[Shaun1979]

The question says to balance the reaction of copper and Oxygen molecule.

I found this on the internet as answers:

2Cu + O2 = CuO (Copper (II) Oxide) OR
4Cu + O2 = Cu2O (Copper (I) Oxide)


I understand what's going on ionically, but I don't understand how this is balanced?

Don't the atoms on the right hand side need to equal the left to be balanced? Or is it because it's an ionic equation that it does not need to indicate number of atoms in product?

[Borek]

I found this on the internet as answers:

2Cu + O2 = CuO (Copper (II) Oxide) OR
4Cu + O2 = Cu2O (Copper (I) Oxide)

Don't believe everything on interenet - neither of these reactions is correctly balanced.

[stewie griffin]

Yes you need to have the same number of each element on each side of the equation. So if you start with 4 Cu atoms but the product only has 2 Cu atoms, then it's not a balanced equation.

[Borek]

if you start with 4 Cu atoms but the product only has 2 Cu atoms

It is not a matter of number of Cu atoms in the product - there can be 3 or 7 or 29. However, there must be identical number of atoms on both sides of the equation - so, if there are 4 Cu atoms on the left, and 2 molecules of product containing 2 atoms of Cu, copper is balanced.

[stewie griffin]

As I had already said

Yes you need to have the same number of each element on each side of the equation.

Both sides indeed do need the same number of each element. As the second reaction was written, there are only 2 copper atoms on the right hand side. Since there's only one product, rather than say "but the right hand side only has 2 Cu atoms" I stuck with the phrase "the product only has 2 Cu atoms" since that one product is the entire right hand side of the chemical equation.  
Sorry for any confusion.