Cross-Over & Balancing — study notes

Ask ten students to write the formula for potassium oxide and half will guess — "KO2"? "K2O2"? There's a reliable, mark-scoring trick for building any ionic formula, and once you understand why it works you'll stop guessing for good.

Ions Carry Charge, So Compounds Must Cancel to Zero

Metal salts and many other compounds aren't built from neutral atoms bonding directly — they're built from ions, particles that carry a positive or negative electric charge because they've lost or gained electrons. A finished compound, though, always has to be electrically neutral overall, so the positive charge you bring in has to exactly cancel the negative charge. That's the whole game: find how many of each ion makes the total charge zero.

The shortcut for doing this fast is called the cross-over rule: take the charge number off one ion and drop it in as the subscript of the other ion, doing this both ways at once. Because you're literally borrowing each ion's charge to set the other one's count, the positive and negative totals are guaranteed to match — every single time.

Worked Example — Potassium Oxide

1. Write down the two ions involved: potassium is K⁺ (charge 1), oxide is O²⁻ (charge 2).
2. Cross the charge numbers over: the 2 from oxide lands as potassium's subscript, and the 1 from potassium lands as oxide's subscript, giving K₂O₁.
3. A subscript of 1 is never written, so it disappears: K₂O.
4. Check it cancels: 2 × (+1) + 1 × (−2) = +2 − 2 = 0. Neutral, so the formula is correct.

Skip the check and you risk a silent slip that costs the mark — always run the numbers before moving on.

The rest of the topic — every remaining worked example, balancing full equations, ionic equations and state symbols, plus a listen-along audio walkthrough and a practice worksheet — is in the full lesson below.