Ionic Bonding Part 1 Edexcel

Ionic Bonding part 1 What is ionic bonding? For an ionic bond you simply need a positive metal ion and a negative non-metal ion.

electron transfer from metal to non-metal to form positive and negative ions  strong electrostatic attraction between the ions

The classic example is sodium chloride: Na+ and Cl-  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. Start by going to the periodic table to get the electronic configuration of each atom i.e. Na 2,8,1 and Cl 2,8,7. It’s easiest to keep it in this GCSE style format. Below I have drawn all the electrons around each atom but quite often you only need the outer electrons. The diagram shows Na transferring (giving) and electron to the Cl to form the two ions:

As ions are formed, it is necessary to put brackets around each atom and put the charges on the outside. It is also good practise to keep the transferred electron, for example, on the Cl above, as a dot.

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The Na+ ion: I have left the outer shell empty and in the diagram but you can just remove this as there is no electrons in it. Remember we are forming full shells.

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. Just remember that the charges on the two ions have to cancel each other out to give a neutral product.

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 phrase that you need is ‘giant ionic lattice’. Ionic compounds adopt a cubic like structure, which is given the name lattice. In ionic terms, this is a giant structure with an alternating pattern of positive and negative ions).

A lattice just means 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 Conduction 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.

ions must be free

Melting Point/bond strength 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.

size and charge are the important factors

A small ion with a high charge means the charge is more concentrated on the ion and therefore has a “stronger” charge  stronger attraction. Takes more energy to break the bonds.

Ionic Radius

positive ions are smaller than their atoms and negative ions are larger

Positive ions: has lost the outer shell therefore is smaller immediately and the proton:electron ratio has increased  stronger attraction between nucleus and electrons. Negative ions: has gained electrons therefore the proton:electron ratio has decreased  less attraction between nucleus and electrons. Questions often show isoelectronic ions i.e. same number of electrons e.g. N3-, O2-, F-, Na+, Mg2+, Al3+. So using the theory above, Al3+ is the smallest and N3- is the largest. Down a group: just as with going down any group, the ions get bigger i.e. adding more shells as you go down.

Evidence for ions Migration An ionic solution is connected to a battery. The positive ions “migrate” (move) to the negative electrode and the negative ions to the positive electrode. If ions are coloured then you will see the colours move.

Cr2O72- orange ions migrate to the positive electrode. And Cu2+ ions move to the negative electrode.

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Just remember most ions are not coloured, so the colour might only comes from one of the ions!

Coloured positive ions: most transition metals. Coloured negative ions: dichromate (orange) and manganate (purple). Ions such as groups 1 to 3 and 5 to 7, plus carbonate, hydroxide, sulphate are not coloured.