[0:00]Assalam o alaikum, aaj hum discuss karenge chemistry chapter three chemical bonding topic number six valence bond theory. Valence bond theory Hetler aur London ne propose ki thi 1927 mein aur further isko Pauling ne develop kiya tha. Agar hum iska dekhen ki basically explain kya karta hai toh VBT jo hai woh bond energies bond length aur shape of covalent molecule ko discuss karti hai. Aur iska basic concept yeh hai ki atoms jo hai woh bond kaise banate hain toh usne kaha ki agar atom mein koi unpaired electron hoga. Toh basically woh unpaired electron bonding mein involved hoga aur do atoms ke unpaired electron jab overlap karte hain toh covalent bond ki formation hoti hai. Jab usne kaha ki jab overlapping hoti hai toh do atoms ek common region share karte hain for example yeh do atoms hain aur inhone aapas mein ek bond banana hai toh yeh is tarah overlap karenge aur inmein ek ye common region hai jo yeh dono aapas mein share karenge.

[1:18]Aur isne second point yeh bataya tha ki orbital jo hai after bond formation apni identity ko maintain karte hain matlab ki bond formation ke baad unki identity change nahi hoti. Next hai formation of bond, jaisa ki humne discuss kiya according to VBT jab do atoms bond banate hain toh usme hota yeh hai ki dono atoms ka jo half-filled orbital hota hai basically woh overlap karta hai. Aur bonding ke baad orbital apni identity ko maintain karte hain. Second point tha electron spin, woh yeh hai ki agar for example do atoms bond bana rahe hain aur ek atom ka jo unpaired electron hai agar uski clockwise spin hai toh second ki anti-clockwise spin hogi matlab bond banane ke liye do orbital ki spin ko opposite hona chahiye. Second yeh hai ki number of bonds, number of bonds do atoms ke darmiyan kitne bante hain, depend karta hai ek atom kitne bond banata hai yeh depend karta hai ki usmein kitne unpaired electrons hain. Jaise agar ek atom mein ek unpaired electron hai it means woh ek bond banayega aur agar do unpaired electron hain woh do banayega and so on. Types of bond. Agar do orbitals jo hain woh single bond banate hain toh it means ki unke bond ko hum sigma bond kehte hain aur agar do atoms ke darmiyan zyada additional overlapping hoti hai, zyada orbitals overlapping karte hain toh fir multiple bonds ki formation bhi ho sakti hai jaise double aur triple bond bhi ban sakta hai. Symmetry requirement, isme yeh hai ki jab bhi do atoms bonding mein involved hote hain unki symmetry same honi chahiye matlab unka bond axis same hona chahiye. Bond axis kya hota hai woh line jo dono atoms ko jodti hai nuclei ke woh same honi chahiye. For example yeh ek atom hai aur yeh dusra atom hai. Theek hai, uska nucleus yeh hai aur dusra ka nucleus yeh hai. Ab in dono ko join karne ke liye jo line hai it must be same, matlab in dono ko ek dusre ke samne aana padega jo hai bond banane ki unki symmetry same honi chahiye matlab aise. Iska matlab hai ki woh line jo dono nuclei ko join kar rahi hai us line ko hum kehte hain bond axis. Dono atom ke nuclei ko join karne wali line ko hum bond axis kehte hain aur jab bhi do orbitals overlap karen toh unki jo bond axis honi chahiye woh same honi chahiye. Tabhi overlapping possible hogi. Energy considerations, jab bhi overlapping hoti hai toh energy release hoti hai kyuki humein pata hai jab unpaired electrons paired honge toh woh stable honge aur stable hone se atom energy ko release karta hai. Secondly yeh hai ki overlapping agar zyada hogi toh zyada energy release hogi matlab jitni achi overlapping hogi energy bhi utni release hogi aur bond bhi utna zyada strong hoga. Agar hum discuss karein ki atoms kitne type ke bonds bana sakte hain according to VBT, atoms do type ke bond bana sakte hain sigma aur pi bond. Sigma bond woh bond hota hai jab do atoms aapas mein pehla bond banate hain ya single bond banate hain usko hum sigma bond kehte hain. Sigma bond banta hai jab do atoms ke orbital head-to-head overlap karein. Head-to-head overlap ka matlab kya hai? Jaise ki agar do s-orbitals hain, ek for example atom a mein s-orbital unpaired hai aur dusre mein bhi s-orbital mein ek unpaired electron hai. Theek hai toh ab inmein agar sigma bond ki formation hogi toh is tarah head-to-head overlap hoga ek ka head dusre ke sath overlap karega aur yeh region darmiyan mein hai jo yeh dono aapas mein share karte hain. Agar do atoms ke darmiyan jo hai woh p aur dono atoms mein p-orbital mein electron unpaired hai toh p-p mein bhi possible hai ki sigma bond ki