Illustrated below are incomplete Lewis structures for some simple molecules/ions.Which of the four structures shown have trigonal planar geometry?[Fill in the blank]

Question

BlackTom AI · Accepted Answer

Trigonal planar geometry requires a central atom with three regions of electron density (steric number 3) and no lone pairs on the central atom (AX3), giving sp2 hybridization and 120° bond angles.

1) Relevant concepts
- VSEPR: molecular shape depends on number of bonding regions and lone pairs (steric number).
- Steric number = number of bonds (count each bond region) + number of lone pairs on the central atom.
- Steric number 3 with zero lone pairs → trigonal planar; steric number 4 → tetrahedral electron geometry (can give trigonal pyramidal if one lone pair); steric number 2 → linear.

2) Step-by-step analysis of each structure (i–v)
(i) NH3: central N has three N–H bonds and one lone pair. Steric number = 3 bonds + 1 lone pair = 4 → electron geometry tetrahedral; molecular shape = trigonal pyramidal (not trigonal planar).

(ii) NH4+ (ammonium): central N has four N–H bonds, no lone pairs, steric number = 4 → tetrahedral geometry (bond angles ≈ 109.5°). Not trigonal planar.

(iii) H3O+ (hydronium): central O has three O–H bonds and one lone pair. Steric number = 3 + 1 = 4 → tetrahedral electron geometry; molecular shape = trigonal pyramidal (lone pair pushes the H atoms down). Not trigonal planar.

(iv) BeH2 (as shown): central Be has two Be–H bonds and no lone pairs. Steric number = 2 → linear geometry (180°). Not trigonal planar.

(v) BH4– (borohydride) or any BH4 species: central B with four B–H bonds and no lone pairs, steric number = 4 → tetrahedral. Not trigonal planar.

3) Intermediate counts (summary)
- (i) N: bonds = 3, lone pairs = 1 → steric 4 → trigonal pyramidal.
- (ii) N+: bonds = 4, lone pairs = 0 → steric 4 → tetrahedral.
- (iii) O+: bonds = 3, lone pairs = 1 → steric 4 → trigonal pyramidal.
- (iv) Be: bonds = 2, lone pairs = 0 → steric 2 → linear.
- (v) B–: bonds = 4, lone pairs = 0 → steric 4 → tetrahedral.

4) Why the correct answer is correct
None of the shown central atoms have the required combination (three bonding regions and zero lone pairs). Each structure’s steric number and lone-pair count produce geometries other than trigonal planar (pyramidal, tetrahedral or linear).

5) Why the other options are incorrect
- Option a (i and iii): both i and iii are trigonal pyramidal, not trigonal planar.
- Option b (i, iii, iv): i and iii are pyramidal; iv is linear — none are trigonal planar.
- Option c (iii): hydronium (iii) is trigonal pyramidal due to a lone pair, so it is not trigonal planar.

6) Conclusion
All analyses confirm that none of the molecules shown are trigonal planar. This matches the provided correct answer: d. None of the molecules shown.

Illustrated below are incomplete Lewis structures for some simple molecules/ions.Which of the four structures shown have trigonal planar geometry?[Fill in the blank]单项选择题

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