Shapes with Four Electron Groups Tetrahedral Arrangement

Molecular Shapes with Four Electron Groups (Tetrahedral Arrangement) 

Picturing molecular shapes is a great way to visualize what happens during a reaction. For instance, when ammonia accepts the proton from an acid, the lone pair on the N atom of trigonal pyramidal NH3 forms a covalent bond to the and yields the ammonium ion (NH4 ), one of many tetrahedral polyatomic ions. Note how the H—N—H bond angle expands from 107.3° in NH3 to 109.5° in NH4, as the lone pair becomes another bonding pair  

for similar molecules within a given electron-group arrangement, electron-pair repulsions cause deviations from ideal bond angles in the following order  

Lone pair-lone pair lone pair-bonding pair bonding pair-bonding pair (10.2) 

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All molecules or ions with four electron groups around a central atom adopt the tetrahedral arrangement. There are three shapes with this arrangement ... 

Figure 11.5 shows the bonding in other molecular shapes with the tetrahedral electron-group arrangement. The trigonal pyramidal shape of NH3 arises when a lone pair fills one of the four sp orbitals of N, and the bent shape of H2O arises when lone pairs fill two of the sp orbitals of O. 

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