# What are some common mistakes students make when determining formulas?

I would say a very common error students make is forgetting to balance the charges of the ions (positive and negative) in ionic compounds.

#### Explanation:

For example, when aluminum and oxygen react they form the compound aluminum oxide.

This is considered an ionic compound because it contains a metallic ion ($A {l}^{+ 3}$) and a nonmetallic ion (${O}^{- 2}$)

So the formula of aluminum oxide needs to be $A {l}_{2} {O}_{3}$ which means there are 2 Al ions paired with 3 O ions.

2 x ($A {l}^{+ 3}$) = +6
3 x (${O}^{- 2}$) = -6
net charge = 0

For other formula/nomenclature tips, hints and reminders check out the video below.

Video from: Noel Pauller