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Chemical Bonding

Chemical Bonding
Chapter 10 Chemical BondingThe 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. www.ThesisScientist.comBonding Theories • Central theme in chemistry: How and Why atoms attach together • This will help us understand how to: 1. Predict the shapes of molecules. 2. Predict properties of substances. 3. Design and build molecules with particular sets of chemical and physical properties. www.ThesisScientist.comLewis Bonding Theory • Built on the idea of valence electrons. • Atoms share there valence electrons. • Gives atoms stablity. www.ThesisScientist.comLewis 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. • •• •• •• •• Li• Be• •B• •C• •N• •O: :F: :Ne: • • • • • • •• www.ThesisScientist.comPractice—Write the Lewis Symbol for Arsenic, Continued. • As is in group 15, therefore it has 5 valence electrons.   As  www.ThesisScientist.comLewis 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. www.ThesisScientist.comLewis 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. •• •• +− Li• Li :F: :F: • •• www.ThesisScientist.comPractice—Show How the Electrons Are Transferred and the Bond Is Formed When Na Reacts with S. www.ThesisScientist.comPractice—Show How the Electrons Are Transferred and the Bond Is Formed When Na Reacts with S, Continued.  2 Na  2 Na  2 e 2     S  2 e  S      www.ThesisScientist.comIonic 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. www.ThesisScientist.comExample 10.3—Using Lewis Theory to Predict Chemical Formulas of Ionic Compounds Predict the formula of the compound that forms between calcium and chlorine. Ca Cl ∙ ∙ Draw the Lewis dot symbols ∙ ∙ ∙ of the elements. Transfer all the valance electrons Ca Cl Cl ∙ ∙ ∙ ∙ ∙ ∙ ∙ from the metal to the nonmetal, ∙           adding more of each atom as you 2+     Ca : Cl : : Cl : go, until all electrons are lost             from the metal atoms and all nonmetal atoms have 8 electrons. CaCl 2 www.ThesisScientist.com ∙ ∙ ∙ ∙ ∙ ∙ ∙ ∙ ∙ ∙ ∙ ∙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. www.ThesisScientist.comPractice—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.   X M   M     X M  M X 3 2  www.ThesisScientist.comCovalent 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. www.ThesisScientist.comUsing 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. www.ThesisScientist.com•• • •• •• Single Covalent Bonds • Two atoms share one pair of electrons.  2 electrons. • One atom may have more than one single bond. •• •• •• F • • F H H O • • • •• •• •• •• •• •• F F H H O •• •• •• F F www.ThesisScientist.com •• •• •• ••• • Double Covalent Bond • Two atoms sharing two pairs of electrons. 4 electrons. • Shorter and stronger than single bond. •• •• O O • • •• •• •• O O •• O O www.ThesisScientist.com• • Triple Covalent Bond • Two atoms sharing 3 pairs of electrons. 6 electrons. • Shorter and stronger than single or double bond. •• •• N N • • • • •• N N •• •• N N www.ThesisScientist.comBonding 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. •• •• •• • •• • • Bonding pairs Lone pairs O S O • •• • • •• www.ThesisScientist.comMultiplicity 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 Trww o's w In.tr Todu hesic stor Sci y e n C th ise t. m co ism try, Chapter 10Trends in Bond Length and Energy Bond Length Energy (pm) (kJ/mol) CC 154 346 C=C 134 602 CC 120 835 CN 147 305 C=N 128 615 CN 116 887 CO 143 358 www.ThesisScientist.com C=O 123 799Polyatomic 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. www.ThesisScientist.comLewis 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. www.ThesisScientist.comLewis 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. C B N O F www.ThesisScientist.comWriting Lewis Structures for Covalent Molecules 1. Attach the atoms together in a skeletal structure.  