How to write chemical formulas?
Writing formulas is a skill that all of chemistry in grades 7 and 8 is based on — from oxides to salts. Good news: you don't have to memorize formulas. All you need is valency and one method.
Empirical vs. structural formula
An empirical formula tells you how many atoms of each element are in a molecule (H₂O). A structural formula additionally shows how these atoms are connected (H–O–H).
Both are usually required on tests — so it's worth understanding the structure of the molecule, not just the notation.
Criss-cross method step by step
Let's write the formula for phosphorus(V) oxide:
- Write down the symbols and valencies: P(V) and O(II).
- Transfer the valencies criss-cross — the valency of phosphorus becomes the subscript of oxygen and vice versa.
- If possible, simplify the subscripts to the smallest whole numbers.
↘ ↙
P2O5
Second example — calcium oxide: Ca(II) and O(II) give Ca₂O₂ criss-cross, and after simplification CaO.
Checking the result
Multiply the number of atoms by the valency — the result must be the same on both sides:
| Formula | Element 1 | Element 2 | Correct? |
|---|---|---|---|
| H₂O | 2 × I = 2 | 1 × II = 2 | ✔ |
| P₂O₅ | 2 × V = 10 | 5 × II = 10 | ✔ |
| Al₂O₃ | 2 × III = 6 | 3 × II = 6 | ✔ |
| „HO₂" | 1 × I = 1 | 2 × II = 4 | ✘ — such a compound will not form |
Common mistakes
- Confusing subscript with coefficient. H₂O is one water molecule with two hydrogens; 2H₂O is two water molecules. The subscript (small number) belongs to the formula, the coefficient (large number in front) — to the reaction equation.
- Forgetting to simplify. Ca₂O₂ is formally "criss-cross", but the correct formula is CaO.
- Memorizing formulas without valencies. There are hundreds of formulas, but only a dozen or so valencies. It pays to know those dozen.
- Ignoring that some elements have multiple valencies. Sulfur forms both SO₂ and SO₃ — that's why the valency is always given in parentheses in the problem: sulfur(IV) oxide or (VI).
Note: the criss-cross method works mechanically — it gives the correct notation, but it doesn't explain why it is so. Understanding comes when you see the bonds: each "hook" of an atom must have a pair. More on this in the article about element valency.
Practice on real molecules
On Cząsteczki.pl, you can build molecules from atoms like building blocks — and the program takes care of the valency for you. If the formula is incorrect, the atoms simply won't connect. This is the fastest way for the criss-cross method to stop being a magic trick and become obvious.