Introductory Chemistry, 3rd EditionNivaldo Tro : Roy Kennedy
Massachusetts Bay Community College
Wellesley Hills, MA Introductory Chemistry, 3rd EditionNivaldo Tro Chapter 10
Chemical
Bonding 2009, Prentice Hall
Slide 2 : The two teams are joined together because both are holding onto the same rope. In a similar way, two atoms are bonded together when both hold onto the same electrons. A covalent bond is a bond formed when atoms share electrons.
Bonding Theories : Tro's Introductory Chemistry, Chapter 10 3 Bonding Theories Central theme in chemistry: How and Why atoms attach together
This will help us understand how to:
Predict the shapes of molecules.
Predict properties of substances.
Design and build molecules with particular sets of chemical and physical properties.
Lewis Bonding Theory : Tro's Introductory Chemistry, Chapter 10 4 Lewis Bonding Theory Built on the idea of valence electrons.
Atoms share there valence electrons.
Gives atoms stablity.
Lewis Symbols of Atoms : Tro's Introductory Chemistry, Chapter 10 5 Lewis Symbols of Atoms Uses symbol of element to represent nucleus and inner electrons.
Uses dots around the symbol to represent valence electrons.
Puts one electron on each side first, then pair.
Remember that elements in the same group have the same number of valence electrons; therefore, their Lewis dot symbols will look alike.
Practice—Write the Lewis Symbol for Arsenic, Continued. : Tro's Introductory Chemistry, Chapter 10 6 Practice—Write the Lewis Symbol for Arsenic, Continued. As is in group 15, therefore it has 5 valence electrons.
Lewis Bonding Theory : Tro's Introductory Chemistry, Chapter 10 7 Lewis Bonding Theory Atoms ONLY come together for a single reason:
to produce a more stable electron configuration.
Atoms bond together by either transferring or sharing electrons.
A lot of atoms like to have 8 electrons in their outer shell.
Octet rule.
There are some exceptions to this rule—the key to remember is to try to get an electron configuration like a noble gas.
Li and Be try to achieve the He electron arrangement.
Lewis Symbols of Ions : Tro's Introductory Chemistry, Chapter 10 8 Lewis Symbols of Ions Cations have Lewis symbols without valence electrons.
Lost in the cation formation.
They now have a full “outer” shell that was the previous second highest energy shell.
Anions have Lewis symbols with 8 valence electrons.
Electrons gained in the formation of the anion.
Practice—Show How the Electrons Are Transferred and the Bond Is Formed When Na Reacts with S. : Tro's Introductory Chemistry, Chapter 10 9 Practice—Show How the Electrons Are Transferred and the Bond Is Formed When Na Reacts with S.
Practice—Show How the Electrons Are Transferred and the Bond Is Formed When Na Reacts with S, Continued. : Tro's Introductory Chemistry, Chapter 10 10 Practice—Show How the Electrons Are Transferred and the Bond Is Formed When Na Reacts with S, Continued.
Ionic Bonds : Tro's Introductory Chemistry, Chapter 10 11 Ionic Bonds Metal to nonmetal.
Metal loses electrons to form cation.
Nonmetal gains electrons to form anion.
Ionic bond results from + to − attraction.
Larger charge = stronger attraction.
Smaller ion = stronger attraction.
Lewis theory allows us to predict the correct formulas of ionic compounds.
Example 10.3—Using Lewis Theory to Predict Chemical Formulas of Ionic Compounds : Tro's Introductory Chemistry, Chapter 10 12 Example 10.3—Using Lewis Theory to Predict Chemical Formulas of Ionic Compounds Predict the formula of the compound that forms between
calcium and chlorine. Draw the Lewis dot symbols
of the elements. Transfer all the valance electrons
from the metal to the nonmetal,
adding more of each atom as you
go, until all electrons are lost
from the metal atoms and all
nonmetal atoms have 8 electrons. Ca2+ CaCl2
Practice—Use Lewis Symbols to Predict the Formula of an Ionic Compound Made from Reacting a Metal, M, that Has 2 Valence Electrons with a Nonmetal, X, that Has 5 Valence Electrons. : Tro's Introductory Chemistry, Chapter 10 13 Practice—Use Lewis Symbols to Predict the Formula of an Ionic Compound Made from Reacting a Metal, M, that Has 2 Valence Electrons with a Nonmetal, X, that Has 5 Valence Electrons.