Most metallic element is generally central.  In PCl , the P is central because it is further left on the periodic 3 table and therefore more metallic.  Halogens and hydrogen are generally terminal.  In C Cl , the Cs are attached together in the center and the Cls are 2 4 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 C Cl , there are two Cls on each C. 2 4 : Cl :  In oxyacids, the acid hydrogens are attached to an oxygen.   In H SO , the S is central, the Os are attached to the S, and each H 2 4 : Cl — C C — Cl : is attached to a different O.  : Cl : www.ThesisScientist.com Writing Lewis Structures for 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 PCl , P has 5 e and each Cl has 7 e for a total of 26 e . 3 − − − −  In ClO , Cl has 7 e and each O has 6 e for a total of 25 e . 3 − −  Add 1 e for the negative charge to get a grand total of 26 e www.ThesisScientist.comThere 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 www.ThesisScientist.comExample HNO 3 1. Write skeletal structure. O  Since this is an oxyacid, H on outside attached to one of the H O N O Os; N is central. 2. Count valence electrons. N = 5 H = 1 O = 3∙6 = 18 3 Total = 24 e www.ThesisScientist.comExample HNO , Continued 3 3. Attach atoms with pairs of O electrons and subtract from  H— O— N— O the total. N = 5 Electrons H = 1 Start 24 O = 3∙6 = 18 Used 8 3 Total = 24 e Left 16 Trww o's w In.tr Todu hesic stor Sci y e n C th ise t. m co ism try, 30 Chapter 10Example HNO , Continued 3 4. Complete octets, outsidein.  O : O :  H is already complete with 2.    1 bond. H— O— N— O H— O— N— O :  Keep going until all atoms  have an octet or you run out of electrons. N = 5 Electrons Electrons H = 1 Start 24 Start 16 O = 3∙6 = 18 Used 8 Used 16 3 Total = 24 e Left 16 Left 0 www.ThesisScientist.comExample HNO , Continued 3  5. If central atom does not have : O : octet, bring in electron pairs  from outside atoms to share. H— O— N— O :  Follow common bonding patterns  if possible.  : O :   H— O— N O :  www.ThesisScientist.comExample 10.4—Writing Lewis Structures for Covalent Compounds Tro's Introductory Chemistry, 33 Chapter 10Example 10.4: • Write the Lewis structure of CO . 2 www.ThesisScientist.comInformation: Example: Given: CO 2 Write the Lewis structure of Find: Lewis structure CO . 2 Solution Map: formula → skeletal → electron distribution → Lewis • Apply the solution map. Write skeletal structure. Least metallic atom central. H terminal. Symmetry. O— C— O www.ThesisScientist.comInformation: Example: Given: CO 2 Write the Lewis structure of Find: Lewis structure CO . 2 Solution Map: formula → skeletal → electron distribution → Lewis • Apply the solution map.  Count and distribute the valence electrons. Count valence electrons. O— C— O 1A 8A 3A4A5A6A7A 2A C = 4 C O O = 2 ∙ 6 Total CO = 16 2 www.ThesisScientist.comInformation: Example: Given: CO 2 Write the Lewis structure of Find: Lewis structure CO . 2 Solution Map: formula → skeletal → electron distribution → Lewis • Apply the solution map.  Count and distribute the valence electrons. Attach atoms. C = 4 O = 2 ∙ 6 Total CO = 16 2 O— C— O Start = 16 e Used = 4 e Left = 12 e www.ThesisScientist.comInformation: Example: Given: CO 2 Write the Lewis structure of Find: Lewis structure CO . 2 Solution Map: formula → skeletal → electron distribution → Lewis • Apply the solution map. C = 4  Count and distribute the valence electrons. O = 2 ∙ 6 Total CO = 16 Complete octets. 2 – Outside atoms first. Start = 16 e Used = 4 e  Left = 12 e : O O—— C C—— O O : Start = 12 e  Used = 12 e Left = 0 e www.ThesisScientist.comInformation: Example: Given: CO 2 Write the Lewis structure of Find: Lewis structure CO . 