Five-Domain Lone-Pair Shapes

Introduction

Replacing equatorial bonded atoms of a trigonal bipyramid with lone pairs changes the visible shape while retaining five electron domains. SF₄ becomes seesaw, ClF₃ becomes T-shaped and XeF₂ becomes linear in the familiar VSEPR descriptions. The pattern follows domain count and lone-pair placement.

Core explanation

SF₄ has four S–F bonding regions and one central lone pair in the standard Lewis/VSEPR model, commonly written AX₄E. Five total domains give trigonal bipyramidal electron geometry. Put the lone pair at an equatorial site to reduce close 90° interactions. The four F atoms occupy two axial and two remaining equatorial sites, producing a seesaw molecular shape. Bond angles deviate from ideal 90° and 120° because the lone pair and different positions repel unequally.

ClF₃ has three Cl–F bonding domains and two central lone pairs, AX₃E₂. The two lone pairs occupy equatorial sites in the basic model. The three F atoms then occupy two axial sites and one equatorial site, making a T-shaped molecule. Calling it trigonal planar because it has three fluorines would ignore the lone pairs; calling it trigonal bipyramidal as the molecular shape would include invisible electron regions as if they were atoms.

XeF₂ has two Xe–F bonds and three central lone pairs, AX₂E₃. All three lone pairs take equatorial positions in the simple five-domain scaffold, leaving the two F atoms axial and opposite each other. Its molecular shape is linear with F–Xe–F angle about 180°, even though the electron-domain geometry is trigonal bipyramidal. A linear atom arrangement does not imply only two electron domains; CO₂ and XeF₂ reach linear shapes through different domain counts.

Why prefer equatorial lone pairs? Each equatorial position has two 90° contacts to axial sites; an axial site has three 90° contacts to equatorial sites. Lone-pair electron density generally exerts a strong repulsive effect in the qualitative model, so placing it equatorially reduces the most unfavourable close contacts. With multiple lone pairs, equatorial sites also separate them around the plane.

These are qualitative structural assignments. Real bonds have measured angles and electronic descriptions beyond hard lobe repulsion. Hypervalent bonding should not be reduced to literal d-orbital promotion just because five domains are drawn. VSEPR is the shape tool here; electronic-structure theory addresses bond character.

Step-by-step reasoning

1. Draw a Lewis model and count central bonds and lone pairs. 2. Confirm total domains equals five. 3. Draw the trigonal bipyramidal electron scaffold. 4. Place lone pairs equatorially where possible. 5. Name the remaining atom positions: seesaw, T-shaped or linear.

Visual explanation

Show one trigonal bipyramid three times. Replace one, then two, then three equatorial atom markers with shaded lone-pair regions. Label the surviving atom-only silhouettes SF₄ seesaw, ClF₃ T and XeF₂ straight line.

Real-world analogy

Five positions on a stage can be occupied by performers or equipment. The stage layout remains five-position, but the visible performers form different patterns as equipment takes some positions. Lone pairs influence geometry without appearing as atoms in molecular-shape names.

Real-world example

XeF₂ is a striking counterexample to the idea that linear molecules must have only two electron domains. Its three equatorial lone pairs and two axial bonds show why Lewis counting must precede shape classification.

Why?

Why is ClF₃ T-shaped rather than trigonal planar? Its chlorine centre has five domains, including two lone pairs. Equatorial placement of those lone pairs leaves two axial and one equatorial F positions, which make a T.

Common misconception

“XeF₂ has two bonds, so it is AX₂ with two domains like CO₂.” Xenon has three lone pairs in the conventional Lewis/VSEPR count, giving AX₂E₃ and five domains despite its linear atom shape.

Worked example

Predict XeF₂ shape. Xe supplies eight valence electrons and two F atoms fourteen, total twenty-two. Two Xe–F bonds use four electrons; the two F atoms use twelve in lone pairs, leaving six electrons as three Xe lone pairs. Xenon has two bonding plus three nonbonding domains. Place the three lone pairs equatorially in a trigonal bipyramid; F atoms are axial, 180° apart. Electron geometry is trigonal bipyramidal, molecular shape linear.

Quick check

1. What molecular shape follows AX₄E in a five-domain arrangement? Answer: Seesaw, as in the common SF₄ model.

Exam focus

Show electron-domain and molecular-shape names separately. Put lone pairs equatorially in these trigonal bipyramidal examples. State angle values as ideal or approximate and avoid unsupported orbital-mechanism claims.

Advanced insight

Multicentre bonding and delocalised molecular orbitals give a richer account of XeF₂ than five localised electron-pair lobes. VSEPR succeeds at predicting its linear geometry without needing to be a full electronic theory.

Summary

Five-domain molecules with one, two or three lone pairs commonly have seesaw, T-shaped or linear atom arrangements. Equatorial lone-pair placement minimises close 90° interactions in the simple model, while the electron scaffold remains trigonal bipyramidal.

Practice questions

1. What are the bond/lone-pair counts for SF₄? Answer: Four bonding domains and one lone pair, AX₄E. 2. What is ClF₃'s molecular shape? Answer: T-shaped, with two equatorial lone pairs. 3. Why is XeF₂ linear? Answer: Three lone pairs occupy equatorial positions, leaving two opposite axial Xe–F bonds. 4. Do SF₄, ClF₃ and XeF₂ share an electron-domain geometry? Answer: Yes. All have five domains with a trigonal bipyramidal electron arrangement.