Practice—Use Lewis Symbols to Predict the Formula of an Ionic Compound Made from Reacting a Metal, M, that Has 2 Valence Electrons with a Nonmetal, X, that Has 5 Valence Electrons, Continued. : Tro's Introductory Chemistry, Chapter 10 14 Practice—Use Lewis Symbols to Predict the Formula of an Ionic Compound Made from Reacting a Metal, M, that Has 2 Valence Electrons with a Nonmetal, X, that Has 5 Valence Electrons, Continued. M3X2
Covalent Bonds : Tro's Introductory Chemistry, Chapter 10 15 Covalent Bonds Often found between two nonmetals.
Typical of molecular species.
Atoms bonded together to form molecules.
Strong attraction.
Atoms share pairs of electrons to attain octets.
Molecules generally weakly attracted to each other.
Observed physical properties of molecular substance due to these attractions.
Using Lewis Atomic Structures to Predict Bonding Between Nonmetal Atoms : Using Lewis Atomic Structures to Predict Bonding Between Nonmetal Atoms Nonmetal atoms often bond to achieve an octet of valence electrons by sharing electrons.
Though there are many exceptions to the Octet rule.
In Lewis theory, atoms share electrons to complete their octet.
This may involve sharing electrons with multiple atoms or sharing multiple pairs of electrons with the same atom. Tro's Introductory Chemistry, Chapter 10 16
Single Covalent Bonds : Tro's Introductory Chemistry, Chapter 10 17 Single Covalent Bonds Two atoms share one pair of electrons.
2 electrons.
One atom may have more than one single bond. H H O •• •• •• •• H • H • O •• • • ••
Double Covalent Bond : Tro's Introductory Chemistry, Chapter 10 18 Double Covalent Bond Two atoms sharing two pairs of electrons.
4 electrons.
Shorter and stronger than single bond.
Triple Covalent Bond : Tro's Introductory Chemistry, Chapter 10 19 Triple Covalent Bond Two atoms sharing 3 pairs of electrons.
6 electrons.
Shorter and stronger than single or double bond.
Bonding and Lone Pair Electrons : Tro's Introductory Chemistry, Chapter 10 20 Bonding and Lone Pair Electrons Electrons that are shared by atoms are called bonding pairs.
Electrons that are not shared by atoms but belong to a particular atom are called lone pairs.
Also known as nonbonding pairs. O S O ••
•• •
• •
• •
• •• •• Lone pairs Bonding pairs
Multiplicity and Bond Properties : Multiplicity and Bond Properties The more electrons two atoms share, the stronger they are bonded together.
This explains the observation that triple bonds are stronger than similar double bonds, which are stronger than single bonds.
C≡N is stronger than C=N, C=N is stronger than C─N.
This explains the observation that triple bonds are shorter than similar double bonds, which are shorter than single bonds.
C≡N is shorter than C=N, C=N is shorter than C─N 21 Tro's Introductory Chemistry, Chapter 10
Trends in Bond Length and Energy : Trends in Bond Length and Energy
Polyatomic Ions : Tro's Introductory Chemistry, Chapter 10 23 Polyatomic Ions The polyatomic ions are attracted to opposite ions by ionic bonds.
Form crystal lattices.
Atoms in the polyatomic ion are held together by covalent bonds.
Lewis Formulas of Molecules : Tro's Introductory Chemistry, Chapter 10 24 Lewis Formulas of Molecules Shows patterns of valence electron distribution in the molecules.
Allows us to predict shapes of molecules.
Allows us to predict properties of molecules and how they will interact together.
Lewis Structures : Tro's Introductory Chemistry, Chapter 10 25 Lewis Structures Some common bonding patterns.
C = 4 bonds & 0 lone pairs.
4 bonds = 4 single, or 2 double, or single + triple, or 2 single + double.
N = 3 bonds & 1 lone pair.
O = 2 bonds & 2 lone pairs.
H and halogen = 1 bond.
Be = 2 bonds & 0 lone pairs.
B = 3 bonds & 0 lone pairs.
Writing Lewis Structuresfor Covalent Molecules : Tro's Introductory Chemistry, Chapter 10 26 Writing Lewis Structuresfor Covalent Molecules 1. Attach the atoms together in a skeletal structure.
Most metallic element is generally central.
In PCl3, the P is central because it is further left on the periodic table and therefore more metallic.
Halogens and hydrogen are generally terminal.
In C2Cl4, the Cs are attached together in the center and the Cls are surrounding them.
Many molecules tend to be symmetrical.
Though there are many exceptions to this, chemical formulas are often written to indicate the order of atom attachment.
In C2Cl4, there are two Cls on each C.
In oxyacids, the acid hydrogens are attached to an oxygen.
In H2SO4, the S is central, the Os are attached to the S, and each H is attached to a different O.