2 Solution Map: formula → skeletal → electron distribution → Lewis • 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.  Start = 12 e : O— C— O : : O C O : Used = 12 e  Left = 0 e www.ThesisScientist.comInformation: Example: Given: CO 2 Write the Lewis structure of Find: Lewis structure CO . 2 Solution Map: formula → skeletal → electron distribution → Lewis • Check: Start  C = 4 e : O C O : O = 2 ∙ 6 e Total CO = 16 e 2 The skeletal structure is symmetrical. All the electrons are accounted for. End Bonding = 4 ∙ 2 e Lone pairs = 4 ∙ 2 e Total CO = 16 e 2 www.ThesisScientist.comWriting Lewis Structures for Polyatomic 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. www.ThesisScientist.com─ Example NO 3 1. Write skeletal structure. O  N is central because it is the most metallic. O N O 2. Count valence electrons. N = 5 O = 3∙6 = 18 3 () = 1 Total = 24 e www.ThesisScientist.com─ Example NO , Continued 3 3. Attach atoms with pairs of O electrons and subtract from  the total. O— N— O Electrons N = 5 Start 24 O = 3∙6 = 18 3 Used 6 () = 1 Left 18 Total = 24 e Trww o's w In.tr Todu hesic stor Sci y e n C th ise t. m co ism try, 46 Chapter 10─ Example NO , Continued 3 3. Complete octets, outsidein.   Keep going until all atoms : O : have an octet or you run out  of electrons. : O— N— O :  N = 5 Electrons Electrons O = 3∙6 = 18 Start 24 Start 18 3 () = 1 Used 6 Used 18 Total = 24 e Left 18 Left 0 www.ThesisScientist.com─ Example NO , Continued 3  5. If central atom does not have : O : octet, bring in electron pairs  from outside atoms to share. : O— N— O :  Follow common bonding patterns  if possible.  : O :   : O— N O :  www.ThesisScientist.comPractice—Lewis Structures • NClO • H PO 3 4 2 • H BO • SO 3 3 3 1 • NO • P H 2 2 4 Trww o's w In.tr Todu hesic stor Sci y e n C th ise t. m co ism try, 49 Chapter 10Practice—Lewis Structures, Continued •• • • O • • • NClO • H PO 3 4 •• •• H O P O H •• •• •• •• •• 32 e • • 18 e O N Cl • • • O H •• • •• 2 •• • H BO • SO 3 3 3 • • •• O • • • O H • •• •• 24 e 26 e •• •• • • O S O • • H O B O H •• •• •• •• •• 1 • NO • P H 2 2 4 H H •• •• •• • • O N O 18 e 14 e • • H P P H •• •• •• Trww o's w In.tr Todu hesic stor Sci y e n C th ise t. m co ism try, 50 Chapter 10Exceptions 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 : N O : www.ThesisScientist.comResonance • 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. www.ThesisScientist.comResonance, Continued • Lewis structures often do not accurately represent the electron distribution in a molecule.  Lewis structures imply that O has a single (147 pm) and 3 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. + + O O O O O O www.ThesisScientist.comDrawing Resonance Structures 1. Draw first Lewis structure that ·· · · O maximizes octets. · · ·· 2. Move electron pairs from O N O · · · · outside atoms to share with ·· ·· central atoms. ·· nd 3. If central atoms, 2 row, only · · O · · move in electrons, you can ·· ·· · O N O move out electron pairs from · ·· ·· multiple bonds. www.ThesisScientist.comPractice—Draw Lewis Resonance − Structures for CNO (C Is Central with N and O Attached) Trww o's w In.tr Todu hesic stor Sci y e n C th ise t. m co ism try, 55 Chapter 10Practice—Draw Lewis Resonance − Structures for CNO (C Is Central with N and O Attached), Continued C = 4 N = 5 O = 6 •• •• •• • • • • () = 1 N C O N C O • • • • Total = 16 e •• Trww o's w In.tr Todu hesic stor Sci y e n C th ise t. m co ism try, 56 Chapter 10Molecular Geometry • Molecules are threedimensional 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. www.ThesisScientist.comElectron Pairs Practice drawing these shapes below Linear TP Tetra TBP Octa www.ThesisScientist.com• 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. www.ThesisScientist.comPredict 