High School Chemical Bonding
IN THIS LESSON
Learn about a theory that is relevant and helpful when understanding chemical bonding.
1/4/25: I consider this lesson complete right now, but I intend to add diagrams to help understand the different bond shapes visually.
VSEPR Theory stands for Valence Shared Electrons Pair Repulsion Theory. It is based in the idea that electron pairs will repel each other and therefore will try to get as far away from each other as possible, creating unique shapes.
In orbital geometry, where there are no molecules bonded using any electron pairs, the pairs will be distributed around the nucleus symmetrically depending on how many electron pairs there are.
When an atom has two electron pairs, it will form a linear orbital shape. The two electron pairs will be on opposite sides of the nucleus, 180° apart.
When an atom has three electron pairs, it will form a trigonal planar orbital shape. The three electron pairs will form a sort of triangle shape around the nucleus, all 120° apart from each other.
When an atom has four electron pairs, it will form a tetrahedral orbital shape. This is a 3D shape. All the electron pairs will form a triangular pyramid with the nucleus in the center of the shape; each electron pair will be 109.5° apart from each other. This is the most important and common orbital shape.
When an atom has five electron pairs, it will form a trigonal bipyramid orbital shape. This is another 3D shape. Three of the electron pairs will form into a flat triangle around the nucleus, while the other two will be over and under the nucleus, making the shape of two triangular pyramids with one of each of their faces pressed together. This combined shape will have the nucleus in the direct center. The electron pairs in the flat triangle will be 120° apart and each of those pairs will be 90° apart from the other two electron pairs.
When an atom has six electron pairs, it will form an octahedral orbital shape. This is also a 3D shape. It is similar to the trigonal bipyramid except the flat shape in the center is a square, not a triangle. The nucleus will be in the center of the shape like the rest. Each electron pair will be 90° away from the next closest pairs.
In molecular geometry, the shapes are determined by the orbital geometry of an atom. However, it focuses on the 3D arrangement of the atoms, not including the lone pairs. The lone pairs still have an impact on molecular shapes, though. The number of orbitals that need to be accounted for in the molecular shape of a molecule is the number of sigma (σ) bonds plus the number of lone electron pairs.
Molecular geometry contains the same basic molecular shapes as the orbital shapes, all with the same names and properties. These are formed when all electron pairs are bonded to another atom. There are also other shapes for the same amounts of electron pairs for when there are still lone electron pairs. Lone pairs repel slightly more than bonded pairs.
When there are three electron pairs and one of those pairs is unbonded, the molecule forms a bent molecular shape, away from the lone pair.
When there are four electron pairs and one of those pairs is unbonded, the molecule forms a trigonal pyramidal molecular shape. This forms a triangular pyramid shape where the central atom is at the top of the shape, not in the center.
When there are four electron pairs and two of those pairs are unbonded, the molecule forms a bent molecular shape, similar to the other bent molecular shape.
When there are five electron pairs and one of those pairs is unbonded, the molecule forms a seesaw molecular shape. This shape is like the trigonal bipyramid, but one of the molecules in the flat triangle in the center is gone.
When there are five electron pairs and two of those pairs are unbonded, the molecule forms a T-shaped molecular shape. This shape is like the trigonal bipyramid, but two of the molecules in the flat triangle in the center are gone. What is left is a flat, T-shaped arrangement of four atoms.
When there are five electron pairs and three of those pairs are unbonded, the molecule forms a linear molecular shape similar to the standard molecular shape of a molecule with two electron pairs.
When there are six electron pairs and one of those pairs is unbonded, the molecule forms a square pyramidal molecular shape. This is a square pyramid shape with the central atom at the center of the square face.
When there are six electron pairs and two of those pairs are unbonded, the molecule forms a square planar molecular shape. This is just a flat square with the central atom in the middle.