Writing Lewis Structuresfor Covalent Molecules, Continued : Tro's Introductory Chemistry, Chapter 10 27 Writing Lewis Structuresfor Covalent Molecules, Continued 2. Calculate the total number of valence electrons available for bonding.
Use group number of periodic table to find number of valence electrons for each atom.
If you have a cation, subtract 1 electron for each + charge.
If you have an anion, add 1 electron for each − charge. In PCl3, P has 5 e− and each Cl has 7 e− for a total of 26 e−. In ClO3−, Cl has 7 e− and each O has 6 e− for a total of 25 e−.
Add 1 e − for the negative charge to get a grand total of 26 e−
Slide 28 : 28 There are steps to apply when working with VSEPR models:
Step 1: Draw a basic skeleton keeping the North, South, East, and West positions of the atomic symbol in mind.
Step 2: Count the number of valence electrons.
Step 3: Place the electrons on the OUTSIDE first.
Step 4: Remaining electrons go on the central atom.
Step 5: Check for the Octet
Step 6: Check for Resonances Structures
Step 7: Predict electron pair shape name, the molecular shape name and bond angles.
TURN TO Lewis Structures in your Lab Manual
Example HNO3 : Tro's Introductory Chemistry, Chapter 10 29 Example HNO3 1. Write skeletal structure.
Since this is an oxyacid, H on outside attached to one of the Os; N is central. 2. Count valence electrons. N = 5
H = 1
O3 = 3∙6 = 18
Total = 24 e-
Example HNO3 , Continued : Tro's Introductory Chemistry, Chapter 10 30 Example HNO3 , Continued 3. Attach atoms with pairs of electrons and subtract from the total. Electrons
Start 24
Used 8
Left 16 N = 5
H = 1
O3 = 3∙6 = 18
Total = 24 e-
Example HNO3 , Continued : Tro's Introductory Chemistry, Chapter 10 31 Example HNO3 , Continued 4. Complete octets, outside-in.
H is already complete with 2.
1 bond.
Keep going until all atoms have an octet or you run out of electrons. N = 5
H = 1
O3 = 3∙6 = 18
Total = 24 e- Electrons
Start 24
Used 8
Left 16 Electrons
Start 16
Used 16
Left 0
Example HNO3 , Continued : Tro's Introductory Chemistry, Chapter 10 32 Example HNO3 , Continued 5. If central atom does not have octet, bring in electron pairs from outside atoms to share.
Follow common bonding patterns if possible.
Example 10.4—Writing Lewis Structures forCovalent Compounds : Example 10.4—Writing Lewis Structures forCovalent Compounds Tro's Introductory Chemistry, Chapter 10 33
Slide 34 : Tro's Introductory Chemistry, Chapter 10 34 Example 10.4:
Write the Lewis structure of CO2.
Example:Write the Lewis structure of CO2. : Tro's Introductory Chemistry, Chapter 10 35 Example:Write the Lewis structure of CO2. Write down the given quantity and its units.
Given: CO2
Example:Write the Lewis structure of CO2. : Tro's Introductory Chemistry, Chapter 10 36 Write down the quantity to find and/or its units.
Find: Lewis structure Information:
Given: CO2 Example:Write the Lewis structure of CO2.
Example:Write the Lewis structure of CO2. : Tro's Introductory Chemistry, Chapter 10 37 Design a solution map. Information:
Given: CO2
Find: Lewis structure Formula of compound Example:Write the Lewis structure of CO2. Skeletal structure Count and
distribute electrons Lewis structure
Example:Write the Lewis structure of CO2. : Tro's Introductory Chemistry, Chapter 10 38 Apply the solution map.
Write skeletal structure.
Least metallic atom central.
H terminal.
Symmetry. Information:
Given: CO2
Find: Lewis structure
Solution Map: formula → skeletal → electron distribution → Lewis Example:Write the Lewis structure of CO2.
Example:Write the Lewis structure of CO2. : Tro's Introductory Chemistry, Chapter 10 39 Apply the solution map.
Count and distribute the valence electrons.
Count valence electrons. Information:
Given: CO2
Find: Lewis structure
Solution Map: formula → skeletal → electron distribution → Lewis Example:Write the Lewis structure of CO2. C O 1A 2A 3A 4A 5A 6A 7A 8A
Example:Write the Lewis structure of CO2. : Tro's Introductory Chemistry, Chapter 10 40 Apply the solution map.
Count and distribute the valence electrons.
Attach atoms. Information:
Given: CO2
Find: Lewis structure
Solution Map: formula → skeletal → electron distribution → Lewis Example:Write the Lewis structure of CO2.
Example:Write the Lewis structure of CO2. : Tro's Introductory Chemistry, Chapter 10 41 Apply the solution map.
Count and distribute the valence electrons.
