Geometry and real life are full of surprising alignments. Down to the scale of molecules, geometry still holds. We see the structure of molecules in chemistry connecting to geometry in the field of molecular geometry. Some might even say molecules developed geometry first. We humble humans "discovered" geometry long after the universe had been putting it to use shaping the molecules that define our world.

Table Of Contents

  1. Molecular Geometry
  2. Valence Shell Electron Pair Repulsion Theory (VSEPR)
  3. Other Arrangements
  4. Tetrahedral Molecules Around Us
  5. Review

Molecular Geometry

As chemical bonds form, individual atoms and their orbiting electrons move into specific shapes, called their electron domain geometry:

  • Linear -- SP hybridized, like CO2, with oxygen atoms 180° apart from one another, with the carbon atom between them
  • Trigonal planar -- SP2 hybridized, like sulfur trioxide, SO3, with the oxygen atoms 120° apart in one plane, the sulfur atom at their center
  • Tetrahedral -- SP3 hybridized, like methane, CH4, with the hydrogen atoms arrayed around the carbon atom at 109.5° bond angles in three dimensions

Many shapes exist beyond tetrahedrals, but we are concentrating on that shape here. It pushes the molecule into a three-dimensional structure.

[suggest animation of rotating tetrahedral molecule like CH4, so viewers can understand the three-dimensional structure]

Chemists have worked hard to explain the actual structure of molecules, developing a theory connecting geometry, energy, and atoms.

Valence Shell Electron Pair Repulsion Theory (VSEPR)

The many shapes of molecules are affected by the number of atomic bonds and lone electron pairs. Lone pairs are the valence electrons of the atom that are not shared with another atom.

Valence Shell Electron Pair Repulsion Theory, or VSEPR (pronounced "Vesper") predicts the molecular geometry of individual molecules. All the bonds to the central atom, plus all the lone pairs, equals the molecule's steric number. For tetrahedral molecules like methane or xenon tetroxide, their steric number is four; four bonds atom to atom and no lone electron pairs.

VSEPR theorizes that the lone pairs perform the same task as the bonds, repelling electrons to distribute joined atoms at equal angles around the central atom. The repulsion seeks its lowest energy level, providing the widest possible dispersal of the surrounding atoms.

Other Arrangements

Molecular geometry, thanks to natural forces, seeks the lowest energy solutions to every bond, so some molecules with central atoms and four connected, surrounding atoms are not tetrahedral.

Xenon tetrafluoride, XeF4, has a steric number of six, not four; it has two lone pairs that array themselves at 90° from the fluorine atoms (above and below the xenon atom) and 180° from each other. The four fluorine atoms take positions at four corners of a square. The result: a square planar molecule, not a tetrahedral.

[insert molecular structure diagram of XeF4]

Tetrahedral Molecules Around Us

Methane is perhaps the most commonly found and familiar tetrahedral molecule. But it is not the only molecule to make use of the familiar pyramid structure.

Silane, SiH4, has a terrible smell, but a delightful molecular geometry -- tetrahedral! You can also find the three-dimensional shape in thiazyl trifluoride, NSF3, and ions of phosphate (PO43-), sulfate (SO42-), and perchlorate (ClO4-).

[if possible, insert diagrams showing the structures of these five molecules. Sites for inspiration include Wikipedia and Chem.Libretexts.org]

Review

Molecular geometry is the study of the physical shape of molecules. Molecules achieve their shapes from the atomic bonds and lone pairs of electrons. The total of bonds and lone pairs is a molecule's steric number. Valence Shell Electron Pair Repulsion (VESPR) Theory attempts to explain the natural repelling forces of these electron arrangements.

Tetrahedral molecules array four atoms around a central atom, every atom oriented 109.5° from the others. The steric number of tetrahedral molecules is four (no lone pairs; four atomic bonds).

Tetrahedral molecular structure is seen in several molecules, the most common of which is methane, CH4. Other molecules include silane, SiH4, and thiazyl trifluoride, NSF3. Tetrahedral structure is also found in the phosphate ion, PO43-, sulfate ion, SO42-, and perchlorate ion, ClO4-.

Next Lesson:

Square Matrix

Instructor: Malcolm M.
Malcolm has a Master's Degree in education and holds four teaching certificates. He has been a public school teacher for 27 years, including 15 years as a mathematics teacher.
Tutors online
Ashburn, VA

20+ Math Tutors are available to help

Get better grades with tutoring from top-rated professional tutors. 1-to-1 tailored lessons, flexible scheduling. Get help fast. Want to see the math tutors near you?