CHAPTER 4A CHEMICAL BONDING.pdf

CHAPTER 4A CHAPTER 4A OUTLINE 1. Chemical bond 2. Lewis's structure 3. Ionic bond 4. Covalent bond 5. Covalent compounds 6. Dative bond 7. Bond length 8. Formal charge 2  LEARNING OBJECTIVES Explain the Lewis theory of bonding. Interpret the octet rule as it p...

CHAPTER 4A CHAPTER 4A OUTLINE 1. Chemical bond 2. Lewis's structure 3. Ionic bond 4. Covalent bond 5. Covalent compounds 6. Dative bond 7. Bond length 8. Formal charge 2  LEARNING OBJECTIVES Explain the Lewis theory of bonding. Interpret the octet rule as it pertains to Lewis structures of compounds. Understand the differences of ionic, covalent and dative bonds. Determine Lewis dots symbols of elements. Summarize ionic and dative bonding and provide examples of compounds containing respective ionic and dative bonds. Utilize the guidelines for writing Lewis structures of compounds and polyatomic ions. Evaluate the average bond lengths of some common single, double and triple bonds. Calculate formal charges on atom in Lewis structure. 3  LEARNING OBJECTIVES (CONTINUE..) Employ formal charges to identify the most likely structure of a compound when multiple Lewis structure are possible. Write Lewis structures for species that do not obey the octet of rule. Draw Lewis structures with and without expanded octets for species where both are possible. Evaluate the concept of resonance and draw resonance structures of a given compound or polyatomic ion. 4  Rule of thumb: Ionic (Metal - Nonmetal) 1.0 CHEMICAL Covalent (Nonmetal – BONDS Nonmetal) Metallic (Metal – Metal) 5  element dots or cross is used to represent the valence electrons in an atom 2.0 LEWIS of the element STRUCTURE Elements in the same group have the same valence electronic configurations 6 LEWIS DOT SYMBOLS FOR THE REPRESENTATIVE ELEMENTS & NOBLE GASES Elements in the same group have the same valence electronic configurations. 7 REMEMBER: T H E AT O M AT E ( 8 ) U N T I L I T WA S F U L L. ( L O L ) 2.1 OCTET RULE 8 This Photo by Unknown Author is licensed under CC BY-NC-ND WHICH OF THE FOLLOWING DEFINES THE OCTET RULE? [A] A rule that refers to the tendency of an atom to have empty valence shells [B] A rule that refers to the tendency of an atom to have 8 electrons in the valence shell [C] A rule that refers to the tendency of electrons to orbit around an atom in energy shells [D] A rule that refers to the tendency of electrons to reside in the nucleus of an atom 9 https://www.nagwa.com/en/videos/584157494787/  WHICH OF THE FOLLOWING DEFINES THE OCTET RULE? [B] A rule that refers to the tendency of an atom to have 8 electrons in the valence shell https://www.nagwa.com/en/videos/584157494787/ 10 2.2 STABLE OCTET RULE Atoms tend to either gain or lose electrons in their highest energy level to form ions Atoms prefer having 8 electrons in their highest energy level Examples Na atom 1s2 2s2 2p6 3s1 One electron extra Cl atom 1s2 2s2 2p6 3s2 3p5 One electron short of a stable octet Na+ Ion 1s2 2s2 2p6 Stable octet Cl- Ion 1s2 2s2 2p6 3s2 3p6 Stable octet 11 Example of ionic bond: Sodium Chloride, NaCl Sodium tends to remove its valence electron to obtain noble gas electronic configuration (Ne). Chlorine tend to accept electron from Na to obtain noble gas electronic configuration (Ar). 12 The electrostatic forces between These two processes occur Na+ and Cl- produce ionic bond simultaneously + 13  Example of ionic bond: Calcium Chloride The electrostatic attraction formed ionic bond between the ions. + + 2 14 15  Example of Covalent Bond : F + F F F 7e- 7e- 8e- 8e- Lewis structure of F2 single covalent bond lone pairs F F lone pairs single covalent bond lone pairs F F lone pairs Hydrogen is an exception: Row 1 of the periodic table, only has the 1s orbital available in the ground state, which can only hold two electrons! 