Structure and Properties

GIANT IONIC STRUCTURES

Ionic compounds have giant structures in which many strong electrostatic forces hold the ions together.  This means they are solids at room temperature.  A lot of energy is needed to overcome the ionic bonds to melt the solids.  Therefore ionic compounds have high melting and boiling points.

However, once an ionic compound has been melted, the ions are free to move.  This allows them to carry electrical charge, so the liquid can conduct electricity.  Some ionic solids dissolve in water because water molecules can split up the lattice.  The ions are free to move in the solutions and so they conduct electricity.

SIMPLE MOLECULES

The atoms within a molecule are held together by strong covalent bonds.  These bonds act between the atoms within the molecule, so simple molecules have little attraction for each other.  Substances made of simple molecules have quite low melting and boiling points.  They do not conduct electricity because the atoms have no overall charge and so they cannot carry electrical charge.  

The forces of attraction between molecules are called intermolecular forces.  They are weak.  The forces are overcome when a molecular substance melts or boils.  This means that substances made of small molecules have low melting and boiling points. . Those with the smallest molecules (eg H2, Cl2 and CH4) have the weakest intermolecular forces and are gases at room temperature.  Larger molecules have stronger attractions and therefore they may be liquids at room temperature (eg Br2, C6H14), or solids with low melting points, eg I2.

GIANT COVALENT STRUCTURES

Atoms of some elements can form several covalent bonds.  These atoms can join together in giant covalent structures or macromolecules.  Every atom in the stucture is joined to several others by strong covalent bonds.  It takes an enormous amount of energy to break down the lattice and so these substances have very high melting points.

Diamond is a form of carbon that has a regular three-dimensional giant structure.  Every carbon atom is covalently bonded to 4 other carbon atoms.  This makes diamond hard.  The compound silica has a simmilar structure.  Graphite is a form of carbon in which the atoms are covalently bonded to 3 other carbon atoms in giant flat two-dimensional layers.  There are no covalent bonds between the layers and they slide over each…