formation ho uske liye dono is tarah apne bond axis ko ek hi angle pe le aate hain. Aur is tarah head-to-head jab overlap hota hai toh hum kehte hain yeh sigma bond hai. Isi tarah agar ek s aur ek p-orbital ho toh unme bhi sigma bond ki formation head-to-head overlap ke through ho sakti hai. Aap note karein ki jab head-to-head overlap hota hai toh electron pair jo hoga woh yahan par darmiyan mein hoga. Do nucleus ke darmiyan mein hoga. Yahan par agar dekhen toh in this region mein jo hai woh electron pair. Is region mein electron pair present hoga toh hum kehte hain in sigma bond jo electron pair hai woh between the two nucleus hai. Yahan par bhi yeh nucleus aa gaya second nucleus aa gaya aur jo electron pair hoga woh yahan par darmiyan mein hoga. Isi liye sigma bond jo hota hai woh zyada strong hota hai kyuki ismein jo electron ki density hoti hai woh between the two nucleus hoti hai. Is tarah agar hum overlaps ko discuss karein toh sigma bond do s-orbital ke darmiyan bhi ban sakta hai, s aur px overlap bhi possible hai, x aur py ke darmiyan, s aur pz aur px aur px matlab do p-orbitals bhi sigma bond ki formation kar sakte hain. Next hai pi bond, jab do atoms pehla bond banate hain toh woh hamesha sigma bond banate hain. Agar wahi do atoms dusra bond bhi banane ki koshish karenge toh woh pi bond hoga aur pi bond jo hai woh kaise banta hai? Woh parallel overlap se banta hai. Wahan par sigma bond mein head-to-head overlap tha lekin yahan par side-wise overlap hota hai. Side-wise overlap kis mein possible hai? S ki toh shape symmetric hoti hai toh wahan par side-wise overlap nahi ho sakta isi liye s-s mein hamesha head-to-head overlap hota hai aur sigma bond ki formation hoti hai jab ki p-p mein jo hai woh pi bond ki formation ho sakti hai. Ab woh kaise hoti hai hum discuss karte hain double bond aur agar triple bond ko dekhen toh double bond mein ek bond sigma hoga aur ek pi hoga aur agar ek atom ke darmiyan triple bond hoga toh it means pehla bond hamesha sigma hoga aur baaki do kya honge pi honge. Ab pi mein side-wise overlap hota kaise hai? Jaise humne abhi discuss kiya do py jo hai ya do pz jo hain woh pi bond bana sakte hain. Ab jo overlap hota hai in case of pi bond woh side-wise hota hai matlab side se woh overlap karte hain. Ab aap note karein ki in case of pi bond jo electron hoga woh ya toh above the nuclei hoga ya toh below the nuclei hoga.

[7:15]Jab ki sigma bond mein jo electron ka jo electron ki density thi woh between the two nuclei thi. Lekin yahan par electron density above and below possible ho sakti hai isko hum kehte hain side-wise overlap. Jaise yeh ethene ka structure hai. Ethene ka formula hota hai CH2 CH2 darmiyan mein carbon-carbon ne do double bonds banaye hue hain. Ab do double bond banaye hue hain iska matlab hai ki pehla bond hoga woh sigma hoga aur second kya hoga pi. Ab sigma bond hai it means woh head-to-head overlap se banega isi liye yahan par ek overlap jo hoga woh head-to-head hoga matlab yahan ka p-orbital iske sath head-to-head overlap banayega. Jabki dusra pi bond hai do jo do p-orbitals hain woh yahan par kya banayenge woh pi bond banayenge toh yahan par jo overlap hoga woh side-wise overlap hoga in case of double bond. Next hai sigma bond strong hai ya pi bond strong hai? Abhi humne discuss kiya ki sigma bond mein electron density hai woh concentrated hoti hai between the two nuclei of atom isi liye woh zyada strong hoti hai. Jabki pi mein electrons density above aur below distribute hui hui hoti hai isi liye distribution ki wajah se woh jo hai woh zyada diffuse hoti hai aur us kam energy se easily hum usko break kar sakte hain. Ya hum keh sakte hain jo pi bond hai woh weak hota hai. Next hai application of valence bond theory, matlab VBT ke through hum kya kar sakte hain? Toh VBT ke through hum single bond ki formation ko discuss kar sakte hain jaise formation of H2 molecule hai. Iske through humein yeh pata lagega ki hydrogen molecule kis tarah bond ki formation karta hai. Humein pata hai ki hydrogen molecule mein do hydrogen atoms hain, agar hum hydrogen ki electronic configuration dekhte hain toh hydrogen ka atomic number one hota hai aur it means ke hydrogen ke valence mein ek hi electron hota hai. Toh yahan par ek hydrogen aa gaya aur yahan par dusra hydrogen. Ab hydrogen ka jo valence shell hai usmein s-orbital hi hota hai it means ke dono ki shape kya hogi? Spherical shape hogi. Darmiyan mein nucleus hoga aur ek hi electron ko surround karega. Toh ab kya