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. www.ThesisScientist.comVSEPR 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. Trww o's w In.tr Todu hesic stor Sci y e n C th ise t. m co ism try, 61 Chapter 10Electron 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. There are 3 electron groups on N. •• •• •• 1 lone pair. • • O N O • • 1 single bond. •• 1 double bond. Trww o's w In.tr Todu hesic stor Sci y e n C th ise t. m co ism try, 62 Chapter 10Linear 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°.         Cl Be Cl  O C O     Trww o's w In.tr Todu hesic stor Sci y e n C th ise t. m co ism try, 63 Chapter 10Trigonal 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°.     F B F      F    Trww o's w In.tr Todu hesic stor Sci y e n C th ise t. m co ism try, 64 Chapter 10Tetrahedral 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°.    F      F C F      F    Trww o's w In.tr Todu hesic stor Sci y e n C th ise t. m co ism try, 65 Chapter 10Sketching a Molecule • Because molecules are threedimensional objects, our drawings should indicate their threedimensional 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. Trww o's w In.tr Todu hesic stor Sci y e n C th ise t. m co ism try, 66 Chapter 10Sketching a Molecule, Continued    F      F C F      F    F C F F F Trww o's w In.tr Todu hesic stor Sci y e n C th ise t. m co ism try, 67 Chapter 10Derivative 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. Trww o's w In.tr Todu hesic stor Sci y e n C th ise t. m co ism try, 68 Chapter 10Derivative 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°.            O S O O S O O S O  www.ThesisScientist.com 69Derivatives 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 triangularbase 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°. Trww o's w In.tr Todu hesic stor Sci y e n C th ise t. m co ism try, 70 Chapter 10Tetrahedral Derivatives   H— N— H H— O— H  H www.ThesisScientist.comMolecular Geometry: Linear • Electron groups rround central atom = 2. • Bonding groups = 2. • Lone pairs = 0. • Electron geometry = linear. • Angle between electron groups = 180°. www.ThesisScientist.comMolecular Geometry: Trigonal Planar • Electron groups around central atom = 3. • Bonding groups = 3. • Lone pairs = 0. • Electron geometry = trigonal planar. • Angle between electron groups = 120°. www.ThesisScientist.comMolecular Geometry: Bent • Electron groups around central atom = 3. • Bonding groups = 2. • Lone pairs = 1. • Electron geometry = trigonal planar. • Angle between electron groups = 120°. www.ThesisScientist.comMolecular Geometry: Tetrahedral • Electron groups around central atom = 4. • Bonding groups = 4. • Lone pairs = 0. • Electron geometry = tetrahedral. • Angle between electron groups = 109.5°. www.ThesisScientist.comMolecular Geometry: Trigonal Pyramid • Electron groups around central atom = 4. • Bonding groups = 3. • Lone pairs = 1. • Electron geometry = tetrahedral. • Angle between electron groups = 109.5°. www.ThesisScientist.comMolecular Geometry: Bent • Electron groups around central atom = 4. • Bonding groups = 2. • Lone pairs = 2. • Electron geometry = tetrahedral. • Angle between electron groups = 109.5°. www.ThesisScientist.comElectron Molecular Bond Example areas Geometry angles 2 bonding. Linear 180° C H , 2 2 1 0 lone pairs SCN 3 bonding, Trigonal 120° C H , BF , 2 4 3 2 0 lone pairs planar CO 3 2 bonding, Trigonal 120° CH O, 2 1 lone pair planar bent SnCl 2 4 bonding, Tetrahedral 109.5° CH , 4 0 lones pairs POCl 3 3 bonding, Pyramidal 107° :NH 3 1 lone pair (trigonal) 2 bonding, Tetrahedral 105° H O, OF 2 2 2 lone pairs bent www.ThesisScientist.com 78 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. Trww