Shapes of Molecules and Ions

Shapes of Molecules and Ions

 

Predict the shape and bond angles for species with five and six negative charge centres using the VSEPR theory

Although most atoms will aim to form an octet of electrons, this rule can be broken in some cases, such as period 3 elements that have empty d orbitals. This bonding also depends on the size of the central atom and the attached atoms.

To determine the shape, and therefore the bond angles, of a molecule, you must first determine how many electrons are in the valence shell of the central atom. Then, find out how many bonds will be formed with the central atom. The difference between these two values tells us how many non-bonding electrons there are.

 

Five Negative Charge Centres

Trigonal Pyramidal

i.e. PCl5

There are five atoms bonded to the central atom, forming the shape of two trigonal pyramids fused at the base. It has six faces. The atoms along the equilateral position form bond angles of 120o. The atoms on the top and bottom of the molecule, or the axial positions, form bond angles of 90 degrees.

Seesaw or Sawhorse

i.e. SF4

These have four bonding pairs and one non-bonding pair. The shape is determined based on the arrangement of the bonded atoms, but is still affected by the presence of a non-bonding pair of electrons.

T-shaped

i.e. ClF3, ICl3

These have three bonding pairs and two non-bonding pairs

 

 

 

 

 

 

Linear

i.e. XeF2

These have two bonding pairs and three non-bonding pairs

 

 

 

 

 

Six Negative Charge Centres

 

Octahedral

i.e. SF6, PF6

 

The twelve valence electrons form six bonding pairs. Four atoms bond on a plane, the equatorial positions, at 90o. The remaining two atoms are on the top and bottom, the axial positions, at 90o. The molecule forms a shape like two square pyramids fused together, called square bipyramidal.

Square Pyramidal

i.e. BrF5

 

These have twelve electrons in the valence shell, with five bonding pairs and one non-bonding pair of electrons

 

 

 

Square Planar

i.e. XeF4

 

These have twelve valence electrons, with four bonding pairs of electrons and two non-bonding pairs. The non-bonding pairs are found in the axial positions at the top and bottom of the molecule. They are symmetrical and therefore non-polar molecules.