hoga jab bond formation hogi toh ek agar uski spin clockwise hai dusri apni spin ko anti-clockwise kar dega aur yeh kya karenge head-to-head overlap karenge kyuki inke darmiyan ek hi bond banta hai toh head-to-head overlap ke through banega aur yahan par jo hai electron pair ban jayega aur is bond ko hum kahenge ki yeh sigma bond ban gaya hai. Toh VBT ke through humein yeh pata lag gaya ki kis tarah sigma bond ki formation hoti hai between two hydrogen atoms. Second hai between two fluorine atoms kis tarah F2 molecule ki formation hoti hai? Agar hum fluorine ki electronic configuration dekhen toh fluorine ka atomic number hota hai nine. It means ki uske pehle shell mein do electrons hote hain baki seven electrons second shell mein hote hain. 2s mein do electrons, 2px mein do, 2py mein do, 2pz mein. Abhi humne discuss kiya according to valence bond theory bond formation mein sirf wohi orbital involved hota hai jo unpaired hai. Toh yahan par ek 2pz unpaired hai aur yahan par dusre fluorine atom mein bhi 2pz unpaired hai. Kyuki fluorine dono aapas mein ek hi bond bana sakte hain kyuki dono mein ek hi unpaired electron hai toh inka jo beech mein bond banega woh sigma bond banega aur sigma bond head-to-head overlap se banta hai. Toh yeh ek 2pz, 2pz orbitals aa gaye hain toh yeh head-to-head overlap karenge aur aise inke darmiyan jo hai woh sigma bond ban jayega. Next hai formation of HF molecule. HF mein humne kya karna hai dono atoms jo bond bana rahe hain uske electronic configuration likhenge.

[10:31]Fluorine ki electronic configuration jaisa abhi humne discuss ki yeh hai aur fluorine mein 2pz orbital jo hai woh unpaired hai. In case of hydrogen jo hai 1s orbital mein ek electron unpaired hota hai. It means ki iska jo unpaired electron hai iske unpaired electron ke sath bond banayega aur inke darmiyan ek hi bond banta hai toh sigma bond hi banega. Pehla atom, bond jo do atoms ke darmiyan banta hai woh sigma bond hota hai. It means ki 2pz fluorine ka bond banayega with 1s of hydrogen aur yeh ho gaya hydrogen ka 1s aur yeh hai 2pz fluorine ka. Toh bond sigma hai isi liye inke darmiyan head-to-head overlap hoga aur is tarah inke darmiyan mein bond ki formation hogi. Next hai formation of O2 molecule. Ab hum dono oxygen atom ki electronic configuration dekhte hain. Oxygen atom ka atomic number hota hai eight. It means ki first shell mein do aur second shell mein six electrons hote hain.

[11:25]Six electrons according to electronic configuration kuch is tarah distribute hote hain. Aap note karein ki har oxygen molecule mein do orbitals jo hain woh unpaired hain jaise yahan par 2py 2pz unpaired hain. Same dusre oxygen mein bhi 2py aur 2pz unpaired hain. It means ki dono oxygen atoms jab bond banayenge toh woh do bond banayenge. Ek unpaired dusre unpaired ke sath isi tarah iska unpaired dusre unpaired ke sath bond banayega. It means total do bonds ki formation hogi toh jo pehla bond dono ke darmiyan banega woh sigma hoga aur dusra kya hoga pi hoga. Ab hum discuss karte hain woh kis tarah uski formation hogi. Toh yeh hai oxygen ka ek simple molecule aap note karein ki yahan par oxygen ne dusre oxygen atom ke sath ek jo hai woh sigma bond ki formation ki hui hai head-to-head overlap aur dusra jo bond hai woh side-wise overlap hai kyuki jab pehla bond hota hai woh head-to-head overlap se banta hai usko hum sigma kehte hain aur agar wahi do atoms dusra bond bhi banaye toh fir woh side-wise overlap hota hai toh yahan par yeh do yeh side-wise overlap kiya hua hai p-y aur p-y ne jis ki wajah se inke darmiyan pi bond ban gaya hai. Next hai formation of N2 molecule. Nitrogen ka atomic number seven hota hai it means iske valence mein total five electrons hote hain. Electronic configuration ke through aapko yeh pata lagega ki nitrogen ke valence mein total teen unpaired electrons hain. Iska matlab hai each nitrogen will form teen bonds. Yahan par second nitrogen mein bhi yehi case hai. It means ki ek bond jo dono ke darmiyan banega woh sigma hoga baki do pi honge. Toh bas yeh nitrogen ke bonds hain aap note karein ek bond jo hai woh inka sigma ban raha hai aur baki do kya ban rahe hain woh pi bond bana rahe hain kyuki unmein side-wise overlap ho raha hai. Unmein jo hai head-to-head overlap possible nahi hai sirf ek hi head-to-head overlap ho sakta hai toh ek sigma bond banega aur do bond jo honge woh pi honge. I hope ki aapko valence bond theory ka basic concept samajh mein aa gaya hoga. Thank you so much Allah Hafiz.