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Session : Session CHEMICAL BONDING www.aptacads.com
Session Objective : www.aptacads.com Session Objective Lewis Theory – Valence electron , Octet rule, Lewis Structure
Geometry of covalent molecules - VSEPR theory
Valence bond theory
Concept of hybridization involving s, p and d orbitals and shapes of some simple molecules
Lewis Structure for Simple molecules : www.aptacads.com Lewis Structure for Simple molecules Selection of central atom :
More electropositive element ( except hydrogen ) , as a rule of thumb Computing bond pairs and lone pairs :
Step 1 : Compute total number of valence electrons (A) in all the atoms present, including the charge on the species.
Step 2 : Compute required no. of valence electrons ( N ) for attaining stable configuration for all the atom in the molecule
Illustrative Problem : www.aptacads.com Illustrative Problem Draw Lewis structure of PH3 Central atom : P Computing bond pairs and lone pairs :
Step 1 : Total number of valence electrons A = 1*5+3*1= 8
Step 2 : Compute required no. of valence electrons
N = 1*8 + 3*2 = 14
Illustrative Problem : www.aptacads.com Illustrative Problem Draw Lewis structure of for hydrogen cyanide gas HCN Central atom : C Computing bond pairs and lone pairs :
Step 1 : Total number of valence electrons A = 1*1+1*4+1*5= 10
Step 2 : Compute required no. of valence electrons
N = 1*2 + 1*8 +1*8= 18
Illustrative Problem : www.aptacads.com Illustrative Problem Central atom : N Computing bond pairs and lone pairs :
Step 1 : Total number of valence electrons A = 1*5+3*6+1= 24
Step 2 : Compute required no. of valence electrons
N = 1*8 + 3*8 = 32
Limitation of octet rule : www.aptacads.com Limitation of octet rule Incomplete Octet of Central Atom In BF3 and AlCl3 ,the central metal atom contains 6 electrons so they are electron deficient species. When central atom has less than four valence electrons , incomplete octet may occur
VSEPR theory : VSEPR theory Explains shape of covalent molecules
Shape of molecule depends on repulsions between electron pairs in valence shell around central atom Pairs of electrons tend to occupy such positions in space that minimize repulsion to increase stability The valence shell is taken as a sphere with the electron pairs localizing on the spherical surface A multiple bond is treated as if it is a single electron pair
VSEPR theory : VSEPR theory Decreasing Order of repulsion lone pair - lone pair > lone pair –bond pair > bond pair - bond pair Why lp- lp repulsion is more compared to lp-bp :
Lone pair electrons are localised on the central atom
Hence , lone pairs occupy more space in a molecule as compared to the bonding pair of electrons.
Therefore to remain at maximum possible distance and thereby attaining stability, lp- lp repulsion is more compared to lp-bp
Structure of the molecule using VSEPR theory : Structure of the molecule using VSEPR theory Draw electron dot structure
Determine no. of electron pairs around the central atom , count multiple bond as one.
Arrange the electron pairs in to regular geometrical shapes without differentiating between lone pair and bond pairs
If lone pairs are present around central atom , use the following concept to finalise structure Decreasing Order of repulsion lone pair - lone pair > lone pair –bond pair > bond pair - bond pair Electron pair repulsion is negligible if angle between two electron pairs >1000
Structure of the molecule using VSEPR theory : Structure of the molecule using VSEPR theory Shortcut for finding electron pair on central atom
Illustrative Problem : Illustrative Problem Trigonal planar
Illustrative Problem : Illustrative Problem 4 single bonds and no lone pair
Illustrative Problem : Shape will be trigonal bipyramidal. Illustrative Problem 5 single bonds and no lone pair
Illustrative Problem : Shape will be octahedral. Illustrative Problem 6 single bonds and no lone pair
Illustrative Problem : Illustrative Problem
Effect of lone pair - CH4 , NH3 , H2O : Effect of lone pair - CH4 , NH3 , H2O Shape will be tetrahedral. 4 single bonds and no lone pair 3 single bonds and one lone pair Shape will be trigonal pyramidal 2 single bonds and two lone pairs Shape will be bent Arrangement will be tetrahedral Arrangement will be tetrahedral Arrangement will be tetrahedral
Illustrative Example : The shape of NH3 is very similar to (a) CH4 (b) CH3–
(c) BH3 (d) CH3+ Illustrative Example Solution :
Correct Option is (b)
Illustrative Problem : Illustrative Problem The bond angle in SnCl2 will be
(a) > 120o (b) < 120o (c) 120o (d) None of these Two bond pairs and one lone pair. Hence, answer is (b). Arrangement of electron pair will be trigonal planar Because of lone pair , bond angle < 1200 Solution :
Illustrative Problem : Illustrative Problem Four single bonds and One lone pair Arrangement of electron pairs will be trigonal bi pyramidal Lp– Bp repulsion - 3 Lp–Bp repulsion - 2 More Stable
Illustrative Problem : Illustrative Problem Three single bonds and Two lone pair Arrangement of electron pairs will be trigonal bi pyramidal Lp– Bp repulsion - 6 Lp–Bp repulsion - 3
Lp-Lp repulsion- 1 Shape – T Lp–Bp repulsion - 4
Illustrative Problem : Illustrative Problem Two single bonds and Three lone pairs Arrangement of electron pairs will be trigonal bi pyramidal Shape – Linear
Illustrative Problem : Illustrative Problem Four single bonds and Two lone pairs Arrangement of electron pairs will be Octahedral Shape – Square Planar
Illustrative Problem : The shape of CH3+ is likely to be
(a) Pyramidal (b)tetrahedral
(c) Linear (d)planar Illustrative Problem Hence, the answer is (d). Shape will be Trigonal Planar. 3 single bonds and no lone pair Solution :
Limitations of VSEPR theory : Cannot determine the shape Multiple bonded species CO2,SO4-2 Limitations of VSEPR theory
Summary of structure of molecules : Summary of structure of molecules
Thank You : Thank You For your doubts
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