Video narration
Exam mein yah relation yaad rakho: DeltaH approx. Sum(bond energies of bonds broken) - Sum(bond energies of bonds formed). Yaani pehle reactants ke bonds todne ki energy, phir products ke bonds banne se released energy ghatao. Covalent bond mein sharing hamesha equal nahin hoti. Samaan ya lagbhag samaan pull wale atoms electrons barabar share karen, to non-polar bond banta hai. Jaise H2. Par oxygen electrons ko hydrogen se zyada kheenchta hai. Isliye O-H bond mein sharing unequal hai; yah polar bond hai. Oxygen side thoda negative, hydrogen side thoda positive ho jaata hai. Ek exam point aur: polar bonds hone par bhi poora molecule hamesha polar nahin hota. Molecular shape dekho. Agar bond effects har direction mein cancel ho jaayen, molecule overall non-polar ho sakta hai. Carbon dioxide, CO2, mein polar C=O bonds hain, par straight shape ke kaaran effects cancel hote hain. Ab molecules ke andar ke bonds aur molecules ke beech ki attraction ko mat milaana. van der Waals forces paas-paas ke molecules, ya atoms, ke beech weak attractions hain. Ye Ionic, Covalent ya Metallic Bonds jaise true chemical bonds nahin hain. Ye kamzor forces boiling point, melting point aur gases ke liquid banne jaise behaviour ko affect karti hain. To salt, water aur metals alag kyon hain? Kyonki atoms stability paane ke liye electrons ko alag dhang se sambhalte hain - kahin transfer, kahin sharing, aur kahin free movement. Isi se unke bonds aur properties badal jaate hain.