What is VSEPR Theory?
VSEPR Theory is short for Valence Shell Electron Pair Repulsion Theory, a method of organizing molecules based on their geometric structures. In chemistry, VSEPR Theory is based on the principle that each atom in a molecule will seek a geometry that maximizes the distance between valence electron pairs, thus minimizing electron-electron repulsion. Valence electrons repel one another because they are negatively charged and like charges repel.
VSEPR Theory of Molecules without lone electron pairs on the central atom
For simplicity and organization, we will separate VSEPR structures into two categories: those with lone pairs on the central atom, and those without lone pairs on the central atom.
BeF2 is an example of a linear molecule. There are 16 total valence electrons in a BeF2 molecule, with three lone pairs (six electrons)on each fluorine atom. In order for these lone pairs on each respective fluorine atom to be the furthest distance possible from the other fluorine atom, the molecule forms a straight line. We refer to this shape as “Linear”. The bond angles in a linear molecule are 180 degrees. CO2 and BeH2 are also linear molecules.
BF3 is an example of a trigonal planar molecule. There are 24 total valence electrons in a BF3 molecule. In order for each fluorine atom to keep its lone pairs as far away as possible from the other fluorine atoms, the molecule forms a triangular, 2-dimensional shape. In molecular geometry, this is known as “trigonal planar”. The bond angles in a trigonal planar molecule are 120 degrees. CO3 (carbonate) is another example of a trigonal planar molecule.
A common example of a tetrahedral molecule is CH4 (methane). There are eight total valence electrons in a methane molecule. In order for the four hydrogens to be furthest apart from one another, we arrive at a tetrahedral shape. Tetrahedral is the 3-dimensional expression of square planar geometry. The H-C-H bond angle in a tetrahedral molecule is 109.5 degrees.
PF5 is an example of a Trigonal Bipyramidal molecule. PF5 has 38 total valence electrons. Each P-F bond uses 2 valence electrons and each fluorine atom has three lone pairs. Phosphorus can expand its octet. Three of the fluorine atoms are in what is called the equatorial position. The other two are in what is called the axial position. In order for the equatorial fluorines to be the furthest distance away from each other as possible, the P-F bonds are at a 120-degree angle with respect to each other, and the bond angles between the equatorial and axial positions are 90 degrees.
SF6 is an example of an octahedral molecule. There are 48 valence electrons in an SF6 molecule. Each S-F bond accounts for two valence electrons, and each fluorine atom carries three lone pairs. Sulfur can expand its octet. The repulsion between the fluorine atoms can be minimized by placing each one at the corner of an octahedron.
VSEPR Theory of molecules with lone electron pairs on the central atom
H2O is an example of a bent molecule. When the central atom in a molecule has lone pairs, these lone pairs repel the bonds rooted in the central atom. In a water molecule, the lone pairs on the oxygen atom force the hydrogen bonds downwards in 2-dimensional space. The bond angle between the hydrogen atoms is 104 degrees.
NH3 is an example of a trigonal pyramidal molecule. In the ammonia molecule, the lone pair on the central nitrogen atom pushes the three N-H bonds downwards due to electron-electron repulsion.
SF4 is an example of a molecule with a seesaw shape. In an SF4 molecule, two of the S-F bonds are situated across from one another in the equatorial plane. The other two S-F bonds direct away from each other in 3-dimensional space. This allows the fluorine atoms to be the greatest distance apart from one another considering there is a lone electron pair on the central sulfur atom. This lone pair pushes the S-F bonds away, much like in bent or trigonal pyramidal geometries.
BrF3 is an example of a T-Shaped molecule. In a BrF3 molecule, there are two lone pairs on the Bromine central atom, forcing more extreme electron-electron repulsion with the Br-F bonds than in other geometries. This shape proceeds bond angles of 86.2 degrees, which is unique to T-Shaped molecules.
In a BrF5 molecule, the geometry closely mimics octahedral geometry. The only difference is that in square pyramidal geometry, one of the axial atoms is replaced with a lone electron pair.
In the square planar molecule XeF4, the geometry closely mimics that of square pyramidal, but for that, the axial bond has been replaced with another lone electron pair. This pushes all four Xe-F bonds into a planar, equatorial arrangement.
The AXE method is an alternative way of expressing molecular geometries. In the AXE model, the A represents the central atom. The X represents the central atom, the X represents the number of single bonds (or coordination number) connected to the central atom, and E represents the number of lone electron pairs located on the central atom.
VSEPR Chart & Steric Number
This VSEPR chart shows you all of the common VSEPR geometries, organized by the steric number and how many lone electron pairs they have. The steric number is how many atoms are bonded to a central atom of a molecule plus the number of lone electron pairs attached to that atom. It is used in valence shell electron pair repulsion theory to find the molecular geometry.
Real-world application of VSEPR Theory
The chemical properties of some molecules often reflect their geometric structures, and VSEPR is the best way to make an educated assertion about the structure of a particular molecule.
Properties often denoted by molecular structure include polarity, color, diamagnetism/paramagnetism, and biological activity. Geometric structures also give chemists and other scientists a productive way to organize molecules.
VSEPR Theory Wrap-Up
Valence shell electron pair repulsion theory is a method of predicting the geometry of molecules. It is based on the core concept that electrons repel one another due to their similar charges, and molecules construct themselves in a way that puts the greatest possible distance between lone electron pairs. Most elementary molecules can fit into 11 different shape categories, and we can predict these accurately simply by knowing the number of valence electrons, recognizing the central atom, and using VSEPR theory.