Ionic Bonding (3) How to build formulas of ionic compounds

Ionic Bonding: How to build formulas of ionic compounds

This blog will be about formulas of ionic compounds. 

It is certainly true that many of us struggle to get the formula right for any particular compound. 

I think many of us envy the clever kid in the class who always has the right formula of the compound on the tip of his tongue and never ever gets them wrong. 

How does she do it?

And she is not alone, there are others just like her, who get it!!! Argh!!!

Now, if you are going to compete against these savants and geniuses, so called, you are going to have to work on this with some blood, sweat and tears, believe me. 

First job is to learn a list of formulae of IONS:

I mean both positive ions and also negative ions.

There really is no short cut to this.

I have no quick fixes to offer you, just damned hard graft but you do want to upstage the smart geeks don’t you — eh!!

So here goes: here is a list of positive ions.

You can find many of these on the internet like I just found this one and copied it over.

Here it is and I like this because it doesn’t do all the work for you!

You are going to have to work out what the name of each positive ion is!

The next thing you need is a list of negative ions.

Again you can get this from many places on the internet and again you are going to have to memorise this list of ion formulas with their names. 

This time I’ve included names because they are not so easy to get right particularly if you are following a school chemistry, GCSE course not a college chemistry, A level or International Bacc course. 

Thing is now how are you going to fit these formulae together to create the formulas of ionic compounds?

Let’s just make the point that these different types of ion attract each other because they have opposite electrical charges

The attraction is electrostatic between them

Question is how many ions attract each other in any given formula?

Let's begin with the simplest examples that is where both the cation and the anion have the same charge like in sodium chloride.

In this compound there is one sodium ion for every chloride ion.

One positive sodium ion attracts one negative chloride ion.

Na+    attracts    Cl^—

So the formula is NaCl

But what about Magnesium chloride?

Magnesium ions are Mg ²⁺   and chloride ions are Cl^—

You can build the formula this way: put the 2 of the positive charge against the chloride ion in the formula and it’s done!

 

So magnesium chloride’s formula is Mg ²⁺       with        Cl^—    like this:

 giving  MgCl₂

Or you can work out that if magnesium carries two positive charges it will attractive two negative charges which will be carried by 2 chloride ions.

Same result: the formula of magnesium chloride looks like this:

Magnesium   2+          Chloride  1—

                                        Chloride  1—

-----------------------------------------------------

                      Mg     Cl₂

If you have a more complex compound like ammonium sulphate then you have to apply brackets to group multiple numbers of the ions that contain more than one element.

So ammonium  NH₄⁺         and sulphate     SO₄^2—    

Now let’s swap the numbers round first which if we do should give us:

Ammonium sulphate       ( NH₄) ₂ SO₄           

So there you have it but all this relies on you remembering the lists of positive ions and negative ions.!!

Why don’t you try working out these formulas once you think you have memorised the lists?

Sodium oxide,

aluminium chloride,

zinc oxide,

zinc hydroxide,

aluminium  oxide,

sodium carbonate,

aluminium sulphate.