Grade 11 → Chemical Bonding and Molecular Structure ↓
VSEPR Theory
Valence shell electron pair repulsion (VSEPR) theory is a simple but effective method used in chemistry to predict the shape or geometry of molecules. This theory, proposed by Ronald Gillespie and Ronald Nyholm in the early 1960s, is essential for understanding molecular structures determined by electron pair interactions around the central atom in the molecule.
Fundamentals of vsepr theory
VSEPR theory is based on several fundamental principles that guide the prediction of molecular shapes:
- Electron pairs, both bonded and unbonded (lone pairs), repel each other and therefore arrange themselves as far apart as possible around the central atom.
- The geometry of a molecule is determined by the number and type of electron pairs present around the central atom.
- Lone pairs exert greater repulsion than bonding pairs, causing the bond angles to deviate from their ideal values.
Types of electron pairs
Before we explore molecular shapes, it is necessary to understand the two types of electron pairs considered in the VSEPR theory:
- Bond pair: These are shared between two atoms to form a covalent bond.
- Non-bonding pairs (lone pairs): These are not shared between atoms but belong to the same atom, which usually affect the shape more due to their repulsion force.
General molecular geometry
Below are common molecular geometries according to VSEPR theory along with visual examples:
Linear geometry
Molecules of the type AB 2
have a linear geometry, where the two B atoms are 180 degrees away from the central A atom. An example of this is carbon dioxide, CO 2
.
Trigonal planar geometry
Molecules like BF 3
have trigonal planar geometry. Here, the B atoms are spread out at 120 degrees in one plane.
Tetrahedral geometry
Tetrahedral geometry, with a bond angle of about 109.5°, is seen in molecules such as methane, CH 4
.
The effect of single joints
Lone pairs are important in determining the shape of the molecule. They occupy more space due to increased repulsion, which leads to variations in bond angles:
Bent geometry
Consider water, H 2 O
The presence of two lone pairs on oxygen compresses the bond angle between the hydrogen atoms to about 104.5 degrees, whereas 109.5 degrees would be expected in a tetrahedral setup.
Triangular pyramid geometry
In ammonia, NH 3
, a lone pair causes the molecule to adopt a trigonal pyramidal shape, reducing the ideal 109.5° angle to about 107°.
Steps to determine molecular geometry using VSEPR
- Write the Lewis structure: Determine the structure by arranging the electrons around the atoms to satisfy their octets.
- Identify electron pairs: Calculate the regions of electron density (bonding and nonbonding) around the central atom.
- Determine the electron pair geometry: Associate the total number of electron pair regions with its corresponding geometry.
- Account for lone pairs: Adjust the geometry to account for repulsion from lone pairs to determine the actual molecular shape.
Examples of molecular shape prediction
Example 1: Methane (CH 4
)
Lewis Structure: Carbon is the central atom with four hydrogen atoms attached to it. No lone pair is found on carbon.
Classification: Four bond pairs, consistent with tetrahedral geometry.
Shape: Tetrahedral, bond angle of about 109.5°.
Example 2: Sulfur dioxide (SO 2
)
Lewis Structure: Sulfur is the central atom with a lone pair and is bonded to two oxygen atoms.
Classification: Two bond pairs and one lone pair give the initial geometry of trigonal planar line.
Shape: Bent or V shaped due to lone pair repulsion, bond angle about 119 degrees.
Applications in chemistry
VSEPR theory is invaluable for predicting molecular geometry in covalent compounds. This insight is important for determining a compound's physical properties, reactivity, and designing new chemicals within a laboratory setting. Understanding molecular shapes aids in discovering interactions between different molecules, which is fundamental in fields such as drug design or the synthesis of new materials.