Short Answer

The shape of a molecule with AX₃ configuration is trigonal planar. In this arrangement, the central atom (A) is bonded to three surrounding atoms (X), and there are no lone pairs on the central atom. Because three electron groups repel each other equally, they arrange themselves at 120° angles in a flat triangular shape.

Examples of AX₃ trigonal planar molecules include boron trifluoride (BF₃), formaldehyde (CH₂O), and sulfur trioxide (SO₃). This geometry is explained by VSEPR theory based on minimum electron pair repulsion.

Detailed Explanation :

Shape of a Molecule with AX₃ Configuration

The AX₃ configuration is used in VSEPR theory to describe molecules where the central atom (A) forms three bonding pairs with surrounding atoms (X) and has no lone pairs. Because all three bonding pairs repel one another equally, they position themselves as far apart as possible. The farthest three points can spread in a single plane is at 120°, creating a trigonal planar geometry.

This arrangement results in a flat, triangular shape with all atoms lying in the same plane. The trigonal planar shape is a direct result of electron pair repulsion and is stable because it minimises repulsion among the three bonding pairs.

1. VSEPR Basis for AX₃ Geometry

According to VSEPR theory:

• Electron pairs repel each other.
• Three electron pairs around the central atom create three electron domains.
• These domains arrange themselves at 120° apart, forming a trigonal planar pattern.

Since AX₃ has no lone pairs, the molecular shape is the same as the electron geometry.

Thus:

• Electron geometry = trigonal planar
• Molecular geometry = trigonal planar

This is unlike AX₃E molecules (like NH₃), where lone pairs distort the shape.

2. Bond Angle in AX₃ Molecules

AX₃ molecules have bond angles of 120°.
These angles are ideal and are not affected by lone pair repulsion because lone pairs are absent.

This balanced arrangement ensures:

• Equal bond lengths
• Symmetrical geometry
• Uniform distribution of electron density

3. Characteristics of Trigonal Planar AX₃ Molecules

(a) Planar Structure

All atoms lie in the same plane.
The molecule looks like a triangle around the central atom.

(b) Symmetry

AX₃ molecules are often highly symmetrical, especially if all X atoms are the same.
Such molecules are often non-polar.

(c) No Lone Pair Distortion

Since AX₃ has no lone pairs, the geometry is not distorted.
The shape remains perfect trigonal planar.

4. Examples of AX₃ Molecules
5. Boron Trifluoride (BF₃)

• Central atom: Boron
• Three F atoms around B
• No lone pair on boron
• Perfect trigonal planar shape
• Bond angle = 120°

2. Formaldehyde (CH₂O)

• Carbon is the central atom
• Two H atoms and one O atom bonded to carbon
• All bonds lie in a plane
• Trigonal planar geometry

3. Sulfur Trioxide (SO₃)

• Central atom S
• Three double-bonded oxygen atoms
• No lone pairs on sulfur
• Molecule is flat and trigonal planar

These examples demonstrate the typical geometry of AX₃ molecules.

5. Why Lone Pairs Are Not Present in AX₃

AX₃ notation specifically means:

• A = central atom
• X₃ = three atoms bonded
• No E = no lone pairs

If lone pairs existed, the configuration would change:

• AX₃E → trigonal pyramidal (like NH₃)
• AX₃E₂ → T-shaped

Therefore, AX₃ must always correspond to trigonal planar.

6. Effect of Hybridization on AX₃ Geometry

The typical hybridization for AX₃ molecules is sp² hybridization.

• Three sp² hybrid orbitals form sigma bonds with X atoms.
• The remaining unhybridized p orbital may participate in pi bonding (e.g., in CH₂O and SO₃).

This hybridization naturally leads to a planar structure with 120° angles.

7. Physical and Chemical Properties Related to Geometry

Polarity

• Symmetrical AX₃ molecules (BF₃) → non-polar
• Unsymmetrical AX₃ molecules (CH₂O) → polar

Reactivity

The trigonal planar arrangement allows easy approach of reagents.
This is important in many organic reactions like nucleophilic addition to carbonyls.

Bond Strength

Even spacing reduces repulsion and increases stability.

Conclusion

The shape of a molecule with AX₃ configuration is trigonal planar, with bond angles of 120° and all atoms lying in the same plane. This shape results from three bonding electron pairs that repel equally and position themselves as far apart as possible. VSEPR theory and sp² hybridization both support this geometry. Molecules like BF₃, CH₂O, and SO₃ clearly show this structure.