o's w In.tr Todu hesic stor Sci y e n C th ise t. m co ism try, 79 Chapter 10Practice—Predict the Molecular Geometry Around the Central Atom •• • • O − • • • ClO • H PO 2 3 4 •• •• H O P O H •• •• •• •• •• • • O Cl O • • • O H •• • •• •• •• 2− •• • H BO • SO 3 3 3 • • •• O • • • O H • •• •• •• •• • • O S O • • H O B O H •• •• •• •• •• 1 • NO • P H 2 2 4 H H •• •• •• • • O N O • • H P P H •• •• •• Trww o's w In.tr Todu hesic stor Sci y e n C th ise t. m co ism try, 80 Chapter 10Practice—Predict the Molecular Geometry Around the Central Atom, Continued •• • • O − • • • ClO • H PO 2 3 4 •• •• H O P O H •• •• •• Tetr. Tetrahedral •• •• • • O Cl O bent • • • O H •• • •• •• •• 2− •• • H BO • SO 3 3 3 • • •• O • • • O H • Trigonal Trig. •• •• •• •• • • pyramidal O S O • • H O B O H •• •• •• •• •• 1 • NO • P H 2 2 4 H H •• •• •• Trig. • • Trig. O N O • • bent H P P H •• pyramidal •• •• Trww o's w In.tr Todu hesic stor Sci y e n C th ise t. m co ism try, 81 Chapter 10Bond 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. • d H Cld • www.ThesisScientist.comElectronegativity • Measure of the pull an atom has on bonding electrons. • Increases across the period (left to right). d+ H — F d • Decreases down the group (top to bottom). • The larger the difference in electronegativity, the more polar the bond. Negative end toward more www.ThesisScientist.com electronegative atom.Electronegativity, Continued 2.1 1.0 1.5 2.0 2.5 3.0 3.5 4.0 0.9 1.2 1.5 1.8 2.1 2.5 3.0 0.8 1.0 1.3 1.5 1.6 1.6 1.5 1.8 1.8 1.8 1.9 1.6 1.6 1.8 2.0 2.4 2.8 0.8 1.0 1.2 1.4 1.8 1.9 2.2 2.2 2.2 1.9 1.7 1.7 1.8 1.9 2.1 2.5 1.6 0.7 0.9 1.1 1.3 1.5 1.7 1.9 2.2 2.2 2.2 2.4 1.9 1.8 1.8 1.9 2.0 2.2 0.7 0.9 1.1 www.ThesisScientist.comElectronegativity, Continued www.ThesisScientist.comBond Polarity 3.03.0 4.02.1 3.00.9 = 0.0 = 1.9 = 2.1 Covalent Ionic Pure Polar 4.0 0 0.4 2.0 Electronegativity difference www.ThesisScientist.comDipole 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. www.ThesisScientist.comFor 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. • PbO • PS • MgCl • HO www.ThesisScientist.comFor 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. • PbO (3.5 1.9) = 1.6 \ polar covalent. • PS (2.5 2.1) = 0.4 \ pure covalent. • MgCl (3.0 1.2) = 1.8 \ ionic. • HO (3.5 2.1) = 1.4 \ polar covalent. www.ThesisScientist.comPolarity 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. www.ThesisScientist.comMolecule 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 Iww ntrodu w.T ch tor esy is S Cch ie em ntiis stt. rc yo , m Chapter 92 10Molecule 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 Iww ntrodu w.T ch tor esy is S Cch ie em ntiis stt. rc yo , m Chapter 93 10Polar Covalent Bonds and Electronegativity This is a picture (EPM) of a chloromethane. The red area is a high concentration of electrons, and blue means low concentration of electrons. www.ThesisScientist.comPolar Covalent Bonds and Electronegativity • What causes the electrons to congregate onto different atoms www.ThesisScientist.comAtoms 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 electronsequallythe 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. www.ThesisScientist.com• Molecules can Ultimately attract each other. This may not seem like much, but this is how DNA is held together. This helps scientists explain the differences in melting point, boiling point, as well as other physical properties. So how do we predict this behavior www.ThesisScientist.comCl Cl Dipole C C Moment H Cl Cl Cl H Cl CH Cl 2 2 CCl 4 m = 2.0 D m = 0.0 D www.ThesisScientist.comAdding Dipole Moments www.ThesisScientist.com• 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. www.ThesisScientist.com• 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 www.ThesisScientist.comExample 10.11—Determining if a Molecule Is Polar Tro's Introductory Chemistry, Chapter 102 10Example 10.11: • Determine if NH is polar. 