When atoms bond to a central atom to form a molecule, they tend to do so in a way that maximizes the distance between bonding electrons. This gives the molecule a particular shape, and when no lone pairs of electrons are present, the electronic geometry is the same as the molecular shape. Things are different when a lone pair is present. A lone pair is a set of two valence electrons that aren't shared among the bonding atoms. Lone pairs occupy more space than bonding electrons, so the net effect is to bend the shape of the molecule, although the electron geometry still conforms to the predicted shape.
In the absence of non-bonding electrons, molecular shape and electronic geometry are the same. A pair of non-bonging electrons, called a lone pair, bends the molecule slightly, but the electronic geometry still conforms to the predicted shape.
Linear Electron Geometry
A linear electron geometry involves a central atom with two pairs of bonding electrons at an angle of 180 degrees. The only possible molecular shape for a linear electron geometry is linear and is three atoms in a straight line. An example of a molecule with a linear molecular shape is carbon dioxide, CO2.
Trigonal Planar Electron Geometry
Trigonal planar electron geometry involves three pairs of bonding electrons at 120-degree angles to each other arranged in a plane. If atoms are bonded at all three locations, the molecular shape is also called trigonal planar; however, if atoms are bonded at only two of the three pairs of electrons, leaving a free pair, the molecular shape is called bent. A bent molecular shape results in the bond angles being something slightly different than 120 degrees.
Tetrahedral Electron Geometry
Tetrahedral electron geometry involves four pairs of bonding electrons at angles of 109.5 degrees from each other, forming a shape that resembles a tetrahedron. If all four pairs of bonding electrons are bonded to atoms, the molecular shape is also called tetrahedral. The name "trigonal pyramidal" is give to the case where there is one pair of free electrons and three other atoms. For the case of only two other atoms, the name "bent" is used, just like the molecular geometry involving two atoms bonded to a central atom with a trigonal planar electron geometry.
Trigonal Bipyramidal Electron Geometry
Trigonal bipyramidal is the name given to the electron geometry involving five pairs of bonding electron pairs. The name comes from the shape of three pairs in a plane at 120-degree angles and the remaining two pairs at 90-degree angles to the plane, which results in a shape that resembles two pyramids attached together. There are four possible molecular shapes for trigonal bipyramidal electron geometries with five, four, three and two atoms bonded to the central atom and are called trigonal bipyramidal, seesaw, t-shaped and linear, respectively. The free electron pairs always fill the three spaces with bond angles at 120 degrees first.
Octahedral Electron Geometry
Octahedral electron geometry involves six pairs of bonding electrons, all of which are at 90 degrees to one another. There are three possible electron geometries with six, five and four atoms bonded to the central atom and are called octahedral, square pyramidal and square planar, respectively.