Tobin Science — Counting Atoms
Subscripts, parentheses, coefficients — then whole equations. How many atoms are really there?
The three counting rules
Every formula on this page — and every formula you will ever meet — is counted with these three rules. Learn them in order, because each one stacks on the one before it.
1 · Subscripts
A subscript counts only the one atom directly in front of it. No subscript means 1 — the atom is still there, it just doesn't need a number to say so.
CO2
1 carbon + 2 oxygen = 3 atoms
The 2 belongs to the O, not the C. And watch the capital letters — CO is carbon and oxygen, but Co is the single element cobalt.
2 · Parentheses
The subscript outside the parentheses multiplies everything inside — every element in the package, not just the last one.
Ca(OH)2
1 calcium + 2 oxygen + 2 hydrogen = 5 atoms
(OH)2 means the whole OH package twice: 2 oxygens AND 2 hydrogens.
3 · Coefficients
The big number out front multiplies the entire formula — every element, parentheses included. It is not glued to the first element.
3H2O
6 hydrogen + 3 oxygen = 9 atoms
Three whole water molecules. The 3 multiplies the O too — 3H2O is not "6 H and 1 O."
Formula X-Ray
Type any formula — coefficient and parentheses welcome — and watch it get counted rule by rule. Or click one below.
Whole equations — the boss level
A chemical equation is just formulas on both sides of an arrow. Reactants on the left, products on the right. To audit one, count every element on each side — coefficients multiply their whole formula, and you add across all the substances on a side.
2H2 + O2 → 2H2O
| Element | Left (reactants) | Right (products) | Match? |
|---|---|---|---|
| H | 2 × 2 = 4 | 2 × 2 = 4 | ✓ |
| O | 2 | 2 × 1 = 2 | ✓ |
Every element matches, so the equation is balanced — atoms were rearranged, not created or destroyed. That is the law of conservation of mass doing its job.
H2 + O2 → H2O
| Element | Left | Right | Match? |
|---|---|---|---|
| H | 2 | 2 | ✓ |
| O | 2 | 1 | ✗ |
An oxygen atom vanished — so this one is not balanced. One mismatched element is all it takes. You don't fix it by touching subscripts (that changes the substance); you fix it with coefficients. But first you have to catch it, and that is what this page trains.
The mistakes that cost the most points
Sign in to practice
First and last name — this is how your teacher gives you credit.
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Printable sheets
Black and white, no color ink needed. Each one opens your printer dialog — choose Save as PDF if you would rather keep a copy. Answer keys print as their own page, so print the student copy for them and the key for you.
Assign a level
Pick a level and share the link. Students sign in with name + period, answer questions built from their own name, and hand you a code. Each question gets one check — the first answer is the one that counts.
Grade assignment codes
Paste every student code below — one big mess is fine. You get a roster sorted by period, then last name.
Check one practice code
For free-practice codes (CAE-…) students earn outside an assignment.
Codes carry the student's name, level, score, and the date, so they cannot be swapped between students. Nothing is uploaded — everything runs on the device, and the page works with the wifi off.
Standards this covers
S8P1 — Matter: atoms, molecules, and the law of conservation of matter. Counting atoms on both sides of an equation is exactly how students verify conservation.
SPS2 — Chemical formulas: what subscripts, parentheses, and formula units actually say about composition.
SPS3 — Conservation of matter in chemical reactions. Level 4 is the auditing skill that comes right before balancing equations by hand.
A note on scope: this page never asks students to balance an equation — only to count and to judge. That is deliberate. Balancing is a separate skill, and it collapses if the counting underneath it is shaky.