Short Answer

The geometry of a molecule is determined mainly by the arrangement of electron pairs around the central atom. These electron pairs include both bonding pairs and lone pairs, and they repel each other. To minimize this repulsion, they arrange themselves as far apart as possible, which creates a specific molecular shape.

Other factors like the number of atoms bonded to the central atom, the presence of lone pairs, and the type of bonding also influence the final geometry. The VSEPR theory is commonly used to predict the shape of molecules based on electron pair repulsion.

Detailed Explanation :

What Determines the Geometry of a Molecule

The geometry of a molecule refers to the three-dimensional arrangement of its atoms in space. This shape is not decided randomly; it depends on several predictable chemical principles. The most important factor is the repulsion between electron pairs around the central atom. Electron pairs naturally try to stay as far apart from each other as possible because they carry negative charges and repel one another. This concept forms the basis of VSEPR (Valence Shell Electron Pair Repulsion) theory, which helps chemists predict the shapes of simple molecules accurately.

Molecular geometry affects many chemical properties, including polarity, reactivity, bond angles, and physical behavior. Understanding what determines molecular geometry also helps explain how molecules interact, fit together, and participate in chemical reactions.

1. Role of Electron Pair Repulsion

Every central atom in a molecule has electron pairs surrounding it. These pairs can be:

• Bonding pairs (shared between atoms)
• Lone pairs (non-bonding pairs on the central atom)

Because electrons repel each other, they try to position themselves as far apart as possible. This repulsion creates a specific geometry.

Examples:

• Two electron pairs → linear arrangement
• Three electron pairs → trigonal planar arrangement
• Four electron pairs → tetrahedral arrangement

The number of electron domains (bonding + lone pairs) is the primary factor determining molecular geometry.

2. Difference Between Electron Geometry and Molecular Geometry

Electron geometry considers all electron pairs, while molecular geometry considers only atoms (bonding pairs).

For example:

• In NH₃, electron geometry is tetrahedral (4 electron pairs).
• Molecular geometry is trigonal pyramidal (because 1 pair is lone).

Thus, lone pairs influence but do not appear directly in the molecular shape.

3. Influence of Lone Pairs on Geometry

Lone pairs occupy more space than bonding pairs because they are held more closely to the central atom and are not shared. This extra repulsion affects bond angles and shapes:

• Lone pair–lone pair repulsion > lone pair–bonding pair > bonding pair–bonding pair

Consequences:

• Lone pairs compress bond angles
• Lone pairs change the expected shape

Examples:

• H₂O: tetrahedral electron geometry → bent molecular geometry
• NH₃: tetrahedral electron geometry → trigonal pyramidal shape
• CO₂: no lone pairs on central atom → linear shape

Thus, lone pairs are a major factor in determining geometry.

4. Number of Bonding Atoms

The number of atoms bonded to the central atom affects the final shape.
Different numbers of bonding atoms lead to different geometries:

• 2 bonded atoms → linear or bent
• 3 bonded atoms → trigonal planar or trigonal pyramidal
• 4 bonded atoms → tetrahedral
• 5 bonded atoms → trigonal bipyramidal
• 6 bonded atoms → octahedral

The arrangement of these bonded atoms depends on electron pair repulsion.

5. Multiple Bonds and Their Effect

Double and triple bonds contain extra electron density. Even though they count as one electron region in VSEPR theory, they exert more repulsion than single bonds.

Effects include:

• Slightly smaller bond angles around multiple bonds
• Distortion of geometry in molecules with a mix of single and multiple bonds

Example:

• In formaldehyde (CH₂O), the double bond between C and O affects the trigonal planar arrangement.

6. Electronegativity and Bond Polarity

Electronegative atoms pull shared electrons closer, changing the distribution of electron density around the central atom. This may slightly adjust bond angles or distort symmetry.

Example:

• In SO₂, the electronegativity of oxygen influences the bent shape.

7. Hybridization and Geometry

Hybridization of orbitals helps explain geometry:

• sp → linear
• sp² → trigonal planar
• sp³ → tetrahedral
• sp³d → trigonal bipyramidal
• sp³d² → octahedral

Hybridization aligns with VSEPR predictions and supports molecular shapes.

8. Real-Life Importance of Molecular Geometry

The shape of a molecule affects:

• Polarity (e.g., H₂O is polar because of its bent shape)
• Biological activity (drug molecules fit into receptors by shape)
• Reactivity and stability
• Physical properties like boiling point and solubility

Thus, molecular geometry is essential in chemistry, biology, and material science.

Conclusion

The geometry of a molecule is determined mainly by the repulsion between electron pairs around the central atom. Both bonding pairs and lone pairs influence the arrangement, with lone pairs causing greater distortion. Additional factors such as hybridization, electronegativity, and the number of bonded atoms also shape the final geometry. Through VSEPR theory, these influences can be predicted and used to understand molecular properties, interactions, and behavior.