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The hybridization

A trigonal bipyramidal shape forms when a central atom is surrounded by five atoms in a molecule. In the geometry, three atoms are in the same plane with bond angles of 120°; the other two atoms are on opposite ends of the molecule. Some elements in Group 15 of the periodic table form compounds of the type AX5; examples include PCl5 and AsF5.


The Lewis dot structure of phosphorous pentachloride.: The three equatorial atoms are in the same plane, with the two axial atoms located on opposite ends of the molecule.


Octa- signifies eight, and -hedral relates to a face of a solid, so “octahedral” literally means “having eight faces.” The bond angles are all 90°, and just as four electron pairs experience minimum repulsion when they are directed toward the corners of a tetrahedron, six electron pairs try to point toward the corners of an octahedron. An example of an octahedral molecule (AX6) is sulfur hexafluoride (SF6).

Interactive: Electron Geometry: Molecules assume different shapes due to patterns of shared and unshared electrons. In these examples all electrons affecting the shape of the molecules are shared in the covalent bonds holding the atoms together to form the molecules.

Lone Electron Pairs

Nonbonding electrons are in orbitals that occupy space, repel the other orbitals, and change a molecule’s shape.


Recognize the effect of lone electron pairs on molecules’ geometries.


Key Points

  • Orbitals containing the various bonding and nonbonding pairs in the valence shell will extend out from the central atom in directions that minimize their repulsions.
  • A nonbonding orbital has no atomic nucleus at its far end to draw the electron cloud toward it; the charge in such an orbital will therefore be concentrated closer to the central atom.
  • Nonbonding orbitals exert more repulsion on other orbitals than do bonding orbitals.

Key Terms

  • coordination number: in chemistry and crystallography, the number of a central atom’s neighbors in a molecule or crystal
  • lone pair: a valence set of two electrons that exists without bonding or sharing with other atoms