Chemical Bonding - Valence Bond Theory (VBT) - PDF

CHEMICAL VALENCE BOND THEORY (VBT) BONDING (B) p—d overlapping (pp – dp bond) CHEMICAL BONDING CHEMICAL VALENCE BOND THEORY (VBT) BONDING (C) d—d overlapping (dp – dp bond) :- CHEMICAL VALENCE BOND THEORY (VBT) BONDING...

CHEMICAL VALENCE BOND THEORY (VBT) BONDING (B) p—d overlapping (pp – dp bond) CHEMICAL BONDING CHEMICAL VALENCE BOND THEORY (VBT) BONDING (C) d—d overlapping (dp – dp bond) :- CHEMICAL VALENCE BOND THEORY (VBT) BONDING Comparison between s and p bond s-bond s-bond 1. Formed by axial overlapping. 1. Formed by side by side overlapping. 2. Involves s-s, s-p, p-p orbitals. 2. Involves p-p, p-d & d-d orbital. 3. 3. Extent of overlapping is more so 3. Extent of overlapping is less so weaker stronger 4. 4. Free rotation around s bond is 4. Free rotation around p bond is possible possible Strength of Covalent Bond  extent of overlapping CHEMICAL QUESTION BONDING Q. Predict the nature of bond in following combinations. Orbitals Internuclear axis Bond s + px z-axis................ px + px x-axis................ py + py y-axis................ pz + pz z-axis................ px + px y or z-axis................ py + py x or z axis................ pz + pz x or y axis................ CHEMICAL BONDING CHEMICAL BONDING CHEMICAL QUESTION BONDING Q. Number of s and p bonds present in CH3—CH =CH—CCH are :- (1) 10s, 3p (2) 10s, 2p (3) 9s, 2p (4) 8s, 3p Ans. (1) CHEMICAL QUESTION BONDING Q. p-p overlapping will be observed in the molecules of :- (A) Hydrogen (B) Hydrogen bromide (C) Hydrogen chloride (D) Chlorine Ans. (D) CHEMICAL VALENCE BOND THEORY (VBT) BONDING Factors affecting overlapping / Bond strength :- (i) Size of overlapping orbital (n is different) :- 1 Bond strength  Size of overlapping orbital E.g. (a) C — C > Si — Si > Ge — Ge > Sn — Sn 2p 2p 3p 3p 4p 4p 5p 5p (b) H — F > H — Cl > H — Br > H — I 1s 2p 1s 3p 1s 4p 1s 5p CHEMICAL VALENCE BOND THEORY (VBT) BONDING (ii) lone pair – lone pair repulsion :- 1 Bond strength  lp – lp repulsion Order of Bond dissociation energy in Halogen molecules  Cl2 > Br2 > F2 > I2 CHEMICAL VALENCE BOND THEORY (VBT) BONDING (iii) Directional nature of orbitals : (n is same) Bond strength  directional nature E.g.:- 2s – 2s < 2s – 2p < 2p – 2p (non-directional) (directional) bond strength ↑ E.g.:- (2p-3d)p > (2p-3p)p bond strength ↓ CHEMICAL QUESTION BONDING Q. At room temperature N2 exist but P2 does not exist, why ? Ans. Strong p bond is formed between 2nd period elements. Remember :- Same explanation for existence of O2 and non existence of S2 at room temperature. CHEMICAL VALENCE BOND THEORY (VBT) BONDING Need for hybridisation :- If C atom uses pure orbitals for bond formation in CH4 molecule – H= 1s 1s 1s 1s CHEMICAL HYBRIDISATION BONDING CHEMICAL BONDING 1 C–H bond by 1s–2s overlapping 3 C–H bond by 1s–2p overlapping Q (1s–2s < 1s–2p) extent of overlapping ↑ \ Three C–H bond (1s–2p) are stronger than one C–H bond (1s–2s) Three C–H bond by (1s–2p) are at 90° and direction of one C–H bond (1s–2s) is uncertain. But fact is that shape of CH4 is tetrahedral and all C–H bond lengths are equal.