Molecular geometry describes the three-dimensional arrangement of atoms within a molecule. This spatial organization is fundamental to chemistry and biology, dictating how molecules interact. A molecule’s shape determines its polarity, affecting its solubility and boiling point, and controls its chemical reactivity. Predicting and understanding molecular shape is an essential scientific tool.
The VSEPR Principle: Why Molecules Have Shape
The model used to predict these shapes is the Valence Shell Electron Pair Repulsion (VSEPR) theory. This theory is based on the idea that electron domains surrounding a central atom repel each other. An electron domain is any region of high electron density, including single, double, or triple bonds, or non-bonding lone pairs. To minimize repulsion, these domains move as far apart as possible, establishing the molecule’s overall electron geometry.
This arrangement of electron domains determines the molecule’s specific shape. It is important to distinguish between electron geometry, which considers all electron domains, and molecular geometry, which considers only the positions of the atoms attached to the central atom. When a central atom has no lone pairs, the geometries are identical. However, lone pairs cause the two geometries to differ, as they exert repulsive forces that influence the atoms’ positions.
The Simplest Structures: Linear and Trigonal Planar
When a central atom is surrounded by only two electron domains, the maximum distance between them is achieved by placing them on opposite sides. This arrangement results in a Linear geometry, characterized by a bond angle of 180 degrees. Carbon dioxide (CO2), where the central carbon atom is double-bonded to two oxygen atoms, is a classic example of this structure.
A central atom with three electron domains arranges them into a plane, pointing towards the corners of an equilateral triangle. This geometry is known as Trigonal Planar, with electron domains separated by bond angles of 120 degrees. Boron trifluoride (BF3) exhibits this shape, as the central boron atom is bonded to three fluorine atoms with no lone pairs.
The Four-Domain Family: Tetrahedral, Bent, and Trigonal Pyramidal
When four electron domains surround a central atom, the electron geometry is always Tetrahedral. This structure involves the domains pointing towards the corners of a tetrahedron, maximizing separation and resulting in ideal bond angles of 109.5 degrees. Methane (CH4), with four single bonds and no lone pairs on the carbon atom, is the parent example of this molecular geometry.
Introducing a single lone pair changes the molecular geometry to Trigonal Pyramidal. The lone pair occupies one domain position, leaving the three remaining atoms positioned in a triangle beneath the central atom, forming a pyramid shape. Ammonia (NH3) has this structure, with three bonding pairs and one lone pair on the nitrogen atom. The lone pair exerts a greater repulsive force, compressing the bond angles to approximately 107 degrees.
If the central atom possesses two lone pairs and two bonding pairs, the molecular geometry becomes Bent, sometimes called V-shaped. The water molecule (H2O) exemplifies this structure, with the oxygen atom having two bonds and two lone pairs. These two lone pairs exert an even stronger repulsive effect than a single lone pair. This increased repulsion further decreases the bond angle to about 104.5 degrees.
Complex Geometries: Trigonal Bipyramidal and Octahedral
A molecule with five electron domains adopts a Trigonal Bipyramidal electron geometry. This shape is characterized by five domains pointing toward the corners of two pyramids joined at their bases, such as in phosphorus pentachloride (PCl5). This geometry is not uniform, featuring three equatorial positions lying in a flat triangle with 120 degree angles, and two axial positions located perpendicular to the plane at 90 degree angles.
Molecules with six electron domains arrange them in an Octahedral electron geometry. This structure, which appears in sulfur hexafluoride (SF6), places all six domains at 90 degree angles relative to their neighbors. The six positions are equivalent, pointing towards the six corners of an octahedron. These larger geometries are found in central atoms from the third period or below, as they have access to d-orbitals, allowing them to exceed the standard octet rule.