1.1 FORMULAE OF COMMON COMPOUNDS AND COMMON IONS AND HOW TO WRITE FORMULAE FOR IONIC COMPOUNDS

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Ok, so let's get started with the first topic from WJEC Chemistry specification. It is the formulae of common compounds and ions. The first thing we are going to have a look at, is what each formula represents.

--> Letters always represent the symbols of atoms from the periodic table.

--> Numbers always represent the ratio of each atom in the compound.

1. In the first instance with water, we can clearly see that hydrogen and oxygen is present, and the little two means there are two hydrogen atoms for each oxygen atom.

2. With sulfuric acid, we have two hydrogen atoms and four oxygen atoms for every sulfur atom.

3. This example is a bit more tricky because we have a 2 in front of carbon dioxide. A big number in front of a compound means that we have two the same molecules. Therefore if we have one carbon and two oxygens in carbon dioxide, we have to multiply it all by two, resulting in two carbons and 4 oxygens.

4. And the last one, is straight forward again, we have 3 hydrogens for every nitrogen atom in ammonia.

At GCSE there was no need for you to remember all of the ions, you just had to familiar with them because they were all on your data sheet. However, now you need to know them all off by heart.

Our periodic table helps us a lot in the process of remembering the ions. From the image above you can see that I drew a rough shape of the periodic table. It shows you that group 1 elements need to loose one electron to become the most stable, therefore they will have a +1 power after them. This means they become positive after loosing one electron. Group 2,3 and 4 behave similarly but they loose more electrons, so their power also increases.

Group 8 element are very happy as they are, so they do not become ions, yet group 5,6 and 7 would much rather accept electrons and become negative and that's how they become stable. So, for example, group 7 element accepts one electron and therefore becomes a -1 ion, and so does group 5 and 6 but their power is bigger.

To summarise, we already know most of the ions that are required to know, just by looking at the peiodic table. Examples of these are hydrogen, lithium, calcium from groups 1 and 2 (H--> +1, Li--> +1, Ca--> +2)or bromide, fluoride, oxide from groups 6 and 7 (Br--> -1, F--> -1, O--> -2).

You also have to know that there are transition metals, and they also become ions and there is no pattern for them, you just have to remember them (Only the ones I put in the box on the image above, there is not that many!!)

Another point that is good to discuss and there were questions about it in exams before, what is the difference between endings -ide and -ate? The answer is very easy, there are two types of negative non-metal ions, the ones with oxygen and without oxygen. Initially a non-metal ion has an -ide ending, such as nitride, fluoride, sulfide but when we add an oxygen to it they change to -ate. For example, nitrate or sulfate.

I am going to have to tell you that there is more to remember, but don't worry, you are going to get used to the new formulaes and you won't even realise when you remebered them all. There is only 6 more I want you to remember and you will ace in writing all of the compounds whatever they ask you for.

My tip for memorising new information, is to write them on post it notes and hung around your bedroom. This way you see them every day, for couple of times a day, so there is no way they will not stick in your head.

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So, let's go further shall we? Now, we are going to learn how to calculate a formual for ionic compounds.

There are three main points to successfully write a formula for ionic compound. I'll thell you them first and then we will go forward to the examples.

• Write the two ions that the compound consists off;

• In ionic compounds the total charge needs to be 0 and for me the easiest way to write the formula was to take the power of one ion and bring it down to the other ion present and vice versa. I feel like this may not make any sense to you right now, but you will get it when I show you examples. I promise!

• Lastly, you just write the formual out and you're done!

1. We've got magnesium with a +2 power and oxygen with a -2 power. They both have the same power, therefore to make a total charge of 0, we just need aone of each (+2+(-2)=0).

2. This is an example where my trick helps, we have calcium with a +2 power and nitrate with a -1 power. Write them out next to each other, we take the power of nitrate down to calcium leaving a single calcium and then we take down the power from calcium (which is +2) and take it down to nitrate giving us two nitartes. ( 1xcalcium+ 2xnitrate=0 or +2+(-1x2)=0 ). As you can see I wrote the nitrate in a bracket and then the 2 next to it, this is because the ion originally has a little number next to it.

3. We have iron(III) and sulfate. Write the ions out, we take the -2 power from sulfate to the iron, leaving us with two irons and then we take the +3 from iron to sulfate, leaving us with three sulfates ( 2x iron+ 3xsulfate=0 or (2x-3)+(3x-2)=0 ) (Please, forgive me, I only wrote one sulfate when writing in grey, the correct formula is the one in the box in blue colour).

4. The last example is very straight forward, both silver and bromide have the same power and this means we just write out the symbols of each ion (+1+(-1)=0 ).

You shoud be happy now, this is all you need to know when it comes to witing formulaes of compounds! See you next time :)

PS. Please remember, I am only a student, and as anyone, I can make mistakes. If you think you can see one, don't hesitate and comment (either here on on my youtube channel) Thank you!