Complete octets.
Outside atoms first. Information:
Given: CO2
Find: Lewis structure
Solution Map: formula → skeletal → electron distribution → Lewis Example:Write the Lewis structure of CO2.
Example:Write the Lewis structure of CO2. : Tro's Introductory Chemistry, Chapter 10 42 Apply the solution map.
Count and distribute the valence electrons.
Complete octets.
If not enough electrons to complete octet of central atom, bring in pairs of electrons from attached atom to make multiple bonds. Information:
Given: CO2
Find: Lewis structure
Solution Map: formula → skeletal → electron distribution → Lewis Example:Write the Lewis structure of CO2.
Example:Write the Lewis structure of CO2. : Tro's Introductory Chemistry, Chapter 10 43 Check: Information:
Given: CO2
Find: Lewis structure
Solution Map: formula → skeletal → electron distribution → Lewis Example:Write the Lewis structure of CO2. The skeletal structure is symmetrical.
All the electrons are accounted for.
Writing Lewis Structures forPolyatomic Ions : Tro's Introductory Chemistry, Chapter 10 44 Writing Lewis Structures forPolyatomic Ions The procedure is the same, the only difference is in counting the valence electrons.
For polyatomic cations, take away one electron from the total for each positive charge.
For polyatomic anions, add one electron to the total for each negative charge.
Example NO3─ : Tro's Introductory Chemistry, Chapter 10 45 Example NO3─ 1. Write skeletal structure.
N is central because it is the most metallic. 2. Count valence electrons. N = 5
O3 = 3∙6 = 18
(-) = 1
Total = 24 e-
Example NO3─ , Continued : Example NO3─ , Continued 3. Attach atoms with pairs of electrons and subtract from the total. N = 5
O3 = 3∙6 = 18
(-) = 1
Total = 24 e- Electrons
Start 24
Used 6
Left 18 Tro's Introductory Chemistry, Chapter 10 46
Example NO3─ , Continued : Tro's Introductory Chemistry, Chapter 10 47 Example NO3─ , Continued 3. Complete octets, outside-in.
Keep going until all atoms have an octet or you run out of electrons. N = 5
O3 = 3∙6 = 18
(-) = 1
Total = 24 e- Electrons
Start 24
Used 6
Left 18 Electrons
Start 18
Used 18
Left 0
Example NO3─ , Continued : Tro's Introductory Chemistry, Chapter 10 48 Example NO3─ , Continued 5. If central atom does not have octet, bring in electron pairs from outside atoms to share.
Follow common bonding patterns if possible.
Practice—Lewis Structures : Practice—Lewis Structures NClO
H3BO3
NO2-1 H3PO4
SO3-2
P2H4 Tro's Introductory Chemistry, Chapter 10 49
Practice—Lewis Structures, Continued : Practice—Lewis Structures, Continued NClO
H3BO3
NO2-1 H3PO4
SO3-2
P2H4 18 e- 26 e- 32 e- 14 e- 24 e- 18 e- Tro's Introductory Chemistry, Chapter 10 50
Exceptions to the Octet Rule : Tro's Introductory Chemistry, Chapter 10 51 Exceptions to the Octet Rule H and Li, lose one electron to form cation.
Li now has electron configuration like He.
H can also share or gain one electron to have configuration like He.
Be shares two electrons to form two single bonds.
B shares three electrons to form three single bonds.
Expanded octets for elements in Period 3 or below.
Using empty valence d orbitals.
Some molecules have odd numbers of electrons.
NO
Resonance : Tro's Introductory Chemistry, Chapter 10 52 Resonance We can often draw more than one valid Lewis structure for a molecule or ion.
In other words, no one Lewis structure can adequately describe the actual structure of the molecule.
The actual molecule will have some characteristics of all the valid Lewis structures we can draw.
Resonance, Continued : Tro's Introductory Chemistry, Chapter 10 53 Resonance, Continued Lewis structures often do not accurately represent the electron distribution in a molecule.
Lewis structures imply that O3 has a single (147 pm) and double (121 pm) bond, but actual bond length is between (128 pm).
Real molecule is a hybrid of all possible Lewis structures.
Resonance stabilizes the molecule.
Maximum stabilization comes when resonance forms contribute equally to the hybrid.
Drawing Resonance Structures : Tro's Introductory Chemistry, Chapter 10 54 Drawing Resonance Structures Draw first Lewis structure that maximizes octets.
Move electron pairs from outside atoms to share with central atoms.
If central atoms, 2nd row, only move in electrons, you can move out electron pairs from multiple bonds.
