Ionic Bonding part 1 The two main types of bonding are ionic and covalent. They are quite different and it is important not to get them mixed up. We will look at covalent in another tutorial. What is ionic bonding? For an ionic bond you simply need a positive metal ion and a negative non metal ion. The ionic bond is the result of ‘electrostatic’ attraction between the positive and negative ions. The classic example is sodium chloride: Na+ and Cl-­‐ à NaCl Dot and cross diagrams The key phrase that you need to remember is ‘transfer of electrons’. To form the two ions, electrons are transferred from the metal to the non-­‐metal. You need to be able to draw ‘dot and cross’ diagrams (also comes up in covalent bonding). This means you need to be able to draw out the electronic structure of each ion involved. For example: sodium chloride, NaCl. From the periodic table, sodium has 11 electrons and chlorine has 17 electrons. Below I have drawn out the dot and cross diagrams for both elements (this was explained in more detail in the electronic structure tutorials). Briefly, sodium’s electrons are arranged: 2,8,1, which means it has 1 electron in its outer shell, and needs to lose this electron to form a full octet i.e. 2,8. Chlorine’s electrons are arranged: 2,8,7, which means it has 7 electrons in its outer shell and needs to gain one electron to form a full octet i.e. 2,8,8. So, we simply transfer one electron from sodium to chlorine as indicated below to form the ionic compound. You can do this for any ionic compound as long as you know how to work out the electronic structure of the two ions involved. Magnesium Chloride: In the above example, magnesium has to lose 2 electrons to form the stable 2,8 configuration, giving Mg2+. Chlorine forms a 1-­‐ ion by gaining one electron but as the magnesium ion has a 2+ charge, we therefore need two Cl-­‐ ions to balance the 2+ charge. So magnesium transfers one electron to each of the two chlorines. This then gives us the formula MgCl2. Ionic Lattice Another magic word that you need is ‘lattice’. Ionic compounds adopt a cubic like structure, which is given the name lattice (an alternating pattern of positive and negative ions). A lattice is a massive structure, it’s not just a couple of repeats like shown above. There are 6.02 x 1023 x 2 ions in one mole of NaCl! So there is going to be a huge number of repeats too. Properties 1. Ionic compounds do not conduct electricity when solid but do conduct in solution or when molten. This is simply due to the ions being free to move in solution/molten and can carry the current. 2. Ionic compounds have very high melting points (hundreds of oC), as the electrostatic forces are very strong. These forces need to be broken when melting or boiling. 3. Ionic compounds are crystalline solids. This is due to the ordered/structured lattice type structure. 4. Ionic compounds can be split up by electrolysis (passing electricity through an ionic solution).