Covalent structures and bonding

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Hi, I need to know the answer to this question; "Draw 'dot and cross' diagrams for the following ionic compound, magnesium nitrate, showing all electrons. Show the charge on each ion in your diagrams." I don't know how to work out the electron configuration of nitrate, nor do I understand how it ionically bonds with the magnesium, please help!!

Corrie writes ....
 
It is difficult for me to draw structures here, but I can tell you what to draw.
 
First of all, magnesium nitrate is an ionic compound, so you will have to draw two separate ions. The (+) ion is Mg²⁺, and there are two ways of drawing this. In both cases you put the Mg inside square brackets and add the charge, like this: [Mg]²⁺.
 
The Mg atom has lost its 2 outer shell electrons, so none are shown as dots or crosses inside the brackets. Some books/teachers prefer to show the next shell of electrons left on the Mg²⁺ ion, so they put 4 pairs of dots or crosses around Mg inside the square brackets. Take your pick!
 
The nitrate ion, NO₃^-, is trickier. Drawing a 'stick' diagram, then doing the 'dot & cross' structure based on that is the easiest way. I can take you through the 'stick' diagram as follows:
 
N can form 3 normal covalent bonds to fill its outer shell, so put down an N atom and join one O atom to it via a double bond (N=O). Then join another O atom to it via a single bond (N-O). Now the nitrogen is 'happy' but the single bonded O is short of an electron. So put a minus sign on it, indicating that it has an extra electron: now you have O=N-O^-. The N atom also has a lone (non-bonding) pair of electrons. You can now join a third O atom onto the N by means of a dative covalent (co-ordinate) bond, using the lone pair on N, like this N:->O. This O now also has a full outer shell, so all four atoms are 'happy'!
 
Now turn this stick' diagram into a dot & cross diagram, using crosses for the N's outer electrons (5) and dots for the O's outer electrons (6). Finally you'll have to add 1 electron (use an 'o') to the outer shell of the O atom attached to the N by only a single bond. Check that each atom now has a full outer shell of 8 electrons. Finally, put a large square bracket around the nitrate ion structure, with a (-) outside. Strictly you should draw two such nitrate ions, to match the 2+ charge on the Mg-ion.
 
Hope this helps.

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updated: 13 November 2007