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4/30/20151 At the end of this lesson, students should be able to : (d)Draw Lewis structure of covalent species with single, double and triple bonds (e)

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Presentation on theme: "4/30/20151 At the end of this lesson, students should be able to : (d)Draw Lewis structure of covalent species with single, double and triple bonds (e)"— Presentation transcript:

1 4/30/20151 At the end of this lesson, students should be able to : (d)Draw Lewis structure of covalent species with single, double and triple bonds (e) Compare the bond length between single, double and triple bonds.

2 4/30/20152 How to draw the Lewis structure (step by step method)

3 4/30/20153 SpeciesStep NF 3 1Total no. of valence electron: 2N is more electropositive than F. Skeletal structure: 3Draw a single bond. Complete the octets of the terminal atoms.

4 4/30/20154 HNO 3 1Total no. of valence electron: 2N is the most electropositive. Skeletal structure: 3Draw a single bond. Complete the octets of the terminal atoms.

5 4/30/ Move a lone pair to form another bond.

6 4/30/20156 Single Bond A covalent bond formed when 2 atoms share a pair of e Represent by dash (-) between 2 atom A single bond is made up of a sigma (σ) bond Example: HCl and HF

7 4/30/20157 Double Bond A covalent bond formed when 2 atoms share 2 pairs of e Represent by double dash (=) between 2 atoms A double bond is made up of sigma bond (σ) and pi bonds (π) Elements that can form double bond : C, N, O, S, Se and P. Example: O 2

8 4/30/20158 Triple bond A covalent bond formed when 2 atoms share 3 pairs of e Represent by triple dash ( ≡ ) between 2 atoms A triple bond is made up of 1 sigma bond (σ) and 2 pi bonds (π) Elements that can form triple bond : C and N. Example : N 2

9 4/30/ e - H H O ++ O HH O HHor 2e - Lewis structure of water Double bond – two atoms share two pairs of electrons single covalent bonds O C O or O C O 8e - double bonds Triple bond – two atoms share three pairs of electrons N N 8e - N N triple bond or

10 4/30/ Exercise Draw the Lewis structure of CO 2..

11 11 Draw the Lewis structure of ClO 3 - Exercise

12 12 Draw the Lewis structure for : a. SO 3 b. CO 2 c. NO 3 - d. SO 4 2- f. CO 3 2- g. N 2 O 4 e. HCN h. CH 2 O E X E R C I S E

13 4/30/ Comparison of the bond length between single, double and triple bonds. Multiple bonds are shorter than single covalent bonds. Bond length is defined as the distance between the nuclei of two covalently bonded atoms in a molecule.

14 4/30/ For a given pair of atoms such as carbon and nitrogen, triple bonds are shorter than double bond, which, in turn are shorter than single bond.

15 4/30/ H2H2 74pm161pm HI Figure : Bond length in H 2 and HI

16 4/30/ Bond TypeBond length (pm) Bond TypeBond length (pm) C-H107C-O143 C=O121C-C154 C=C133C≡CC≡C120 C-N143C=N138 C≡NC≡N116N-0136 N=O122O-H96 Table : Comparison of the bond length

17 4/30/ At the end of this lesson, students should be able to : f)Explain the exception to the octet rule: incomplete octet, expanded octet and odd number electrons

18 4/30/ Exception to the octet rule  In octet configuration, atom should have e configuration of noble gas  But, there’s an exception  * incomplete octet * incomplete octet * expanded octet *odd electron molecule

19 4/30/ Incomplete octet Elements in groups 2 & 13 (Be, B, Al) Period 2 Less metallic character Do not donate e but share e Central atom have less than 8 e (not achieve

20 4/30/ Eg : BeCl 2 Be shared e with Cl (covalent bond) but in BeCl 2 molecule Be only have 4 e in the outer shell (less than 8 e, does not achieve octet configuration).... ClBeCl Be.. ::: : ++

21 Eg : AlCl 3 and BF 3

22 4/30/ Expanded Octet Involves period 3 and onwards ( Has d orbital that involves in bo Central atoms having 10 or even.

23 4/30/ eg : SF 6 S shared e with F (covalent bond) but in SF 6 molecule S have 12 e in the outer shell (more than 8 e, does not obeys octet rule) S F F F F F F.. : :

24 4/30/ P shared e with F (covalent bond) but in PF 5 molecule P have 10 e in the outer shell (more than 8 e, does not obeys octet rule) PF 5 P F F F F F..

25 4/30/ Odd electron molecule Molecule with odd no. of e, complete pairing is impossible and an octet around each atom cannot be achieved Mostly from an odd numbered group ( Nitrogen- Group 15, Chlorine-Group 17) Example : nitric oxide (NO) and nitrogen dioxide (NO 2 ) : Since we need even no. of electrons for complete pairing (to reach 8) The octet rule cannot be satisfied for all the atoms in any of these molecule N=O O = N- O + -

26 26.. :F - N - F: :F: Lewis structure No. of e surrounding a central atom Assumption: e pairs/bonding pair in the valence shell repel one another Study of molecular geometry Gives the position of all atoms VSEPR!!..

27 27 - The 3D arrangement of atoms in a molecule. - affects physical and chemical properties. - predicted by using Valence Shell Electron Pair Repulsion (VSEPR) model. - The study of molecular geometry states “The valence electron pairs surrounding a central atom repel one another. Thus, the orbital containing those electron pairs are oriented to be as far apart as possible”.

28 28 Divide molecules into 2 categories:  Central atom has No Lone Pairs  Central atom has ≥1 Lone Pairs : F - N - F :.... : F : O = C = O..

29 29 BASIC MOLECULAR SHAPE LLinear TTrigonal Planar TTetrahedral TTrigonal bipyramidal OOctahedral

30 30 Cl Be 1. Linear

31 120° 2. Trigonal Planar

32 32 3. Tetrahedral

33 33 4. Trigonal Bipyramidal

34 34 5. Octahedral

35 35 -Geometry of molecule is more complicated if the central atom has both lone pairs and bonding pairs. -3 types of repulsive forces:  those between bonding pairs  those between lone pairs  those between a bonding

36 36 bonding-pair vs. bonding pair repulsion lone-pair vs. lone pair repulsion lone-pair vs. bonding pair repulsion < <

37 37 Molecular Geometry for Compound that contain at least one lone pair

38 38

39 39

40 40

41 41 Predicting Molecular Geometry 1.Draw Lewis structure for molecule. 2.Count number of lone pairs on the central atom and number of atoms bonded to the central atom. 3.Use VSEPR to predict the geometry of the molecule. e.g: What are the molecular geometries of SO 2 and SF 4 ?

42 42 Excercise: Use the VSEPR model to predict the geometry of the following molecules and ions: a)C 2 H 4 b) OF 2 c) AlCl - 4 d) AsH 3

43 43 Predicting Molecular Geometry 1.Draw Lewis structure for molecule. 2.Count number of lone pairs on the central atom and number of atoms bonded to the central atom. 3.Use VSEPR to predict the geometry of the molecule. e.g: What are the molecular geometries of SO 2 and SF 4 ? SO O AB 2 E bent S F F F F

44 44 Excercise: Use the VSEPR model to predict the geometry of the following molecules and ions: a)C 2 H 4 b) OF 2 c) AlCl - 4 d) AsH 3


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