17  COVALENT COMPOUNDS Covalent compounds may consist of: Single bond Double bond Triple bond 18  2.3 BOND LENGTH The distance between nuclei of the atoms involves in the bond C C C C C C 1.54 Å 1.34 Å 1.20 Å As the number of bonds between the carbon increase, the bond length decreases because C are held more closely and tightly together. As the number of bonds between two atoms increases, the bond grows shorter and stronger. 19 20 H + O + H H O H Lewis structure of water 2e- 8e- 2e- single covalent bonds H O H 21 Double bond : two atoms share two pairs of electrons O C O or O C O 8e- 8e- 8e- double bonds double bonds 22 Triple bond : two atoms share three pairs of electrons N N or N N 8e- 8e- triple bond triple bond 23  2.4 DATIVE BOND/Coordinate bond  Dative bond is a bond in which the pair of shared electrons is supplied by one of the two bonded atoms.  Requirement for dative bonds : i. Donor atoms should have at least one lone pair electrons. ii. The atoms that accepts these electrons should have empty orbitals. Coordinate (dative) covalent bonds: formed when both electrons of the shared pair of electrons originate from the same atom. – Example: carbon monoxide (CO) C O coordinate bond  EX AMPLE 2.1: H Dative Bond/ H Coordinate Bond H N + H H N H H H NH3 + H NH4 ammonia ammonium ion 25 EX AMPLE 2.2: H2O + H H3O water hydronium ion 26  2.5 HOW TO DRAW LEWIS STRUCTURES Count Add Draw Complete Form Count total Add 1 for each Draw skeletal Complete an If structure number of negative charge. structure of octet for all contains too valence e- of Subtract 1 for the compound. atoms except many electrons, atoms involved. each positive Place least hydrogen. form double charge. electronegative and triple element in the bonds on center. central atom as needed. 27 STEPS FOR WRITING LEWIS STRUCTURES: 1. Sum the valence electrons from all the atoms present in the compound. Don’t worry about keeping track of which electrons come from which atom. It is the total number of electrons that is important. CH4: 8 total valence electrons 28  STEPS FOR WRITING LEWIS STRUCTURES: 2. Draw a skeletal structure. Use a pair of electrons to form a bond between each pair of bound atoms.(Place least electronegative element in the center!) Hint #1:The atom with the smallest electronegativity is usually the central atom (H2O is a notable exception). COCl2 Electronegativities C = 2.5 O = 3.5 Cl = 3.0 29  STEPS FOR WRITING LEWIS STRUCTURES: 2. Draw a skeletal structure. Use a pair of electrons to form a bond between each pair of bound atoms. Hint #2: Polyatomic species are usually clumped and not spread out. SO2F2 O F–S-F F–O–S–O-F O 30 STEPS FOR WRITING LEWIS STRUCTURES: 3. Arrange the remaining electrons to satisfy the duet rule for H and the octet rule for the second-row elements. CH3Cl.. : Cl : H C H H 31  STEPS FOR WRITING LEWIS STRUCTURES: 4. If electrons remain after the octet rule has been satisfied, then place them on the elements having available d orbitals (elements in Period 3 or beyond, often the central atom, because more orbital available, s,p,d orbitalsmore than 8 e). OR 4. If structure contains too many electrons, form double and triple bonds on central atom as needed. IN SUMMARY: WRITING LEWIS STRUCTURES 1. Draw skeletal structure of compound showing what atoms are bonded to each other. Put least electronegative element in the center. – 2. Count total number of valence negative charge. Add 1 for each e Subtract 1 for each positive charge. 