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The water molecule

Molecular Geometries

The VSEPR theory describes five main shapes of simple molecules: linear, trigonal planar, tetrahedral, trigonal bipyramidal, and octahedral.

LEARNING OBJECTIVES

Apply the VSEPR model to determine the geometry of molecules where the central atom contains one or more lone pairs of electrons.

KEY TAKEAWAYS

Key Points

  • Linear: a simple triatomic molecule of the type AX2; its two bonding orbitals are 180° apart.
  • Trigonal planar: triangular and in one plane, with bond angles of 120°.
  • Tetrahedral: four bonds on one central atom with bond angles of 109.5°.
  • Trigonal bipyramidal: five atoms around the central atom; three in a plane with bond angles of 120° and two on opposite ends of the molecule.
  • Octahedral: six atoms around the central atom, all with bond angles of 90°.

Key Terms

  • VSEPR Theory: the Valence Shell Electron Pair Repulsion (VSEPR) model is used to predict the shape of individual molecules based on the extent of electron-pair electrostatic repulsion

AXE Method

Another way of looking at molecular geometries is through the “AXE method” of electron counting. A in AXE represents the central atom and always has an implied subscript one; X represents the number of sigma bonds between the central and outside atoms (multiple covalent bonds—double, triple, etc.— count as one X); and E represents the number of lone electron pairs surrounding the central atom. The sum of X and E, known as the steric number, is also associated with the total number of hybridized orbitals used by valence bond theory. VSEPR uses the steric number and distribution of X’s and E’s to predict molecular geometric shapes.

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AXE method: The A represents the central atom; the X represents the number of sigma bonds between the central atoms and outside atoms; and the E represents the number of lone electron pairs surrounding the central atom. The sum of X and E, known as the steric number, is also associated with the total number of hybridized orbitals used by valence bond theory.