SHAPES OF MOLECULES AND IONS

LP – LP > LP- BP > BP- BP VSEPR Lewis structures are used to predict the shapes of molecules as they show the number of electron pairs around the central atom. These are called regions of electron density and they arrange themselves so that the potential energy due to their repulsion is a minimum. This is referred to as the valence shell electron pair repulsion theory. Lone pair electrons tend to be closer to the central atom and take up more space than the bonding pairs. Consequently, their repulsive effect will be greater than the bond pair: LP – LP > LP- BP > BP- BP The actual shape of molecules depends on the number of bonded and non bonded pairs around the central atom.

What’s New??? Further to the shapes learnt last year you will be looking at molecules with 5 and 6 electron clouds around the central atom. We will also consider something called axial and equatorial positions around the central atom..

Positions Y = Equatorial Z = Axial Z

Electron Cloud = 5 It is possible for all the 5 electron clouds to be involved in bonding OR some used in bonding and the rest are non bonding pairs around the central atom.

AB5 = Trigonal Bipyramidal All the 5 e- pairs around the central atom are involved in bonding so take up a shape described as a trigonal bipyramid - three of the outer atoms are in a plane at 120° to each other; the other two are at right angles to this plane. The trigonal bipyramid therefore has two different bond angles - 120° and 90°.

AB4 = Seesaw Of the 5 e- pairs around the central atom 4 are involved in bonding and 1 is a lone pair on the central atom. The lone pair e- takes up an equatorial position. The four atoms are arranged around the central atom of the molecule in a distorted tetrahedral or seesaw shape.

AB3 = T- shape Of the 5 e- pairs around the central atom 3 are involved in bonding and 2 are lone pairs on the central atom. The lone pairs e- takes up an equatorial position on same side of the central atom. The 3 atoms are arranged around the central atom of the molecule in a T- Shape.

AB2 = linear Of the 5 e- pairs around the central atom 2 are involved in bonding and 3 are lone pairs on the central atom. The 2 bond pair electrons takes up the axial positions while the 3 lone pair electrons take up the equatorial positions, resulting in a linear shape.

Electron Cloud = 6 It is possible for all the 6 electron clouds to be involved in bonding OR some used in bonding and the rest are non bonding pairs around the central atom.

AB6 = Octahedral All the 6 e- pairs around the central atom are involved in bonding so take up a shape described as a octahedral. All the bond angles are 90o to each other. 4 bond pairs are in a square plane while the other 2 are positioned perpendicular to the square plane at opposite ends of the central atom.

AB5 = square pyramid 5 of the e- pairs around the central atom are involved in bonding and there is 1 lone pair. The 4 bond pairs are in the square planar base and the lone pair is in the perpendicular position giving rise to a shape called square pyramid.

AB4 = square planar 4 of the e- pairs around the central atom are involved in bonding and there are 2 lone pairs. The bond pairs take up positions in the square plane giving the molecule a square planar shape.

Explaining the Shape Mention the total number of electron clouds around the central atom and their initial position. Then state how many of these are bonding electrons and how many are non bonding electrons. Talk about repulsion between all the electron pairs. Mention the bond angle between the bonding pair of electrons which gives the electron pairs least repulsion (for up to tetrahedral). State the final shape which results from this and link it to stability.

Example 1 Draw the Lewis diagrams of the PCl4+ and BrF4- ions and discuss their shapes.

+ -

PCl4+ is tetrahedral because it has 4 electron clouds PCl4+ is tetrahedral because it has 4 electron clouds. All the 4 e- clouds are used to form bonds with the outer Cl atoms. These repel each other so that the electron clouds are as far apart from each other as possible. The electron clouds position themselves in a tetrahedral shape around the central P atom. Since all 4 pairs are bonding the shape is tetrahedral where the bond pairs are 109o apart. This gives the ion stability.

In the BrF4– ion there are 6 electron clouds around the central Br atom. The e- clouds take up an octahedral position. However, only 4 pairs are involved in bonding while 2 pairs are non-bonding. There is repulsion b/w the 2 LP e- , the LP and BP e- clouds as well as the bond pair e-. Since LP e- clouds are closer to the central atom their repulsive effect is the strongest. They push the bond pairs closer and repulsion is minimised when the 4 BP e- arrange themselves in a square planar arrangement to be as far apart as possible.