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What Is the Difference Between Electronic Geometry & Molecular Shape?

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Electron geometry and molecular geometry are the arrangement of electrons or atoms around a central atom in three-dimensional space. This gives a molecule a particular shape and certain bond angles.

Are Electron Geometry and Molecular Geometry Definitions the Same?

The molecular geometry definition in chemistry is the arrangement of atoms in relation to a central atom in three-dimensional space.

Electron geometry is the arrangement of electron groups. If lone pairs of electrons, electrons not bonded to other atoms, are located in the molecule, this will change the molecular geometry, not the electron geometry.

If all the electron groups are bonded, with no lone pairs, then the electron geometry and molecular geometry are the same.

VSEPR and Geometry

Valance-shell electron repulsion (VSEPR) is a theory that proposes the geometry of a molecule based on minimizing the repulsion between electron pairs. This model is useful for most compounds with a central atom.

Electrons, whether bonded or in lone pairs, will repel each other, and they arrange around a central atom in a way that minimizes this repulsion and maximizes the distance between them.

Lone pairs of electrons will repel stronger than bonded ones, and this will alter the bond angles in the molecular geometry, making the angles slightly smaller.

Electron Geometry

Electron geometry is determined by electron group:

  • 2 electron groups, linear
  • 3 electron groups, trigonal-planar
  • 4 electron groups, tetrahedral
  • 5 electron groups, trigonal-bipyramidal
  • 6 electron groups, octahedral

Linear Electron Geometry and Molecular Geometry

Linear geometry involves a central atom with two pairs of bonding electrons at an angle of 180 degrees (a straight line). This is the only possible shape for a linear geometry; the electron geometry and molecular geometry are the same.

Trigonal Planar Electron Geometry

Trigonal planar electron geometry is a central atom with three pairs of bonding electrons at 120-degree angles to each other arranged in a plane or "flat."

Only the trigonal planar has the same electron geometry and molecular geometry:

  • trigonal planar: all three pairs of bonding electrons are bonded to atoms
  • bent: two atoms bonded, one lone pair of electrons (Here, molecular geometry differs from electron geometry, and, as stated above, the bond angle is slightly less than 120 degrees.)

Tetrahedral Electron Geometry

Tetrahedral electron geometry is a central atom surrounded by four pairs of bonding electrons at angles of 109.5 degrees from each other, forming a shape that resembles a tetrahedron.

Only the tetrahedral shape has the same electron geometry and molecular geometry:

  • tetrahedral: all four pairs of bonding electrons are bonded to atoms
  • trigonal pyramidal: three atoms bonded, one lone pair of electrons
  • bent: two atoms bonded, two lone pairs of electrons

Trigonal Bipyramidal Electron Geometry

Trigonal bipyramidal is a central atom with five pairs of bonding electron pairs.

The geometrical name comes from the shape of three pairs in a plane at 120-degree angles (the trigonal planar geometry) and the remaining two pairs at 90-degree angles to the plane. The shape resembles two pyramids with a triangular base attached together.

It is important to note that the lone electron pairs will fill the trigonal planar portion of the geometry first. Only the trigonal bipyramidal shape has the same electron geometry and molecular geometry:

  • trigonal bipyramidal: all five pairs of bonding electrons are bonded to atoms
  • seesaw: four atoms bonded, one lone pair of electrons
  • t-shaped: three atoms bonded, two lone pairs of electrons
  • linear: two atoms bonded (located opposite to each other), three lone pairs of electrons

Octahedral Electron Geometry

Octahedral electron geometry is a central atom with six pairs of bonding electrons, all of which are at 90 degrees to one another. The shape resembles two pyramids with a square base attached together.

Only the octahedral shape has both the electron geometry and molecular geometry the same. Note that there are no other combinations within this geometrical shape than those listed below:

  • octahedral: all six pairs of bonding electrons are bonded to atoms
  • square pyramidal: five atoms bonded, one lone pair of electrons
  • square planar: four atoms bonded, two lone pairs of electrons

References

About the Author

Rosann Kozlowski is currently a freelance writer and tutor. She has a Master's Degree in Chemistry from the University of Oregon and has previously worked in the pharmaceutical industry and has taught at the middle school, high school, and college levels.