To begin, I will teach you how to write formulas correctly with the Criss Cross and Drop Method in Chemistry!

When writing and naming Ionic Compounds, one must take into account the charge of the ion. Metals, or cations, are usually positive. Nonmetals, or anions, are usually negative. 

The charges are indicated by how many electrons an element is willing to donate or accept. For example, Sodium wants to lose one electron to have its outer shell filled which makes it have a charge of +1.

For anions, they are willing to accept or gain electrons in order to have their outer shell filled. Fluorine gains one electron to have 8 in its outer shell and will have a -1 charge. 

Now that we know those basic facts, we can get into how to combine them as a compound. Ionic compounds are made between a metal and a nonmetal. 

These charges from ions must be taken into account when making ionic bonds. Let us take Aluminum and Oxygen as an example. Aluminum has a positive charge of +3 and Oxygen has a charge of -2. 

We now have to criss-cross and drop these charges in order to write this formula correctly.

After doing this, we are left with two atoms of aluminum and three atoms of oxygen. 

Here is the reasoning behind why we need to Criss-Cross and Drop. We need to have no leftover electrons and the only way of doing that is having two aluminums and three oxygens.