The Atom Accountant

Why it matters

Burn a strip of magnesium and it flares with a dazzling white light. Writing that as Mg + O₂ → MgO looks fine… until you count the oxygens. Chemistry keeps accounts — and they must balance.

The idea in plain words

Balance it yourself: set the coefficients so magnesium and oxygen each count equal on both sides.

Predict first

Mg + O₂ → MgO has 2 oxygen atoms on the left but only 1 on the right. How do we fix it?

we may never change WHAT the substances are (formulae) — only HOW MANY of each (coefficients). MgO₂ is a different (wrong) substance.

• Change MgO to MgO₂
• Put coefficients: 2Mg + O₂ → 2MgO — correct
• Cross out one O from O₂
• Nothing — small differences don't matter

What you do

First watch the burn on all three levels. Then you take the accountant's chair.

Check yourself

Why must chemical equations be balanced?

Conservation of mass: every atom on the left must reappear on the right — rearranged, never destroyed.

In 2Mg + O₂ → 2MgO, the small 2 in O₂ and the big 2 before MgO are…

The subscript is part of the substance's identity (O₂ is oxygen gas). The coefficient is the count we adjust. Balance with coefficients only.

Which is the balanced equation for hydrogen burning in oxygen to form water?

2H₂ + O₂ → 2H₂O: H 4=4, O 2=2 ✓. Option (d) balances but H₂O₂ is hydrogen peroxide, not water — formulae carry identity.

• To make them look neat
• Because atoms cannot be created or destroyed in a reaction — correct
• Because the products weigh less than the reactants
• Only exam equations need balancing
• the same kind of number
• subscript = atoms inside one molecule; coefficient = how many units of that substance are in the balanced equation — correct
• both changeable when balancing
• both fixed by nature
• H₂ + O₂ → H₂O
• H₂ + O → H₂O
• 2H₂ + O₂ → 2H₂O — correct
• H₂ + O₂ → H₂O₂ (water)