3. Complete an octet for all atoms except hydrogen. 4. If structure contains too many electrons, form double and triple bonds on central atom as needed.  Write The Lewis Structure For EXAMPLE 2.3 N i t r o g e n Tr i f l u o r i d e ( N F 3 ) Draw the Lewis structure for each of the following compounds: i. HF ii. CH4 iii. CHCl3 iv. NH3 v. H2O 35  3.0 FORMAL CHARGE IS important !For Lewis structure for a molecule (the most stable!) In chemistry, a formal charge (FC) is the charge assigned to an atom in a molecule assuming that electrons in all chemical bonds are shared equally between atoms regardless of relative electronegativity Sometimes atoms will have extra or not enough electron electrons. When determining the best Lewis structure (or predominant resonance structure) for a molecule, the structure is chosen such that the formal charge on each of the atoms is as close to zero as possible. 37 Formal charge is the charge on a certain atom in a Lewis structure. 38 39 The sum of formal charge on each atom should equal: zero for a molecule the charge on the ion for a polyatomic ion Formal charge is used to find the most stable Lewis structure. 41 A stable structure has : formal charge on each atom closest to zero. formal negative charge on a more electronegative atom. formal positive charge on a more electropositive atom. 42 Retrieved from http://www.chem.ucla.edu/~harding/IGOC/O/octet_rule.html 43 4.0 RESONANCE STRUCTURE Retrieved from https://kpu.pressbooks.pub/organicchemistry/chapter/1-3-resonance-structures/ 44  R E S O N A N C R E Q U I R E M E N T : M O L E C U L E S / I O N S M U S T H A V E M U LT I P L E B O N D S M O L E C U L E S / I O N S M U S T H A V E L O N E P A I R S E L E C T R O N S 45 EXAMPLE 4.1 WRITE THE LEWIS STRUCTURE FOR N I T R I C AC I D ( H N O 3)  Formal deh yde I s A Li qu id Wit h A D i s a g r e e a b l e O d o r , Tr a d i t i o n a l l y EXAMPLE 4.2 H a s B e e n U s e d To P r e s e r v e Laboratory Specimens. Draw The M ost Li kely Lew i s St r uct ure For Th e Compound. Wr i t e T h e L e w i s S t r u c t u r e Fo r Fo r m a l d e h yd e ( C H 2O ) EXAMPLE WRITE THE RESONANCE S T R U C T U R E F O R C A R B O N AT E 4.3 I O N ( C O 32-) EXAMPLE WRITE THE RESONANCE S T R U C T U R E F O R C A R B O N AT E 4.3 I O N ( C O 32-) - - - - O C O O C O O C O O O O - - 5.0 Exception to the Octet Rule F F B Three conditions: F 1)Incomplete octet Boron trifluoride 2)Expanded octet S; P; Si; Cl; Xe 3)Odd no. electron F F F S F N O nitric oxide F F sulfur hexafluoride 50  5.1 Incomplete Octet  Occurs when central atom has less than 8 electrons.  Elements that can form incomplete octet are: Boron,B , Beryllium, Be, Aluminium, Al 51 Ex 5.1 52  5.2 Expanded Octet Occurs when central atom has more than 8 electrons. Expanded Octet formed by : non-metals that have d orbitals non-metals of the 3rd, 4th, 5th….rows in the periodic table 54 Ex 5.2 55  5.3 Odd Electron Molecules Nitrogen contain odd number electrons. Example: Nitric oxide, NO Nitrogen dioxide, NO2 57 Retrieved from https://www.chemistrylearner.com/exception-to-octet-rule.html 59  Ex 5.3 Lewis structure of N2O *Choose Lewis structure that have formal charges close to 0 *Choose Lewis structure that has the negative formal charge on the most electronegative element *O is more electronegative than N 60 Ex 5.3 Lewis structure of N2O 61 62  63 This Photo by Unknown Author is licensed under CC BY-SA-NC