilicon chloride3) silicon tetrachloride

B. P2O5 1) phosphorus oxide

2) phosphorus pentoxide3) diphosphorus pentoxide

C. Cl2O7 1) dichlorine heptoxide

2) dichlorine oxide3) chlorine heptoxide

Solution

Select the correct name for each compound.

A. SiCl4 3) silicon tetrachloride

B. P2O5 3) diphosphorus pentoxide

C. Cl2O7 1) dichlorine heptoxide

Learning Check

Write the name of each covalent compound.

B. NO2

C. PF3

D. CCl4

Solution

Step 1 Name the first nonmetal by its element name.

A. CO The first nonmetal (C) is carbon.

B. NO2 The first nonmetal (N) is nitrogen.

C. PF3 The first nonmetal (P) is phosphorus.

D. CCl4 The first nonmetal (C) is carbon.

Solution

Step 2 Name the second element by using the first syllable of its name followed by ide.

A. CO The second nonmetal (O) is oxide.

B. NO2 The second nonmetal (O) is oxide.

C. PF3 The second nonmetal (F) is fluoride.

D. CCl4 The second nonmetal (Cl) is chloride.

Solution

Write the name of each covalent compound.Step 3 Add prefixes to indicate number of atoms.

A. CO Because there is one carbon atom, no prefix isneeded. The subscript 1 for the O atom is shown using the prefix mono.

CO is carbon monoxide.

B. NO2 Because there is one nitrogen atom, no prefix is

needed. The subscript 2 for the O atoms isshown using the prefix di.

NO2 is nitrogen dioxide.

Solution

Write the name of each covalent compound.Step 3 Add prefixes to indicate number of atoms.

C. PF3 Because there is one phosphorus atom, no prefix is needed. The subscript 3 for F atomsis shown using the prefix tri.PF3 is phosphorus trifluoride.

D. CCl4 Because there is one carbon atom, no prefix is

needed. The subscript 4 for the Cl atoms isshown using the prefix tetra.CCl4 is carbon tetrachloride.

Writing Formulas for Covalent Compounds

Writing Formulas for Covalent Compounds – Diboron Trioxide

Write the formula for the covalent compound diboron trioxide.Step 1 Write the symbols in order of the elements in

the name. The first nonmetal is boron (B) and the second nonmetal is oxygen (O). B O

Step 2 Write any prefixes as subscripts. The prefix di in diboron indicates there are two boronatoms and is shown as a subscript after B. Theprefix tri in trioxide indicates there are three oxygen atoms and is shown as a subscript after O. B2O3

Learning Check

Write the correct formula for each of the following:

A. phosphorus pentachloride

B. dinitrogen trioxide

C. sulfur hexafluoride

Solution

Step 1 Write the symbols in order of the elements in the name. A. phosphorus pentachloride

The first nonmetal is phosphorus, (P) and the second

nonmetal is chlorine, (Cl). P Cl

B. dinitrogen trioxide The first nonmetal is nitrogen, (N) and the secondnonmetal is oxygen, (O). N O

C. sulfur hexafluorideThe first nonmetal is sulfur, (S) and the second nonmetal is fluorine, (F). S F

Solution

Step 2 Write any prefixes as subscripts. A. phosphorus pentachloride: No prefix for

phosphorus indicates there is 1 phosphorusatom. The prefix penta in pentachlorideindicates there are 5 chlorine atoms, and isshown as a subscript after Cl. PCl5

B. dinitrogen trioxide: The prefix di in dinitrogenindicates there are 2 nitrogen atoms and is shown as a subscript after N. The prefix tri intrioxide indicates there are 3 oxygen atoms and

isshown as a subscript after O. N2O3

Solution

Step 2 Write any prefixes as subscripts. C. sulfur hexafluoride: No prefix for sulfur

indicates there is 1 sulfur atom. The prefix

hexa in hexafluoride indicates there are 6fluorine atoms and is shown as a subscript

after F. SF6

Summary to Naming Simple Compounds

Learning Check

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

A. SO3

B. BaCl2

C. (NH4)3PO3

D. Cu2CO3

E. N2O4

Study Tip: Ionic or Covalent

A compound is

• ionic if the first element in the formula or the name is a metal or the polyatomic ion NH4

