GCSE C2 Chapter 1

• When two elements react together compound formed, elements chemically bonded together

Ionic bonding:

• Involves a metallic element and a non metallic element
• They react together to achieve gain or lose electrons to obtain the structure of noble gases (full highest energy levels of electrons)
• Electrons from the highest energy level are transfered from the metallic element to the non mentallic element
• The metallic element becomes positively charged because it has lost electrons
• The non metalic element becomes negatively charged because it has gained electrons
• A charged atom is called an ion
• The charge depends on the number of electrons in the highest energy level

Covalent bonding:

• Involves two or more non metallic elements
• They react together to achieve the structure of noble gases
• They do this by sharing electrons in their highest energy level (all non metals need to gain electrons to achieve this)

• Ioninc bonding holds oppositely charged ions together in a giant structure or lattice
• The structure of an ionic compund is very regular because the ions are arranged in a regular pattern of layers which compact very closely together
• Electrostatic forces of attraction act in all directions due to the different charges
• Each ion is surrounded by ions of the opposite charge creating strong forces of attraction which hold each ion firmly in place thus holding the giant lattice together

• The number of ions in the ionic compound is dependent upon the charge which the ion has formed which is the number of electrons in the highest energy level of the atom before it reacted. 
• For example Sodium, Na has one electron in its highest energy level so it bonds with chlorine which has 7 electrons in its highest occupied energy level. When the two elements bond each Na atom loses one electron to form sodium ions with a single positive charge. Chlorine atoms gain one electron to form ions with single negative charges.

• The ionic compound of calcium chloride has a ratio of 2:1 because calcium has a two plus charge whereas chloride only forms a single negative therefore double are needed.

• The formula of an ionic compound can be worked out providing you know the elements in he compounds and the charges they formed. 
• The formula is just the simplist ratio of the ions in the compound
• For ions with opposite charges (e.g. +1 and -1) the ratio of ions is 1:1 so the formula has just one of each ion
• For ions with unequal opposite charges, the overall charge if the ratio was 1:1 would be unblanaced and in an ionic compound the overall charge is zero. Therefore there needs to be ratio of ions with the weakest charge.
• Some ions are made up of more than one element and therefore brackets are used

• Non-metallic elements can also bond together by sharing pairs of electrons because all non-metals need to gain electrons to gain full highest energy levels
• The shared electrons strongly attract the atoms it has bonded to forming covalent bonds
• Substances that have atoms held together by covalent bonds are called molecules

• Covalent bonds only act between the atoms it bonds to so intermolecular forces are weak meaing most covalently bonded substances are small molecules however some atoms covalently bond to many other atoms creating giant covalent structures sometimes know as macromolecules

• Dot and cross diagrams can represent covalent bonds

• The atoms of metals are all the same size and are arranged in a regular pattern and layers

• They can be represented by models of small spheres like marbles all placed in layers together

• When metallic atoms pack together the electrons in the highest energy level delocalise leaving a positive lattice surrounded by a negative "sea" of electrons. The negative electrons strongly attract the positive lattice holding the structure firmly together.