When the central atom has 5 bonding pairs of electrons and no non-bonding pairs the geometry is trigonal bipyramidal. Notice those atoms in the trigonal plane are referred to as equitorial atoms, while the atoms perpendicular to the trigonal plane are called axial atoms.
If a bonding pair of electrons is replaced with a nonbonding pair the geometry is described as a seesaw shape. The nonbonding pair of electrons is always located in the equatorial position never the terminal position to minimize electron-pair repulsions.
When the central atom has 3 bonding pairs of electrons and 2 nonbonding pair the geometry is T–shaped. Again the two nonbonding pairs of electrons are located in equatorial positions.
When the central atom has 2 bonding pairs of electrons and 3 nonbonding pair the geometry is linear.
