Determine the molecular geometry and molecular polarity for phosphine, PH
. – Tetrahedral lonic – Trigonal pyramidal nonpolar – Trigonal planar nonpolar – Linear nonpolar – Trigonal pyramidal lonic – Bent polar – Bent ionic – Tetrahedral polar – Bent nonpolar – Trigonal pyramidal polar – Trigonal planar ionic – Trigonal planar polar – Linzar lonic – Linear polar – Sahedral nonpotar
The Correct Answer and Explanation is:
The correct answer is: Trigonal pyramidal, polar
Phosphine (PH₃) is a molecule composed of one phosphorus (P) atom and three hydrogen (H) atoms. To determine its molecular geometry and polarity, we begin by examining its electron domain geometry.
Phosphorus is in Group 15 of the periodic table and has five valence electrons. In PH₃, three of these electrons are used to form single covalent bonds with three hydrogen atoms, while the remaining two electrons form a lone pair. This gives the phosphorus atom four regions of electron density: three bonding pairs and one lone pair.
According to the Valence Shell Electron Pair Repulsion (VSEPR) theory, these four regions arrange themselves in a way that minimizes repulsion. This leads to a tetrahedral electron geometry. However, because one of these regions is a lone pair and not a bonded atom, the actual molecular geometry of PH₃ is trigonal pyramidal.
The presence of the lone pair affects the bond angles, making them slightly smaller than the ideal tetrahedral angle of 109.5 degrees. This lone pair also contributes to an uneven distribution of electron density in the molecule.
Regarding molecular polarity, each P-H bond is only slightly polar due to the small difference in electronegativity between phosphorus and hydrogen. However, the asymmetrical shape caused by the lone pair prevents the dipole moments of the individual bonds from canceling out. As a result, the molecule has a net dipole moment, making it polar.
In summary, PH₃ has a trigonal pyramidal molecular geometry and is a polar molecule. Its polarity arises not from strongly polar bonds but from its asymmetrical shape and the presence of a lone pair on the central phosphorus atom, which creates an imbalance in electron distribution.
