0.4 Chemical bonding

The crystal lattice structure of ionic compounds

Ionic substances are actually a combination of lots of ions bonded together into a giant molecule. The arrangement of ions in a regular,geometric structure is called a crystal lattice . So in fact $\mathrm{NaCl}$ does not contain one $\mathrm{Na}$ and one $\mathrm{Cl}$ ion, but rather a lot of these two ions arranged in a crystal lattice where the ratio of $\mathrm{Na}$ to $\mathrm{Cl}$ ions is 1:1. The structure of a crystal lattice is shown in [link] .

Properties of ionic compounds

Ionic compounds have a number of properties:

• Ions are arranged in a lattice structure
• Ionic solids are crystalline at room temperature
• The ionic bond is a strong electrical attraction. This means that ionic compounds are often hard and have high melting and boiling points
• Ionic compounds are brittle, and bonds are broken along planes when the compound is stressed
• Solid crystals don't conduct electricity, but ionic solutions do

Metallic bonds

The nature of the metallic bond

The structure of a metallic bond is quite different from covalent and ionic bonds. In a metal bond, the valence electrons are delocalised , meaning that an atom's electrons do not stay around that one nucleus. In a metallic bond, the positive atomic nuclei (sometimes called the 'atomic kernels') are surrounded by a sea of delocalised electrons which are attracted to the nuclei ( [link] ).

Metallic bond

Metallic bonding is the electrostatic attraction between the positively charged atomic nuclei of metal atoms and the delocalised electrons in the metal.

The properties of metals

Metals have several unique properties as a result of this arrangement:

• Thermal conductors Metals are good conductors of heat and are therefore used in cooking utensils such as pots and pans. Because the electrons are loosely bound and are able to move, they can transport heat energy from one part of the material to another.
• Electrical conductors Metals are good conductors of electricity, and are therefore used in electrical conducting wires. The loosely bound electrons are able to move easily and to transfer charge from one part of the material to another.
• Shiny metallic lustre Metals have a characteristic shiny appearance and are often used to make jewellery. The loosely bound electrons are able to absorb and reflect light at all frequencies, making metals look polished and shiny.
• Malleable and ductile This means that they can be bent into shape without breaking (malleable) and can be stretched into thin wires (ductile) such as copper, which can then be used to conduct electricity. Because the bonds are not fixed in a particular direction, atoms can slide easily over one another, making metals easy to shape, mould or draw into threads.
• Melting point Metals usually have a high melting point and can therefore be used to make cooking pots and other equipment that needs to become very hot, without being damaged. The high melting point is due to the high strength of metallic bonds.
• Density Metals have a high density because their atoms are packed closely together.