How can I write the formula for lithium oxide?
This should be straightforward for you :)
This means that Lithium can bond once, and Oxygen can bond twice.
Because Lithium can only bond once, if one atom of lithium were to bond with Oxygen, there would still be one free electron which needs to be bonded. The solution to this is for another Lithium atom to bond with the final free Oxygen electron. So, two Lithium atoms and one Oxygen atom creates:
Hope this helps! :)