Orbital Overlap; Hybrid Orbitals (Sections)
(a) Which geometry and central atom hybridization would you expect in the series BH4 -, CH4, NH4 +? (b) What would you expect for the magnitude and direction of the bond dipoles in this series? (c) Write the formulas for the analogous species of the elements of period 3; would you expect them to have the same hybridization at the central atom?
(a)The most common compound of Boron with hydrogen is borane (BH ). This compound has three 3electrons in its outermost shell, which is sp hybridised. But the compound, BH has four 4- -electrons and so boron combines with four hydrogen atoms to form BH . Hence the central 4 3atom, boron has sp hybridization and so it has a tetrahedral structure.CH ha4four electrons and carbon combines with four hydrogen atoms to form CH . Hence the 4 3central atom, carbon has sp hybridization and so it has a tetrahedral structure.The most common compound of nitrogen with hydrogen is ammonia (NH ). The compound, 3NH + has three electrons and so nitrogen combines with four hydrogen atoms to form NH . + 4 4Hence the central atom, nitrogen has sp hybridization and so it has a tetrahedral structure.BH , CH and NH are isoelectronic AX systems. All the central atom will have sp 3 4 4 4 4hybridization and will have a regular tetrahedral structure with H-A-H bond angle 109.5°. .(b)The electronegativity of the central atom decreases in the order N > C > BFind the electronegativity of H in the series. H is slightly less electronegative than C, and almostthe same as B. The magnitude of the bond dipole decreases in the series N-H > C-H > B-H. Thenegative end of the dipole is towards N, C and B.(c)Analogous species of the elements of period 3 corresponding to B, C and N are Al, Si and P.These elements in period 3 combines with four hydrogen atoms to form analogous species offormula AlH , SiH and PH . + 4 4 4(d)In the species AlH , SiH and PH , the central atom has four electrons and hence the geometry 4 4 4is tetrahedral. Hybridization corresponding to tetrahedral geometry is sp3. Thus, species AlH , 4 + - +SiH an4PH have he s4e hybridization at the central atom as BH , CH and NH . 4 4 4\n