Atomic Bonding

Atoms form bonds between them in order to achieve a full outer shell of electrons. Having a full outer shell makes them stable. However these atoms do not come out of nowhere, they come from atoms that have lost their electrons in a reaction and go to atoms that need to gain electrons in that chemical reaction, OR atoms will share electrons rather than give or take them, meaning there are 2 different types of atomic bonding, ionic and covalent.     

Why Do Atoms Bond?

Ionic Bonding

Ionic bonding occurs between metals and non metals, with the metals losing electrons and the non metals gaining electrons. The correct number of atoms of the reactants are needed so that no metal electrons "go to waste" and the non metals have all the electrons they need.  Al2O3 needs 2 aluminium atoms and 3 oxygen atoms to form an ionic molecule with full/complete outer shells and is overall a neutral molecule (has no charge).  NaCl needs 1 sodium atom and 1 chlorine atom to make an ionic molecule. 

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Caption: : Aluminium Oxide ionic bonding (left) and Sodium Chloride bonding (right).

Ionic Equations

Oxidation is the loss of electrons by a substance in a chemical reaction. Reduction is the gaining of electrons by a substance. This can be remembered with the acronym OILRIG ; Oxidation Is Losing Reduction Is Gaining. The substance which accepts the electrons is the oxidising agent, and the substance which gives the electrons is the reducing agent. In an ionic reaction, the metals are oxidised, they have lost electrons, and the non metals are reduced, they have gained electrons. The metals are the reducing agents and the non metals are the oxidising agents.  Ionic equations show the oxidised and reduced ions in a chemical reaction. For example, zinc metal can displace copper ions from a solution of copper(II) sulphate. Copper metal and zinc(II) sulphate solution are formed. As a normal (unbalanced) equation, it would be set out as... Zn + CuSO4 ---> Cu + ZnSO4. As an ionic equation, it would be set out like... Zn + Cu^(2+)  + SO4^(2-) ---> Cu + Zn^(2+) + SO4^(2-) . The molecules are "separated" and their individual charges are shown before and after the reaction. Before, zinc is a metal so it has no charge, copper has a 2+ charge because it is bonded with sulphate, which has a 2- charge, an since there is only 1 copper the charges must balance out, (also it says copper(II) in the question). After, copper is a metal so it has no charge, zinc has a 2+ because it is bonded with sulphate (full reason above), (also it says zinc(II) is formed in the question).  Zinc is shown to have become +ve i.e. it has lost electrons so it has been oxidised. The copper has become neutral form +ve, i.e. it has gained electrons,so it has been reduced. The sulphate ions have not changed in charge, so they are spectator ions. 

Ionic Lattices