Shapes of Molecules

Ionic bonding is non-directional.

Repel ions with the same charge

Attract ions with opposite charges

Due to this the structure is simply the arrangement of the lattice which maximises attraction and minimises repulsion. 

Highly directional

Molecules with two or more atoms have a definite, three-dimensional shape.

Electron-repulsion theory is based on:

The shape of the molecule depends on the number of electron pairs around the central atom.

Electron pairs repel one another so they stay as far apart as possible.

Two electrons in a pair not involved in bonding.

Lone pairs provide most repulsion. Most repulsion is between two lone pairs, followed by the repulsion between one lone pair and one bonding pair.

Answing a question:

• State the number of lone pairs and bonding pairs
• The effect this has on repulsion
• 'Maximum seperation, minimum repulsion'

A molecule that gains or loses one or more electrons forms an ion. 

The shape can be predicted from electron-pair repulsion theory.

ALKANES

The simplest alkane molecule, methane, has a tetrahedral structure. 

All the carbons, in an alkanes, have a tetrahedral arrangement around them. 

ALKENES

The simplest alkene, C2H4. There is a C=C bond. Then each carbon has three bonds all in the same plane, at angles of 120.

ALCOHOLS

The simplest is methanol. Four bonds around the carbon so the H-C-H angle is 109.5. There are two bonding pairs and two non-bonding pairs around the oxygen, so the C-O-H bond angle is 104. 

CARBOXYLIC ACIDS

Methanoic acid is the only carboxylic acid that is planar.

In all carboxylic acids the -COOH is planar, with the rest of the molecule being based on tetrahedral carbon atoms. 

HALOGENALKANES

Bromomethane, CH3Br is the simplest halogenalkane. It has a central carbon atom with four bonding pairs. The shape of the molecule is tetrahedral. 

ALDEHYES AND KETONES

Propanal is a typical aldehyes and propanone is the simplest ketone. 

In both molecules the central carbon has three groups bonded to it, it is therefore planar with bond angles of 120. 

Allotropes are different forms of the same element that exist in the same physical state. 

The difference in allotropes are down to different ways of bonding between the atoms. 

DIAMOND

Composed entirely of interlocking tetrahedral carbon atoms. This gives it exeptional strength. However, it is a poor conductor of heat and electricity. 

GRAPHITE

Composed of planes of trigonal carbon atoms. Each bonded to three others at 120. 

These planes are arranged in sheets to form three dimensional crystals. Van der waals forces maintain the arrangement of layers.

Because each atom is only bonded to three other, the remaining electron is free to circulate hence its capability to conduct electricity.

Fullerenes are a group of spherical carbon molecules.

The first one was named buckminsterfullerene and had 60 carbon atoms and 32 sides. 

A scientist reported that in the soot produced when making fullerenes he discovered elongated cage-like structures. This work was replicated.

They were named nanotubes. 

These closed-cage carbon structures contain 12 five-membered rings and almost any number of six-membered rings.

They make highly comlex shapes i.e. doughnut shapes, corkscrews and cones.