Which of the following characteristics apply to PH3? 1) trigonal pyramidal molecular geometry 2) sp2 hybridized 3) polar bonds 4) nonpolar molecule

Answer 1

In order to answer this question, you really want to be thinking about what is called the steric number. The steric number calculated by: (number of lone pairs of electrons) + (number of atoms bonded to central atom). This will give you an idea of the number of general locations electrons will reside in the molecule. For this particular molecule, PH₃, the steric number is 4, as phosphorus has one lone pair and is bonded to three hydrogens. Since this molecule has a steric number of 4 but is only bonded to 3 atoms, we can infer a trigonal pyramid geometry. I think of it like this: the steric number of 4 tells you tetrahedral geometry, but you have to take away an atom because there's only three hydrogens, leaving the others roughly in their place, which produces the trigonal pyramidal geometry. In other words, think about what methane, a molecule with tetrahedral geometry, would look like if you lopped off the top hydrogen. The steric number also tells you the hybridization, as you can use that to work up from s (steric number 1), to sp (steric number 2), to sp² (steric number 3), then to sp³ (steric number 4). So it's not sp² hybridized. As for the polarity of the bonds, this is determined by the difference in electronegativity. Hydrogen and phosphorus have approximately the same electronegativity at 2.2 and 2.19 I believe, so the bonds themselves are nonpolar. This, however, tells us nothing about the polarity of the molecule as a whole. For this, we consider the symmetry of the molecule. This molecule is asymmetrical because lone pair at the top of the molecule, and this means it's polar.