2. Is balanced, 3. Is unbalanced, 4. Is balanced. We need the same number of each element on both sides. The reason why 3. Is unbalanced is becauase we have 2N and 2H on the left, but only 1N and 6H on the right. If you have an expression like 3H2, this is 3 H2 molecules, meaning 6 H molecules total.