Practice—Draw Lewis Resonance Structures for CNO−(C Is Central with N and O Attached) : Practice—Draw Lewis Resonance Structures for CNO−(C Is Central with N and O Attached) Tro's Introductory Chemistry, Chapter 10 55
Practice—Draw Lewis Resonance Structures for CNO−(C Is Central with N and O Attached), Continued : Practice—Draw Lewis Resonance Structures for CNO−(C Is Central with N and O Attached), Continued C = 4
N = 5
O = 6
(-) = 1
Total = 16 e- Tro's Introductory Chemistry, Chapter 10 56
Molecular Geometry : Tro's Introductory Chemistry, Chapter 10 57 Molecular Geometry Molecules are three-dimensional objects.
We often describe the shape of a molecule with terms that relate to geometric figures.
These geometric figures have characteristic “corners” that indicate the positions of the surrounding atoms with the central atom in the center of the figure.
The geometric figures also have characteristic angles that we call bond angles.
Slide 58 : Practice drawing these shapes below Linear TP Tetra TBP Octa Electron Pairs
Slide 59 : Prentice Hall © 2007 59 Linear molecules have bond angles of 180°.
Planar triangular molecules have bond angles of 120°.
Tetrahedral molecules have bond angles of 109.5°. Students always forget that tetrahedral
is not flat! Please do not forget.
Slide 60 : Predict the molecular geometry of acetaldehyde. One carbon on acetaldehyde is connected to four other atoms through bonds, resulting in a tetrahedral arrangement. The other carbon has only three bonds, so the arrangement around it is planar triangular. Therefore the overall structure is as shown.
VSEPR Theory : VSEPR Theory Electron groups around the central atom will be most stable when they are as far apart as possible. We call this valence shell electron pair repulsion theory.
Since electrons are negatively charged, they should be most stable when they are separated.
The resulting geometric arrangement will allow us to predict the shapes and bond angles in the molecule. Tro's Introductory Chemistry, Chapter 10 61
Electron Groups : Electron Groups The Lewis structure predicts the arrangement of valence electrons around the central atom(s).
Each lone pair of electrons constitutes one electron group on a central atom.
Each bond constitutes one electron group on a central atom.
Regardless of whether it is single, double, or triple. Tro's Introductory Chemistry, Chapter 10 62
Linear Geometry : Linear Geometry When there are two electron groups around the central atom, they will occupy positions opposite each other around the central atom.
This results in the molecule taking a linear geometry.
The bond angle is 180°. Tro's Introductory Chemistry, Chapter 10 63
Trigonal Geometry : Trigonal Geometry When there are three electron groups around the central atom, they will occupy positions in the shape of a triangle around the central atom.
This results in the molecule taking a trigonal planar geometry.
The bond angle is 120°. Tro's Introductory Chemistry, Chapter 10 64
Tetrahedral Geometry : Tetrahedral Geometry When there are four electron groups around the central atom, they will occupy positions in the shape of a tetrahedron around the central atom.
This results in the molecule taking a tetrahedral geometry.
The bond angle is 109.5°. Tro's Introductory Chemistry, Chapter 10 65
Sketching a Molecule : Sketching a Molecule Because molecules are three-dimensional objects, our drawings should indicate their three-dimensional quality
By convention:
A filled wedge indicates that the attached atom is coming out of the paper toward you.
A dashed wedge indicates that the attached atom is going behind the paper away from you. Tro's Introductory Chemistry, Chapter 10 66
Sketching a Molecule, Continued : Sketching a Molecule, Continued Tro's Introductory Chemistry, Chapter 10 67
Derivative Shapes : Derivative Shapes The molecule’s shape will be one of basic molecular geometries if all the electron groups are bonds and all the bonds are equivalent.
Molecules with lone pairs or different kinds of surrounding atoms will have distorted bond angles and different bond lengths, but the shape will be a derivative of one of the basic shapes. Tro's Introductory Chemistry, Chapter 10 68
Derivative of Trigonal Geometry : Derivative of Trigonal Geometry When there are three electron groups around the central atom, and one of them is a lone pair, the resulting shape of the molecule is called a bent shape.
The bond angle is < 120°. 69
Derivatives of Tetrahedral Geometry : Derivatives of Tetrahedral Geometry When there are four electron groups around the central atom, and one is a lone pair, the result is called a Trigional pyramidal shape.
Because it is a triangular-base pyramid with the central atom at the apex.
When there are four electron groups around the central atom, and two are lone pairs, the result is called a tetrahedral–bent shape.
It is planar.
It looks similar to the trigonal planar bent shape, except the angles are smaller.
