Structure and Bonding (9-1 new AQA GCSEs)

There are three types of strong chemical bonding:

• ionic bonding
• covalent bonding
• metallic bonding

Atoms that have gained or lost electrons are called ions. Ionic bonds occur between positive and negative ions.

Ionic bonding involves a transfer of electrons from metal atoms to non-metal atoms.

The metal atoms lose electrons to become positively charged ions. The non-metal atoms gain electrons to become negatively charged ions.

The ionic bond is a strong electrostatic force of attraction between the positive metal ion and the negative non-metal ion.

Ionic compounds are giant structures of ions, that are held together by strong forces of attraction that act in all directions between oppositely charged ions.

Ionic compounds:

• have high melting and boiling points
  
• do not conduct electricity when solid, because the ions cannot move
• do conduct electricity when molten or in solution, because the charged ions are free to move about and carry a charge

Metallic bonding occurs in:

• metallic elements, such as iron and copper
• alloys, such as stainless steel

Metals have a giant structure in which electrons in the outer shell are delocalised (not bound to one atom)

This produces a regular arrangement of positive ions held together by electrostatic attraction to the delocalised electrons

A metallic bond is an attraction between the positive ions and the delocalised negatively charged electrons

The properties of metals make them very useful:

• Metallic bonds are very strong and most metals have high melting and boiling points. This means that they are useful structural materials

• The delocalised electrons can move around freely and transfer energy. This makes metals good thermal and electrical conductors

• The particles in pure metals have a regular arrangement

• The layers are able to slide over each other quite easily, which means that metals can be bent and shaped

• Traditionally, copper is used to make water pipes because it is an unreactive metal (so it does not react with water) and it can be easily shaped

Most metals objects are made from alloys - mixtures that contain a metal and at least one other element.

• Pure metals are too soft for many uses

• In alloys, the added element disturbs the regular arrangement of the metal atoms so the layer do not slide over each other so easily

• This means alloys are usually stronger and harder than pure metal

• An example of an alloy would be stainless steel
  

A covalent bond is a shared pair of electrons between atoms

Covalent bonds occur in:

• non-metallic elements
• compounds of non-metals

For example, a chlorine atom has seven electrons in its outer shell. In order to bond with another chlorine atom: 

• an electron from each atom is shared
• this gives each chlorine atom eight electrons in the outer shell
• each atom now has a complete outer shell

Some covalently bonded structures have simple structures, others have giant covalent structures.

Simple molecules contain a relatively small number of non-metal atoms joined together by covalent bonds

• The molecules have no overall electrical charge, so they cannot conduct electricity

• substances that consist of simple molecules are usually liquids and gases that have relatively low melting and boiling points

• This is because they have weak intermolecular forces

• These intermolecular forces are very weak compared to the strength of the covalent bonds in the molecules themselves

• All the atoms in giant covalent structures are linked by strong covalent bonds.

• These bonds must be broken for the substance to melt or boil.

• This means that giant covalent structures are solids with very high melting and boiling points.

• They form lattices where each atom is linked to other atoms

• Examples of giant covalent structures include: diamond, silicon dioxide, and graphite

• They do not conduct electricity (except graphite)

• They are insoluble in water

Diamond is a form of carbon: 

• It has giant, rigid covalent structures (lattice)

• Each carbon atom forms four strong covalent bonds with other carbon atoms

• All the strong covalent bonds means that it is a very hard substance with a very high melting point

• There are no charged particles, so it does not conduct electricity

Graphite is another form of carbon:

• It also has a giant covalent structure and a very high melting point

• Each carbon atom forms three covalent bonds with other carbon atoms

• This results in a layered, hexagonal structure

• The layers are held together by weak intermolecular forces

• This means that the layers can slide past each other, making graphite soft and slippery

• One electron from each carbon atom in graphite is delocalised

• These delocalised electrons allow graphite to conduct heat and electricity

Silicon dioxide has a lattice similar to diamond:

• Each oxygen atom is joined to two silicon atoms

• Each silicon atoms joined to four oxygen atoms