K2O K is a metal; compound is ionic; potassium

• covalent if the first element in the formula or the name is a nonmetal

N2O N is a nonmetal; compound is covalent;

dinitrogen oxide

Solution

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

A. SO3 covalent – sulfur trioxide

B. BaCl2 ionic – barium chloride

C. (NH4)3PO3 ionic – ammonium phosphite

D. Cu2CO3 ionic – copper(I) carbonate

E. N2O4 covalent – dinitrogen tetroxide

4.6Electronegativity and Bond Polarity

Electronegativity

The electronegativity value• of an element indicates the attraction of an atom for the

shared electrons in a bond• 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

Electronegativity and the Periodic Table

Comparing Ionic, Polar, and Nonpolar Covalent Bonds

Comparing Nonpolar and Polar Bonds

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

Nonpolar Covalent Bonds

A nonpolar covalent bond • occurs between nonmetals• is an equal or almost equal sharing of electrons• has a very small electronegativity difference

Examples: Electronegativity Atoms Difference Type of BondN-N 3.0 3.0 = 0.0 Nonpolar covalent

Cl-Br 3.0 2.8 = 0.2 Nonpolar covalent

H-Si 2.1 1.8 = 0.3 Nonpolar covalent

Polar Covalent Bonds

A polar covalent bond • occurs between nonmetal atoms• is an unequal sharing of electrons• has a moderate electronegativity difference

Examples: Electronegativity

Atoms Difference Type of Bond

O-Cl 3.5 3.0 = 0.5 Polar covalent

Cl-C 3.0 2.5 = 0.5 Polar covalent

O-S 3.5 2.5 = 1.0 Polar covalent

Ionic 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 Bond

Cl-K 3.0 – 0.8 = 2.2 Ionic

N-Na 3.0 – 0.9 = 2.1 Ionic

S-Cs 2.5 – 0.7 = 1.8 Ionic

Predicting Bond Types118

Learning Check

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. H-Cl

Solution

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

Difference Type of bond A. K-N 2.2 ionic (I)

B. N-O 0.5 polar covalent (P)C. Cl-Cl 0.0 nonpolar covalent (NP) D. H-Cl 0.9 polar covalent (P)

4.7Shapes and Polarity of Molecules

VSEPR Theory

Valence Shell Electron-Pair Repulsion Theory (VSEPR)

• describes the orientation of electron groups around the central atom• states that electron groups are arranged as far apart as possible around the central atom• states that specific shape of a molecule is determined by the number of atoms attached to the central atom

Central Atom with Two Electron Groups

In a molecule of BeCl2

• there are only two electron groups around the central atom, Be, which is an exception to octet rule

• repulsion is minimized by placing the two groups on opposite sides of the Be atom, giving this a linear arrangement with bond angles of 180°

• the shape with two electron groups around the central atom is linear

Central Atom with Three Electron Groups

In a molecule of BF3

• there are only three electron groups around the central atom, B, which is an exception to octet rule

• repulsion is minimized by placing the three groups as far apart as possible at bond angles of 120°

• the shape with three electron groups around the central atom is trigonal planar

Central Atom with Three Electron Groups

In a molecule of SO2

• there are three electron groups around the central atom S:

a single-bonded O atom, a double-bonded O atom, and a lone pair of electrons

• repulsion is minimized by placing the three groups as far apart as possible

• since one group is a lone pair, the shape is determined by the two O atoms bonded to S, giving the molecule its bent shape

Central Atom with Four Electron Groups

In a molecule of CH4

• there are four electron groups attached to H atoms around the central atom, C

• repulsion is minimized by placing the four groups at corners of a tetrahedron with bond angles of 109°

• the shape with four bonds on the central atom is called tetrahedral

In a molecule of NH3

• there are three electron groups attached to H atoms and a lone pair around the central atom, N

• the shape with three bonds on the central atom is called trigonal pyrimidal

In a molecule of H2O

• there are two electron groups attached to H atoms and two lone pairs around the central atom, O

• the shape with two bonds on the central atom is called bent

Molecular Shapes for Molecules129

Guide to Predicting Molecular Shape

Predict Molecular Shape of H2S

Predict the shape of H2S.