For both shapes, the bond angle is < 109.5°. Tro's Introductory Chemistry, Chapter 10 70
Tetrahedral Derivatives : Tro's Introductory Chemistry, Chapter 10 71 Tetrahedral Derivatives
Molecular Geometry: Linear : Tro's Introductory Chemistry, Chapter 10 72 Molecular Geometry: Linear Electron groups rround central atom = 2.
Bonding groups = 2.
Lone pairs = 0.
Electron geometry = linear.
Angle between electron groups = 180°.
Slide 73 : 73 Molecular Geometry: Trigonal Planar Electron groups around central atom = 3.
Bonding groups = 3.
Lone pairs = 0.
Electron geometry = trigonal planar.
Angle between electron groups = 120°.
Slide 74 : Tro's Introductory Chemistry, Chapter 10 74 Molecular Geometry: Bent Electron groups around central atom = 3.
Bonding groups = 2.
Lone pairs = 1.
Electron geometry = trigonal planar.
Angle between electron groups = 120°.
Slide 75 : 75 Molecular Geometry: Tetrahedral Electron groups around central atom = 4.
Bonding groups = 4.
Lone pairs = 0.
Electron geometry = tetrahedral.
Angle between electron groups = 109.5°.
Slide 76 : 76 Molecular Geometry: Trigonal Pyramid Electron groups around central atom = 4.
Bonding groups = 3.
Lone pairs = 1.
Electron geometry = tetrahedral.
Angle between electron groups = 109.5°.
Slide 77 : Tro's Introductory Chemistry, Chapter 10 77 Molecular Geometry: Bent Electron groups around central atom = 4.
Bonding groups = 2.
Lone pairs = 2.
Electron geometry = tetrahedral.
Angle between electron groups = 109.5°.
Slide 78 : 78
Predicting the Molecular Shapes Around Central Atoms : Predicting the Molecular Shapes Around Central Atoms 1. Draw the Lewis structure.
2. Determine the number of electron groups around the central atom.
3. Classify each electron group as bonding or lone pair, and count each type.
Remember: Multiple bonds count as one group.
4. Use the previous slide’s table to determine the shape and bond angles. Tro's Introductory Chemistry, Chapter 10 79
Practice—Predict the Molecular Geometry Around the Central Atom : Practice—Predict the Molecular Geometry Around the Central Atom ClO2−
H3BO3
NO2-1 H3PO4
SO32−
P2H4 Tro's Introductory Chemistry, Chapter 10 80
Practice—Predict the Molecular Geometry Around the Central Atom, Continued : Practice—Predict the Molecular Geometry Around the Central Atom, Continued ClO2−
H3BO3
NO2-1 H3PO4
SO32−
P2H4 Tro's Introductory Chemistry, Chapter 10 81 Tetr.
bent Tetrahedral Trigonal Trig.
bent Trig.
pyramidal Trig.
pyramidal
Bond Polarity : Tro's Introductory Chemistry, Chapter 10 82 Bond Polarity Bonding between unlike atoms results in unequal sharing of the electrons.
One atom pulls the electrons in the bond closer to its side.
One end of the bond has larger electron density than the other.
The result is bond polarity.
The end with the larger electron density gets a partial negative charge and the end that is electron deficient gets a partial positive charge.
Electronegativity : 83 Electronegativity Measure of the pull an atom has on bonding electrons.
Increases across the period (left to right).
Decreases down the group (top to bottom).
The larger the difference in electronegativity, the more polar the bond.
Negative end toward more electronegative atom.
Electronegativity, Continued : Tro's Introductory Chemistry, Chapter 10 84 Electronegativity, Continued
Electronegativity, Continued : Tro's Introductory Chemistry, Chapter 10 85 Electronegativity, Continued
Electronegativity and Bond Polarity : Electronegativity and Bond Polarity If the difference in electronegativity between bonded atoms is 0 to 0.3, the bond is pure covalent.
If the difference in electronegativity between bonded atoms 0.4 to 1.9, the bond is polar covalent.
If the difference in electronegativity between bonded atoms larger than or equal to 2.0, the bond is ionic. Tro's Introductory Chemistry, Chapter 10 86
Bond Polarity : Tro's Introductory Chemistry, Chapter 10 87 Bond Polarity 0 0.4 2.0 4.0 Electronegativity difference Covalent Ionic Polar Pure 3.0-3.0
= 0.0 4.0-2.1
= 1.9 3.0-0.9
= 2.1
Dipole Moments : Tro's Introductory Chemistry, Chapter 10 88 Dipole Moments A dipole is a material with positively and negatively charged ends.
Polar bonds or molecules have one end slightly positive, d+, and the other slightly negative, d-.
Not “full” charges, come from nonsymmetrical electron distribution.
Dipole moment, m, is a measure of the size of the polarity.
