Molec Geometry Worksheets | PDF | Molecular Orbital | Chemical Bond - Free Printable

Let’s solve each molecule step by step. We’ll draw the Lewis structure, count electron pairs (bonding and lone), then use that to find the shape and bond angle.

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1. SeO₃

- Selenium (Se) is in group 16 → 6 valence electrons.
- Oxygen (O) is also group 16 → 6 valence electrons each.
- Total valence electrons = 6 + 3×6 = 24

Lewis Structure:
- Put Se in center, surrounded by 3 O atoms.
- Make single bonds first: Se–O (uses 6 electrons).
- Remaining 18 electrons go as lone pairs on O atoms (each O gets 3 lone pairs → 6 electrons per O × 3 = 18).
- But now Se has only 6 electrons — not enough! So we make double bonds.
- Best structure: Se forms double bonds with all 3 O atoms. Each double bond uses 4 electrons → 3×4 = 12 electrons used in bonding.
- Remaining 12 electrons go as lone pairs on O atoms (each O gets 2 lone pairs → 4 electrons per O × 3 = 12).
- Se has no lone pairs, 3 bonding regions (all double bonds count as one region each).

e⁻ Tally:
- Bonding pairs: 3 regions (even though they’re double bonds, for geometry we count them as one “domain” each)
- Lone pairs on central atom (Se): 0

Shape: Trigonal planar
Bond Angle: 120°

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2. AsH₃

- Arsenic (As) is group 15 → 5 valence electrons.
- Hydrogen (H) is group 1 → 1 valence electron each.
- Total valence electrons = 5 + 3×1 = 8

Lewis Structure:
- As in center, bonded to 3 H atoms (single bonds → 6 electrons used).
- Remaining 2 electrons go as a lone pair on As.
- So As has 3 bonding pairs + 1 lone pair.

e⁻ Tally:
- Bonding pairs: 3
- Lone pairs on central atom: 1

Shape: Trigonal pyramidal (because of the lone pair pushing down)
Bond Angle: Less than 109.5° → about 107° (like ammonia, NH₃)

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3. NO₂⁻

- Nitrogen (N) is group 15 → 5 valence electrons.
- Oxygen (O) is group 16 → 6 each → 2×6 = 12
- Add 1 extra electron because of the negative charge → total = 5 + 12 + 1 = 18

Lewis Structure:
- N in center, bonded to two O atoms.
- Try single bonds first: N–O and N–O → 4 electrons used.
- Remaining 14 electrons: give each O 3 lone pairs (6 electrons each → 12 total), leaving 2 electrons → put as lone pair on N.
- Now N has 2 bonds + 1 lone pair → only 5 electrons? Not good.
- Better: make one double bond and one single bond.
- Double bond to one O (uses 4 electrons), single bond to other O (2 electrons) → total bonding = 6 electrons.
- Remaining 12 electrons:
- The double-bonded O gets 2 lone pairs (4 electrons)
- The single-bonded O gets 3 lone pairs (6 electrons) → but it needs a negative charge? Wait...
- Actually, best structure: N has one double bond, one single bond, and one lone pair. The single-bonded O carries the negative charge (has 3 lone pairs).
- Central N: 2 bonding regions (one double, one single — both count as one domain each) + 1 lone pair → total 3 electron domains.

Wait — correction: In VSEPR, double bonds still count as ONE electron domain. So:

e⁻ Tally:
- Bonding domains: 2 (one to each O)
- Lone pairs on central atom (N): 1

But wait — let’s recount properly.

Total valence electrons: 18

Structure:
- N central
- One O double bonded to N (N=O)
- One O single bonded to N (N–O⁻) → this O has 3 lone pairs and a negative charge
- N has one lone pair

So around N:
- 2 atoms attached → 2 bonding domains
- 1 lone pair → 1 nonbonding domain
→ Total 3 electron domains → trigonal planar electron geometry, but molecular shape is bent.

Shape: Bent (or angular)
Bond Angle: Less than 120° → about 115° (due to lone pair repulsion)

Actually, standard value for NO₂⁻ is approximately 115°.

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4. BeF₂

- Beryllium (Be) is group 2 → 2 valence electrons.
- Fluorine (F) is group 17 → 7 each → 2×7 = 14
- Total valence electrons = 2 + 14 = 16

Lewis Structure:
- Be in center, bonded to two F atoms with single bonds → 4 electrons used.
- Remaining 12 electrons go as lone pairs on F atoms (each F gets 3 lone pairs → 6 electrons each × 2 = 12).
- Be has only 4 electrons — but Be is an exception! It can be electron-deficient. No lone pairs on Be.

e⁻ Tally:
- Bonding pairs: 2
- Lone pairs on central atom: 0

Shape: Linear
Bond Angle: 180°

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Final Answer:

1. SeO₃
- Lewis Diagram: Se double bonded to three O atoms, no lone pairs on Se
- e⁻ Tally: 3 bonding domains, 0 lone pairs
- Shape: Trigonal planar
- Bond Angle: 120°

2. AsH₃
- Lewis Diagram: As bonded to three H atoms, one lone pair on As
- e⁻ Tally: 3 bonding domains, 1 lone pair
- Shape: Trigonal pyramidal
- Bond Angle: ~107°

3. NO₂⁻
- Lewis Diagram: N bonded to one O with double bond, one O with single bond (with negative charge), one lone pair on N
- e⁻ Tally: 2 bonding domains, 1 lone pair
- Shape: Bent
- Bond Angle: ~115°

4. BeF₂
- Lewis Diagram: Be bonded to two F atoms with single bonds, no lone pairs on Be
- e⁻ Tally: 2 bonding domains, 0 lone pairs
- Shape: Linear
- Bond Angle: 180°