Questions: Molecular Geometry: VSEPR Theory and 3D Structure

Question 1

Nitrogen in ammonia (NH₃) has 3 bonding pairs and 1 lone pair. A student predicts trigonal planar molecular geometry, reasoning that nitrogen forms 3 bonds. What is the student's error?

• A. NH₃ should be tetrahedral because all four electron groups, including the lone pair, point to atom positions
• B. The lone pair occupies one position of the tetrahedral electron geometry, making the molecular geometry trigonal pyramidal, not planar
• C. Nitrogen's 3 bonds do produce trigonal planar geometry; the student's prediction is correct
• D. Lone pairs do not count as electron groups in VSEPR theory, so the student should use only the 3 bonds

Answer: (B)

The student confused molecular geometry (where atoms are) with electron geometry (where all electron pairs are). NH₃ has 4 electron groups (3 bonds + 1 lone pair), giving tetrahedral electron geometry. But the molecular geometry describes only where the atoms are: the lone pair occupies one tetrahedral corner and is 'invisible,' so the 3 N–H bonds point to the other three corners, producing a trigonal pyramidal shape. Option 3 is the opposite error — lone pairs absolutely count as electron groups; ignoring them is precisely the mistake.

Question 2

Water (H₂O) has an H–O–H bond angle of 104.5°, less than the ideal tetrahedral angle of 109.5°. What best explains this compression?

• A. Oxygen's high electronegativity pulls bonding electrons toward itself, drawing the hydrogen atoms closer together
• B. The two lone pairs exert greater repulsion on the bonding pairs than bonding pairs exert on each other, compressing the bond angle
• C. Water has trigonal planar electron geometry with an ideal 120° angle, reduced by oxygen's electronegativity to 104.5°
• D. The hydrogen atoms are too small to maintain 109.5° separation, so they fall closer together

Answer: (B)

Water has tetrahedral electron geometry (4 groups: 2 bonds + 2 lone pairs). Lone pairs are held closer to the central atom and spread out over a wider angular region than bonding pairs, exerting stronger repulsion on neighboring groups. The two lone pairs squeeze the two O–H bonds closer together, compressing the bond angle below the ideal 109.5°. Each additional lone pair adds more compression: NH₃ (1 lone pair) has 107°; H₂O (2 lone pairs) has 104.5°. Option 2 is wrong — water does not have trigonal planar electron geometry (which requires 3 electron groups, not 4).

Question 3

The electron geometry and molecular geometry of a molecule are typically identical.

• T. True
• F. False

Answer: False

They differ whenever the central atom has lone pairs. Electron geometry includes lone pairs in determining spatial arrangement; molecular geometry describes only where the atoms are. For example, NH₃ has tetrahedral electron geometry but trigonal pyramidal molecular geometry, and H₂O has tetrahedral electron geometry but bent molecular geometry. Electron and molecular geometry are the same only when all electron groups are bonding pairs (e.g., CH₄ is tetrahedral in both).

Question 4

A molecule with 4 electron groups and 1 lone pair on the central atom has trigonal pyramidal molecular geometry.

• T. True
• F. False

Answer: True

Four electron groups give tetrahedral electron geometry (109.5° ideal angles). With 1 of those groups being a lone pair, only 3 groups are bonds to atoms. The three bonded atoms sit at three corners of the tetrahedron, with the lone pair occupying the fourth — producing a trigonal pyramidal molecular shape. NH₃ is the canonical example. This is distinct from trigonal planar geometry (3 electron groups, all bonding, 120° angles, no lone pair).

Question 5

What is the difference between electron geometry and molecular geometry? Use SF₄ (5 electron groups: 4 bonds and 1 lone pair) to illustrate why the distinction matters.

Model answer: Electron geometry describes the spatial arrangement of ALL electron groups around the central atom, including lone pairs. Molecular geometry describes only where the bonded atoms are, ignoring lone pairs. For SF₄, 5 electron groups give trigonal bipyramidal electron geometry. The lone pair preferentially occupies an equatorial position (less repulsion there), leaving 4 S–F bonds: 2 axial and 2 equatorial. This produces a 'see-saw' molecular geometry — not trigonal bipyramidal. Without the distinction, you would incorrectly predict a symmetric trigonal bipyramidal shape and make wrong predictions about polarity and reactivity.

The lone pair's position in SF₄ is not arbitrary — it occupies equatorial rather than axial because equatorial positions have fewer 90° interactions with other groups (which are the most repulsive). This preference for equatorial lone pairs is a specific application of VSEPR logic that is only visible once you distinguish electron from molecular geometry and think carefully about where repulsion is minimized.