# WRITING FORMULA FOR IONIC COMPOUNDS

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WRITING FORMULA FOR IONIC COMPOUNDS
(NH4)3PO4 All you need to know about writing formula for Y11 to Y13 Science

IONIC COMPOUNDS Ionic compounds are very common.
Some examples are common salt (Sodium Chloride) found in the sea and Limestone (Calcium Carbonate)

SOME BACKGROUND Ionic compounds form when metal and non-metal atoms react together and form ions. The METALS form POSITIVE ions. The NON-METALS form NEGATIVE ions. There are also COMPLEX IONS formed from more than one atom.

POSITIVE IONS These are formed when atoms lose electrons and become positively charged. The superscript in the symbol tells us the size of the positive charge. Na+ has a positive one charge. Al3+ has a positive three charge

Here are some examples of some Positive ions
Sodium Na+ Magnesium Mg2+ Ferric (Iron III) Fe3+ Aluminium Al3+ Calcium Ca2+ Copper Cu2+

NEGATIVE IONS These are formed when atoms gain electrons.
The superscript tells you the size of the negative charge. Cl- has a negative one charge. O-2 has a negative two charge.

COMPLEX IONS These are groups of atoms made up of more than one type of atom. They may have a positive or negative charge. Some examples are the Ammonium ion (NH4+) and the Carbonate ion (CO32-)

To find out the symbol and charge on an ion
Look up the symbol and charge on the table of ions on your worksheet Use this to write the formula of the compound using the method described in the following slides

TO WRITE THE FORMULA FOR A COMPOUND (eg Sodium Chloride) YOU MUST KNOW THE
Positive ion and the Negative ion Na+ Cl-

THE IMPORTANT THING TO REMEMBER IS…..
The total of the charge on the positive ions MUST equal the total of the charge on the negative ions so that they cancel each other out!

NOW, Back to Sodium Chloride
The Sodium ion (Na+) has a charge of +1 The Chloride ion (Cl-) has a charge of The charge on the positive ion is exactly equal but opposite to the charge on the negative ion. So one positive ion exactly cancels the charge on a negative ion. The formula is written as NaCl

SO NOW……... The charge on the one positive ion (Sodium) is exactly balanced by the size of the charge on the one negative ion (Chloride) Na Cl NaCl The compound Sodium Chloride is stable like this.

Now, try to write the formula for these on your worksheet...
Magnesium Oxide Potassium Chloride Calcium Oxide Zinc Oxide Calcium Sulfide Copper Oxide Lead Oxide Click the left mouse button when you have finished.

Here are the answers... Magnesium Oxide MgO Potassium Chloride KCl
Calcium Oxide CaO Zinc Oxide ZnO Calcium Sulphide CaS Copper Oxide CuO Lead Oxide PbO If you got 5 or better you are doing well!

Now, a more complicated example. Write the formula for Calcium Chloride
Look up the formula for the calcium ion Look up the formula for the Chloride ion Cl- Ca+2

REMEMBER !….. The total of the charge on the positive ions MUST equal the total of the charge on the negative ions so that they cancel each other out!

You need two Chloride ions to balance the charge on a Sodium ion
Cl- Total charge = -2 Ca2+ Total charge = +2

THIS TIME……... The charge on the one positive ion (Calcium) is exactly balanced by the size of the charge on the two negative ions (Chlorine) The compound Calcium Chloride is stable like this. Cl Ca Cl CaCl2

How we write the formula for Calcium Chloride
There is one calcium ion, this is shown by writing the symbol- Ca There are two Chloride ions, this is shown by writing the symbol for chloride with a subscript ‘2’- Cl2 So the symbol for the whole compound Calcium Chloride is CaCl2

Try to write the formula for these compounds on your worksheet
Sodium oxide Magnesium Chloride Potassium oxide Copper chloride Lead Chloride Sodium Sulfide aluminium Chloride Iron (II) Chloride Iron (III) Chloride Click the left mouse button when you have finished.

And here are the correct answers …
Sodium oxide Na2O Magnesium Chloride MgCl2 Potassium oxide K2O Copper chloride CuCl2 Lead Chloride PbCl2 Sodium Sulphide Na2S Aluminium Chloride AlCl3 Iron (II) Chloride FeCl2 Iron (III) Chloride FeCl3 If you got 6 or better you are doing well!

WRITING FORMULA FOR COMPOUNDS CONTAINING COMPLEX IONS
If a compound contains a complex ion writing its formula can be confusing. If a single complex ion is required it is simply written as its symbol eg Nitrate NO3-

WRITING FORMULA FOR COMPOUNDS CONTAINING COMPLEX IONS
If more than one complex ion is required then brackets are put around the whole symbol, then a subscript to indicate the number of ions If you need two nitrates: (NO3)2 If you need three nitrates: (NO3)3 Confused ! It does gets easier with practise.

Now here’s an example: Write the formula for Magnesium Nitrate
The symbol for the magnesium ion is : The symbol for the Nitrate ion is : Mg+2 NO3-

You need two Nitrate ions to balance the charge on a Magnesium ion
NO3- Total charge = -2 Mg2+ Total charge = +2

So the formula is written…..
Mg(NO3)2

Now try these examples on your worksheet
Ammonium Carbonate Ammonium Sulfate Ammonium Phosphate Sodium Carbonate Copper Nitrate Calcium Nitrate aluminium Nitrate Click the left mouse button when you have finished.

Ammonium Carbonate (NH4)2CO3 Ammonium Sulfate (NH4)2SO4 Ammonium Phosphate (NH4)2SO4 Sodium Carbonate Na2CO3 Copper Nitrate Cu(NO3)2 Calcium Nitrate Ca(NO3)2 aluminium Nitrate Al(NO3)3 If you got 5 or better you are making excellent progress!

Sometimes it is not easy to get the charges to balance!
An example is aluminium oxide. The charge on the aluminium ion is 3+ The charge on the oxide ion is 2- At first glance these do not easily balance Do it as follows………...

You need three oxide ions to balance the charge on two aluminium ions
O-2 O-2 Total charge = -6 Al3+ Total charge = +6

THIS TIME……... The charge on the two positive ions (aluminium) is exactly balanced by the size of the charge on the three negative ions (Oxygen) The compound Aluminium Oxide is stable like this. O Al O Al O Al2O3

Here are some examples to try..
Copper Phosphate Iron (III) Oxide Iron (III) Sulfide aluminium Sulfide Magnesium Phosphate Calcium Phosphate Click the left mouse button when you have finished.