Determining Molecular Shape: Pyramidal Geometry

To identify which species has a pyramidal shape, we need to analyze their molecular geometry using VSEPR theory. Pyramidal geometry occurs when a molecule has 4 electron domains (3 bonding pairs + 1 lone pair) around the central atom.

Step 1: Determine Electron Domains for Each Option

Option 1: SiO₃^2–

Central atom: Si (Group 14, 4 valence electrons)

O atoms contribute 0 electrons (each O forms double bond)

Charge: +2 electrons (from 2– charge)

Total valence electrons: 4 + 2 = 6

Electron domains: 3 (all bonding, no lone pairs)

Geometry: Trigonal planar

Option 2: OSF₂

Central atom: S (Group 16, 6 valence electrons)

O contributes 0 electrons (double bond), F atoms contribute 1 each

Total valence electrons: 6 + 2 = 8

Electron domains: 3 bonding + 1 lone pair = 4 domains

Geometry: Pyramidal

Option 3: SO₃

Central atom: S (Group 16, 6 valence electrons)

O atoms contribute 0 electrons (all form double bonds)

Total valence electrons: 6

Electron domains: 3 (all bonding)

Geometry: Trigonal planar

Option 4: BrF₃

Central atom: Br (Group 17, 7 valence electrons)

F atoms contribute 1 each: 3 electrons

Total valence electrons: 7 + 3 = 10

Electron domains: 3 bonding + 2 lone pairs = 5 domains

Geometry: T-shaped

Step 2: Identify Pyramidal Geometry

Only OSF₂ has the required 3 bonding pairs + 1 lone pair configuration that gives pyramidal geometry.

Final Answer

The species with pyramidal shape is OSF₂

Related Concepts

VSEPR Theory: Valence Shell Electron Pair Repulsion theory predicts molecular geometry based on electron domain arrangement around central atoms.

Electron Domain Formula:

Electron domains = bonding pairs + lone pairs

Common Molecular Geometries

2 domains: Linear
3 domains: Trigonal planar
4 domains: Tetrahedral (if no lone pairs), Pyramidal (if 1 lone pair), Bent (if 2 lone pairs)
5 domains: Trigonal bipyramidal
6 domains: Octahedral