Step 1:
Find out the chemical symbol [e.g: Na for sodium, Ca for calcium ] of the element from the Periodic Table (which is given in the test/exam)
Step 2:
Find out the charge of the element from the Periodic Table
Look at the group number on top of the Periodic Table and link it to the charges by using the triangle below:
For example, given the Periodic Table in a test/exam: 
Look out for the elements you are supposed to write the chemical formula for [Eg: Potassium oxide, (K) and (O)]
Charge for K: +1
Charge for O: -2
Elements/Atoms with a charge are called ions.
You will need 2 K+ to balance the negative charge of O2-.
Step 3:
REMEMBER that CHARGES are written ON TOP. The NUMBER OF IONS you need should be written ON THE BOTTOM.
Thus the chemical formula of Potassium oxide is K2O.
Now, get out your Periodic Table, follow the steps and try writing the chemical formula for:
- Aluminium chloride
- Calcium Oxide
- Lithium Iodide
- Magnesium Fluoride
- Beryllium Nitride
Answer:
- AlCl3
- CaO
- LiI
- MgF2
- Be3N2
**Note **
Every new capital letter is a new element !!!
[E.g: CO is carbon monoxide, carbon and oxygen but Co is cobalt, found in the Periodic Table]
A roman numeral behind the element refers to the charge.
[E.g: Iron(III) chloride --> Fe3+ and Cl-. Thus the chemical formula is FeCl3]
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