What is the molecular formula of the compound produced when lithium reacts with oxygen gas?
This question is asking us for the molecular formula for the product of the reaction between lithium and oxygen. As you may already know, this reaction of lithium, an alkali metal, with oxygen in the air occurs very quickly. For example, a piece of lithium metal, when cut, will tarnish very quickly, when in contact with air. This is as a result of its reaction with the oxygen in the air. You will find alkali metals, like lithium, being stored under oil. This is in order to prevent its reaction with both the oxygen in the air and any water, since the reaction between lithium and water is very violent.
We’re given the two reactants in this case, lithium and oxygen. So the product must be a result of combining lithium and oxygen atoms in some way. Now we just need to work out how many lithium atoms and how many oxygen atoms in each molecule. Let’s start by looking at the oxidation states of both lithium and oxygen. Lithium is an alkali metal in group one of the periodic table. It forms a plus one oxidation state.
Oxygen is in group 16 on the right-hand side of the periodic table and most commonly forms a minus two oxidation state. We want our resulting compound to be neutral overall. So all of the charges on both the lithium and the oxygen must add up together to give us zero. If our molecular formula was one lithium and one oxygen atom, we would not have a neutral compound. As we can see, we need two lithium atoms to balance out the two minus charge on the one oxygen atom. So the molecular formula for our final compound is Li₂O, two lithium atoms to one oxygen atom. And this is the formula for lithium oxide.