3 www.ThesisScientist.comExample: Determine if NH is polar. 3 • Write down the given quantity and its units. Given: NH 3 www.ThesisScientist.comInformation: Example: Determine if NH is polar. Given: NH 3 3 Find: If polar Solution Map: formula → Lewis → polarity and shape → molecule polarity • Apply the solution map.  Draw the Lewis structure. Write skeletal structure. H N H H www.ThesisScientist.comInformation: Example: Determine if NH is polar. Given: NH 3 3 Find: If polar Solution Map: formula → Lewis → polarity and shape → molecule polarity • Apply the solution map. N = 5  Draw the Lewis structure. H = 3 ∙ 1 Count valence electrons. Total NH = 8 3 H N H H www.ThesisScientist.comInformation: Example: Determine if NH is polar. Given: NH 3 3 Find: If polar Solution Map: formula → Lewis → polarity and shape → molecule polarity • Apply the solution map. N = 5  Draw the Lewis structure. H = 3 ∙ 1 Attach atoms. Total NH = 8 3 H N H Start 8 e H Used 6 e Left 2 e www.ThesisScientist.comInformation: Example: Determine if NH is polar. Given: NH 3 3 Find: If polar Solution Map: formula → Lewis → polarity and shape → molecule polarity • Apply the solution map. N = 5  Draw the Lewis structure. H = 3 ∙ 1 Complete octets. Total NH = 8 3 ∙∙ H N H Start 2 e Used 2 e H Left 0 e www.ThesisScientist.comInformation: Example: Determine if NH is polar. Given: NH 3 3 Find: If polar Solution Map: formula → Lewis → polarity and shape → molecule polarity • Apply the solution map. Electronegativity  Determine if bonds are polar. N = 3.0 H = 2.1 ∙∙ H N H 3.0 – 2.1 = 0.9 H \ polar covalent www.ThesisScientist.comInformation: Example: Determine if NH is polar. Given: NH 3 3 Find: If polar Solution Map: formula → Lewis → polarity and shape → molecule polarity • Apply the solution map.  Determine shape of molecule. 4 areas of electrons around N; ∙∙ N H N H H 3 bonding areas H 1 lone pair H H Shape = trigonal pyramid www.ThesisScientist.comInformation: Example: Determine if NH is polar. Given: NH 3 3 Find: If polar Solution Map: formula → Lewis → polarity and shape → molecule polarity • Apply the solution map.  Determine molecular polarity. Bonds = polar Shape = trigonal pyramid N H H H Molecule = polar www.ThesisScientist.comInformation: Example: Determine if NH is polar. Given: NH 3 3 Find: If polar Solution Map: formula → Lewis → polarity and shape → molecule polarity ∙∙ Bonds = polar • Check: H N H Shape = trigonal pyramid N = 5 H N H = 3 ∙ 1 H H Total NH = 8 3 The Lewis structure H is correct. The bonds Bonding = 3 ∙ 2 e Molecule = polar are polar and the Lone pairs = 1 ∙ 2 e shape is unsymmetrical, Total NH = 8 e 3 so the molecule should be polar. www.ThesisScientist.comPractice—Decide Whether Each of the Following Molecules Is Polar •• • • O EN • • •• •• •• • • •• •• O = 3.5 O N Cl • • • • •• O S O N = 3.0 • • •• Cl = 3.0 S = 2.5 Tro's Iww ntrodu w.T ch tor esy is S Cch ie em ntiis stt. rc yo , m Chapter 115 10Practice—Decide Whether the Each of the Following Molecules Is Polar, Continued •• • • •• •• •• O • • • • O N Cl • • •• •• •• • • O S O Trigonal • • 3.0 •• bent Trigonal N 3.5 3.0 O planar O Cl 2.5 3.5 S 3.5 3.5 O O 1. Polar bonds, N—O 1. Polar bonds, all S—O 2. Asymmetrical shape 2. Symmetrical shape Polar Nonpolar Tro's Iww ntrodu w.T ch tor esy is S Cch ie em ntiis stt. rc yo , m Chapter 116 10Molecular 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 Iww ntrodu w.T ch tor esy is S Cch ie em ntiis stt. rc yo , m Chapter 117 10• 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. www.ThesisScientist.com
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