9. What is the molecular geometry of PH_(3) __

The molecular geometry of \( PH_3 \) (phosphine) is trigonal pyramidal. <br /><br />Here's the reasoning:<br /><br />1. **Valence Electrons**: Phosphorus (P) has 5 valence electrons, and each hydrogen (H) atom contributes 1 valence electron. Therefore, the total number of valence electrons in \( PH_3 \) is \( 5 + 3 \times 1 = 8 \).<br /><br />2. **Lewis Structure**: In the Lewis structure of \( PH_3 \), phosphorus is the central atom bonded to three hydrogen atoms. This uses up 6 of the 8 valence electrons (2 electrons per bond), leaving 2 electrons as a lone pair on the phosphorus atom.<br /><br />3. **Electron Pair Geometry**: The electron pair geometry around phosphorus is tetrahedral because there are four regions of electron density (three P-H bonds and one lone pair).<br /><br />4. **Molecular Geometry**: The presence of the lone pair affects the molecular shape. The lone pair-bond pair repulsion is greater than bond pair-bond pair repulsion, which pushes the hydrogen atoms closer together, resulting in a trigonal pyramidal shape.<br /><br />Therefore, the molecular geometry of \( PH_3 \) is trigonal pyramidal.