This is a question, not a home work. The book I'm reading explains how to balance equations step-by step. UNBALANCED EQUATION IS THIS: C2H6 + O2 = CO2 + H2O The book gets to this: C2H6 + O2 = 2CO2 + 3H2O The book says "There are 7 oxygen on the right-hand-side but only 2 on the left". All fine so far. Then the book says "Oxygen's HAVE to go around in PAIRS so how can you get an odd number (7) of them on the left side". Then the book goes on to balance the equation: 2C2H6 + 7O2 = 4CO2 + 6H2O My questions are this: 1. Do Oxygen's HAVE to go around in PAIRS? What if I balanced an equation (any equation) and there were say 7 oxygen on left side and 7 on the right side - would that be incorrect? Would I have to get 14 on both sides (or an even number on both sides)? 2. If the book is correct and "Oxygen's HAVE to go around in PAIRS" then what other elements HAVE to go around in pairs or triplets etc (for the purpose of balancing equations)? [This is HIGH SCHOOL CHEMISTRY] Thank you.