Step 1 Draw the electron-dot formula.

Step 2 Arrange electron groups around the central atom tominimize repulsion. To minimize repulsion, electron groups have a tetrahedral arrangement.

Step 3 Use the atoms bonded to the central atom determinethe molecular shape. With two bonds and two lone pairs, the shape is bent.

Learning Check

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

A. PF3

B. H2S

C. CCl4

Solution

Step 1 Draw the electron-dot formula.

A. PF3 B. H2S C. CCl4

Solution

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

Step 2 Arrange electron groups around the central atom tominimize repulsion.

A. PF3 To minimize repulsion electron groups have a tetrahedral arrangement.

B. H2S To minimize repulsion electron groups have a tetrahedral arrangement.

C. CCl4 To minimize repulsion electron groups have a tetrahedral arrangement.

Solution

Step 3 Use the atoms bonded to the central atom todetermine the molecular shape. A. PF3 With three bonds and one lone pair, the

shape is trigonal planar.

B. H2S With two bonds and two lone pairs, the shape is bent.

C. CCl4 With four bonds the shape is tetrahedral.

Nonpolar Molecules

Nonpolar molecules• such as H2, Cl2, and O2 are nonpolar because they contain

nonpolar bonds• with polar bonds such as CO2 can be nonpolar if the polar

bonds cancel each other in a symmetrical arrangement• dipoles cancel out, which makes the molecule nonpolar

Polar Molecules

Polar molecules such as HCl are polar• because one end of the molecule is more negatively

charged than the other• when polar bonds in the molecule do not cancel each

other• because the electrons are shared unequally in the polar

covalent bond

Polar Molecules

In polar molecules• with three or more atoms, the shape of the molecule determines

whether the dipoles cancel or not• there are often lone pairs around the central atom• such as H2O, the dipoles do not cancel, making the molecule

positive at one end and negative at the other end • such as H2O, there is a dipole on the central atom

Learning Check

Identify each of the following molecules as polar or nonpolar. Explain.

A. PBr3

B. HBr

C. CF4

Solution

Identify each of the following molecules as polar or nonpolar.

Explain.

Number Number

Bonds Lone Pairs Geometry

A. PBr3 3 1 trigonal pyrimidal

The dipoles do not cancel; the molecule is polar.

B. HBr 1 0 linear

The bond has a dipole; the molecule is polar.

C. CF4 4 0 tetrahedral

The bonds have dipoles; the dipoles cancel and the molecule is nonpolar.

4.8Attractive Forces in Compounds

Dipole-Dipole Attractions and Hydrogen Bonds

In covalent compounds, polar molecules • exert attractive forces called dipole-dipole attractions• form strong dipole attractions called hydrogen bonds between hydrogen atoms bonded to very electronegative atoms of F, O, or N

Hydrogen bonds are the strongest force betweenmolecules and play a major role in the shape of DNA.

Dispersion Forces

Dispersion forces are

• weak attractions between nonpolar molecules

• caused by temporary dipoles that develop when molecules bump into each other

• weak, but make it possible for nonpolar molecules to form liquids and solids

Comparison of Bonding and Attractive Forces

Melting Points and Attractive Forces

Melting points

• of compounds are related to the strength of attractive forces between molecules or compounds

• are lower due to weak forces such as dispersion forces

• are higher due to stronger attractive forces such as hydrogen bonding

• are highest in ionic compounds due to the strong attractive forces between ions in the compound

Melting Points of Selected Substances146

Learning Check

Identify the main type of attractive forces that are present in liquids of the following compounds: ionic bonds, dipole-dipole, hydrogen bonds, or dispersion forces.

A. NCl3

B. H2O

C. Br-BrD. KClE. NH3

Solution

Identify the main type of attractive forces that are present in liquids of the following compounds: ionic bonds, dipole-dipole, hydrogen bonds, or dispersion forces.

A. NCl3 dipole-dipole forcesB. H2O hydrogen bondsC. Br-Br dispersion forcesD. KCl ionic bondsE. NH3 hydrogen bonds

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