Molecular Shape and Geometry
Molecular geometry describes the arrangement of atoms in a molecule. Bonding and the distribution of electrons help determine that arrangement, which affects properties such as polarity and how molecules interact.
THE IDEA
Molecular Shape and Geometry
Life is a relationship among molecules and not a property of any molecule.
—Linus Pauling
Molecular geometry describes the three-dimensional arrangement of atoms in a molecule. Bonding and the distribution of electrons, including lone pairs, help determine its shape. Composition and geometry both affect a molecule’s properties.
A bond angle is the angle between two bonds that share an atom. Common molecular shapes include:
- Linear: The atoms lie on a straight line. For a central atom bonded to two neighbors, the bond angle is 180 degrees, as in carbon dioxide (CO2). A diatomic molecule such as nitric oxide (NO) is also linear, but has no bond angle.
- Trigonal planar: A central atom and its three bonded neighbors lie in one plane, with the neighbors arranged around the center. They do not all lie on one line.
- Trigonal pyramidal: Three bonded atoms form a triangular base, with the central atom outside that plane. Ammonia (NH3) is an example.
- Tetrahedral: Four atoms occupy the corners of a tetrahedron around a central atom, as in methane (CH4).
- Square planar: Four bonded atoms lie in a plane around the central atom. In xenon tetrafluoride (XeF4), two lone pairs occupy opposite positions outside that plane. The electron-domain arrangement is octahedral, while the molecular shape is square planar, with adjacent bond angles of 90 degrees.
- Trigonal bipyramidal: Three bonded atoms occupy equatorial positions with angles of 120 degrees; two more occupy axial positions on opposite sides. Molecular phosphorus pentachloride (PCl5) illustrates this shape.
- Octahedral: Six atoms surround a central atom at the vertices of an octahedron. Adjacent bonds form angles of 90 degrees. Sulfur hexafluoride (SF6) is an example.
When to use it
Predicting the physical properties (boiling point, solubility) of new chemical compounds based on their molecular shape.
Designing pharmaceutical drugs that fit precisely into protein binding sites due to specific molecular geometries.
Explaining why some CO2 vibrations absorb infrared radiation by changing the molecule’s dipole moment, unlike the ordinary vibrations of O2 and N2.
Engineering polymers with desired flexibility or strength by choosing monomers with specific geometric arrangements.
FROM THEORY TO EVERYDAY LIFE
In practice
Linear: Carbon dioxide (CO2)
Carbon dioxide has a central carbon atom double-bonded to two oxygen atoms, resulting in a linear shape with 180° bond angles. This geometry makes CO2 nonpolar despite having polar bonds.
Trigonal pyramidal: Ammonia (NH3)
Ammonia has a central nitrogen atom bonded to three hydrogen atoms and one lone pair, creating a trigonal pyramidal shape. The lone pair pushes the bonds slightly closer, giving a bond angle of about 107°.
Tetrahedral: Methane (CH4)
Methane has a carbon atom bonded to four hydrogen atoms, forming a symmetrical tetrahedron with 109.5° bond angles. This perfect symmetry results in a nonpolar molecule.
Square planar: Xenon tetrafluoride (XeF4)
Xenon tetrafluoride has four fluorine atoms bonded to xenon in a plane, with two lone pairs above and below. Adjacent fluorine–xenon–fluorine bond angles are 90°, and the symmetric molecule is nonpolar.
Trigonal bipyramidal: Phosphorus pentachloride (PCl5)
In its molecular gas-phase form, phosphorus pentachloride has five chlorine atoms around phosphorus: three in equatorial positions (120° apart) and two in axial positions (180° apart). The molecule overall is nonpolar due to symmetry.
Octahedral: Sulfur hexafluoride (SF6)
Sulfur hexafluoride has six fluorine atoms around sulfur in an octahedral arrangement, with 90° angles between adjacent bonds. Its symmetric geometry makes the molecule nonpolar.
A MOMENT TO REFLECT
Put it to work.
How would you predict the molecular geometry of water (H2O) based on the VSEPR theory? What properties arise from that shape?
Explain why carbon dioxide is linear while sulfur dioxide is bent. How does geometry affect polarity?
If you replace one atom in a tetrahedral molecule with a different element, how might the bond angles change and why?
CONTINUE THE CONVERSATION
Source texts
- Wisdom Theory 100 Mental Models (2020) (Wisdom Theory)