Question

Consider the following AB3 molecules and ions: PCl3 , SO3 , AlCl3 , SO3 2– , and CH3 + . How many of these molecules and ions do you predict to have a trigonal-planar molecular geometry? (a) 1 (b) 2 (c) 3 (d) 4 (e) 5

Answer 1

Out of PCl3, SO3, AlCl3, SO32−, and CH3+, SO3, AlCl3, and CH3+ are predicted to have a trigonal-planar molecular geometry, a total of 3, so the correct answer is (c) 3.

The student has listed various AB3 molecules and ions (PCl3, SO3, AlCl3, SO3 2−, and CH3 +) and is asking how many of these have a trigonal-planar molecular geometry. To answer this, we need to consider the steric number and presence of lone pairs on the central atom in each structure.

• PCl3: The central atom P has three bonded atoms and one lone pair, giving it a trigonal pyramidal geometry, not trigonal planar.
• SO3: The central atom S has three bonded atoms and no lone pairs, resulting in a trigonal-planar molecular geometry.
• AlCl3: Al has three bonded atoms and no lone pairs indicating a trigonal-planar molecular geometry.
• SO3 2−: The central atom S has three bonded atoms and one lone pair, which leads to a bent molecular shape, even though it initially seems to have a trigonal planar electron-pair geometry.
• CH3+: The central atom C has three bonded atoms and no lone pairs, which means it has a trigonal-planar molecular geometry.

Therefore, the molecules and ions predicted to have a trigonal-planar molecular geometry are SO3, AlCl3, and CH3+, which makes a total of 3. So the correct answer is (c) 3.