Measured in debyes, D.
For Each of the Following Bonds, Determine Whether the Bond Is Ionic or Covalent. If Covalent, Determine if It Is Polar or Pure. If Polar, Indicate the Direction of the Dipole. : For Each of the Following Bonds, Determine Whether the Bond Is Ionic or Covalent. If Covalent, Determine if It Is Polar or Pure. If Polar, Indicate the Direction of the Dipole. Pb-O
P-S
Mg-Cl
H-O
For Each of the Following Bonds, Determine Whether the Bond Is Ionic or Covalent. If Covalent, Determine if It Is Polar or Pure. If Polar, Indicate the Direction of the Dipole, Continued. : For Each of the Following Bonds, Determine Whether the Bond Is Ionic or Covalent. If Covalent, Determine if It Is Polar or Pure. If Polar, Indicate the Direction of the Dipole, Continued. Pb-O (3.5 - 1.9) = 1.6 \ polar covalent.
P-S (2.5 - 2.1) = 0.4 \ pure covalent.
Mg-Cl (3.0 - 1.2) = 1.8 \ ionic.
H-O (3.5 - 2.1) = 1.4 \ polar covalent.
Polarity of Molecules : Tro's Introductory Chemistry, Chapter 10 91 Polarity of Molecules In order for a molecule to be polar it must:
1. Have polar bonds.
Electronegativity difference—theory.
Bond dipole moments—measured.
2. Have an unsymmetrical shape.
Vector addition.
Polarity effects the intermolecular forces of attraction.
Molecule Polarity : Molecule Polarity The O—C bond is polar. The bonding electrons are pulled equally toward both O ends of the molecule. The net result is a nonpolar molecule. Tro's Introductory Chemistry, Chapter 10 92
Molecule Polarity, Continued : Molecule Polarity, Continued The H—O bond is polar. Both sets of bonding electrons are pulled toward the O end of the molecule. The net result is a polar molecule. Tro's Introductory Chemistry, Chapter 10 93
Polar Covalent Bonds and Electronegativity : Polar Covalent Bonds and Electronegativity Prentice Hall © 2007 Chapter Five 94 This is a picture (EPM) of a chloromethane. The red area is a high concentration of electrons, and blue means low concentration of electrons.
Polar Covalent Bonds and Electronegativity : Polar Covalent Bonds and Electronegativity What causes the electrons to congregate onto different atoms? Prentice Hall © 2007 Chapter Five 95
Slide 96 : Chapter Five 96 Atoms like chlorine want electrons more then hydrogen. As chlorine pulls electrons away from they hydrogen atom, a charge builds up on the chlorine. Instead of sharing electrons-equally-the molecule begins to look, like an ionic compound. The chemistry term is the molecule has a dipole moment and is said to be polar; just like earth has a North and South Pole.
Ultimately : Ultimately Molecules can attract each other. This may not seem like much, but this is how DNA is held together. This helps scientists Prentice Hall © 2007 Chapter Five 97 explain the differences in melting point, boiling point, as well as other physical properties.
So how do we predict this behavior?
DipoleMoment : Tro's Introductory Chemistry, Chapter 10 98 CH2Cl2
m = 2.0 D CCl4
m = 0.0 D DipoleMoment
Adding Dipole Moments : 99 Adding Dipole Moments
Slide 100 : Prentice Hall © 2007 Chapter Five 100 Dipoles or polarity can be represented by an arrow pointing to the negative end of the molecule with a cross at the positive end resembling a + sign.
Notice: the molecules have a “Pole,” like a North and South Pole.
Slide 101 : 101 Just because a molecule has polar covalent bonds does not mean that the molecule is polar overall.
Carbon dioxide and tetrachloromethane molecules have no net polarity because their symmetrical shapes cause the individual bond polarities to cancel each other out.
Notice: these do Not have a North/South Pole
Example 10.11—Determining if a Molecule Is Polar : Example 10.11—Determining if a Molecule Is Polar Tro's Introductory Chemistry, Chapter 10 102
Slide 103 : Tro's Introductory Chemistry, Chapter 10 103 Example 10.11:
Determine if NH3 is polar.
Example:Determine if NH3 is polar. : Tro's Introductory Chemistry, Chapter 10 104 Example:Determine if NH3 is polar. Write down the given quantity and its units.
Given: NH3
Example:Determine if NH3 is polar. : Tro's Introductory Chemistry, Chapter 10 105 Example:Determine if NH3 is polar. Write down the quantity to find and/or its units.
