16 A Textbook of Inorganic Chemistry – Volume I iii) H 2 O: The central atom in H 2 O molecule is O which has six valence electrons (2, 6). Now because each hydrogen atom needs one electron to complete its duplet (1), the O atom uses its two valence electrons to create two bond pair domains, while four electrons are left as two lone pair domains. Now though the geometry for four electron-pair domains is tetrahedral with 109°28ґ, in this case, the two non-bonding domains would require more space than the bonding domains.
This, in turn, would result in a greater lone-pair–lone-pair repulsion yielding a V-shaped geometry with H−O−H bond angle (104.5°) less than the normal 109°28ґ of a perfectly tetrahedral system. It is also worthy to note that the bond angle in water is also less than NH 3 which is obviously due to the presence of one extra lone pair in H 2 O. Figure 9. Structure of H 2 O molecule from the VSEPR model. 4. Five electron-pair domains: i) PF 5 : The central atom in PF 5 molecule is P which has five valence electrons (2, 8, 5).
Now because each fluorine atom needs one electron to complete its octet (2, 7), the P atom uses its all five valence electrons in sharing to create five bond pair domains only. Hence the geometry for minimum repulsion will be perfect trigonal bipyramidal and the normal bond angle between equatorial groups should be 120°, while the normal bond angle between axial fluorine should be 180°. Now because each axial position has three neighbors at 90° while every equatorial position has only two 90° neighbors, we can conclude that the more crowding at the axial position would lead to longer axial bonds comparatively. Figure 10. Structure of PF 5 molecule from the VSEPR model. ii) SF 4 :
The central atom in SF 4 molecule is S which has six valence electrons (2, 8, 6). Now because each fluorine atom needs one electron to complete its octet (2, 7), the S atom uses its four valence electrons to create four bond pair domains, while two electrons are left as lone pair domain. Now, although the geometry for five Buy the complete book with TOC navigation, high resolution images and no watermark. Copyright © Mandeep Dalal