Find: If polar Information:
Given: NH3
Example:Determine if NH3 is polar. : Tro's Introductory Chemistry, Chapter 10 106 Example:Determine if NH3 is polar. Design a solution map. Information:
Given: NH3
Find: If polar Formula of compound Lewis structure Molecular polarity
Example:Determine if NH3 is polar. : Tro's Introductory Chemistry, Chapter 10 107 Example:Determine if NH3 is polar. Apply the solution map.
Draw the Lewis structure.
Write skeletal structure. Information:
Given: NH3
Find: If polar
Solution Map:
formula → Lewis → polarity and shape → molecule polarity
Example:Determine if NH3 is polar. : Tro's Introductory Chemistry, Chapter 10 108 Example:Determine if NH3 is polar. Apply the solution map.
Draw the Lewis structure.
Count valence electrons. Information:
Given: NH3
Find: If polar
Solution Map:
formula → Lewis → polarity and shape → molecule polarity
Example:Determine if NH3 is polar. : Tro's Introductory Chemistry, Chapter 10 109 Example:Determine if NH3 is polar. Apply the solution map.
Draw the Lewis structure.
Attach atoms. Information:
Given: NH3
Find: If polar
Solution Map:
formula → Lewis → polarity and shape → molecule polarity Start 8 e-
Used 6 e-
Left 2 e-
Example:Determine if NH3 is polar. : Tro's Introductory Chemistry, Chapter 10 110 Example:Determine if NH3 is polar. Apply the solution map.
Draw the Lewis structure.
Complete octets. Information:
Given: NH3
Find: If polar
Solution Map:
formula → Lewis → polarity and shape → molecule polarity Start 2 e-
Used 2 e-
Left 0 e- ∙∙
Example:Determine if NH3 is polar. : Tro's Introductory Chemistry, Chapter 10 111 Example:Determine if NH3 is polar. Apply the solution map.
Determine if bonds are polar. Information:
Given: NH3
Find: If polar
Solution Map:
formula → Lewis → polarity and shape → molecule polarity ∙∙ Electronegativity
N = 3.0
H = 2.1 3.0 – 2.1 = 0.9
polar covalent
Example:Determine if NH3 is polar. : Tro's Introductory Chemistry, Chapter 10 112 Example:Determine if NH3 is polar. Apply the solution map.
Determine shape of molecule. Information:
Given: NH3
Find: If polar
Solution Map:
formula → Lewis → polarity and shape → molecule polarity 4 areas of electrons
around N;
3 bonding areas
1 lone pair Shape = trigonal pyramid
Example:Determine if NH3 is polar. : Tro's Introductory Chemistry, Chapter 10 113 Example:Determine if NH3 is polar. Apply the solution map.
Determine molecular polarity. Information:
Given: NH3
Find: If polar
Solution Map:
formula → Lewis → polarity and shape → molecule polarity Bonds = polar
Shape = trigonal pyramid Molecule = polar
Example:Determine if NH3 is polar. : Tro's Introductory Chemistry, Chapter 10 114 Example:Determine if NH3 is polar. Check: Information:
Given: NH3
Find: If polar
Solution Map:
formula → Lewis → polarity and shape → molecule polarity Bonds = polar
Shape = trigonal pyramid Molecule = polar ∙∙ The Lewis structure
is correct. The bonds
are polar and the
shape is unsymmetrical,
so the molecule should be polar.
Practice—Decide Whether Each of the Following Molecules Is Polar : Practice—Decide Whether Each of the Following Molecules Is Polar EN
O = 3.5
N = 3.0
Cl = 3.0
S = 2.5 Tro's Introductory Chemistry, Chapter 10 115
Practice—Decide Whether the Each of the Following Molecules Is Polar, Continued : Practice—Decide Whether the Each of the Following Molecules Is Polar, Continued Polar Nonpolar 1. Polar bonds, N—O
2. Asymmetrical shape 1. Polar bonds, all S—O
2. Symmetrical shape Trigonal
bent Trigonal
planar 2.5 Tro's Introductory Chemistry, Chapter 10 116
Molecular Polarity Affects Solubility in Water : Molecular Polarity Affects Solubility in Water Polar molecules are attracted to other polar molecules.
Since water is a polar molecule, other polar molecules dissolve well in water.
And ionic compounds as well.
Some molecules have both polar and nonpolar parts. Tro's Introductory Chemistry, Chapter 10 117
Slide 118 : Prentice Hall © 2007 Chapter Five 118 In HCl, electrons spend more time near the chlorine than the hydrogen. Although the molecule is overall neutral, the chlorine is more negative than the hydrogen, resulting in partial charges on the atoms.
Partial charges are represented by a d- on the more negative atom and d+ on the more positive atom.
The ability of an atom to attract electrons is called the atom’s electronegativity.