SCIENCE · INVESTIGATE AND EXPLAIN

Read formulas without changing the substance

Distinguish a molecular formula from a coefficient and count atoms across several species.

Learning outcome: Distinguish a molecular formula from a coefficient and count atoms across several species.

Starting recap

A subscript counts atoms within one molecule. A coefficient counts whole molecules or proportional amounts.

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Try

Two water molecules contain four hydrogen atoms and two oxygen atoms. This does not mean the formula of water is H4O2.

Open the experiment

Predict

For five H2 molecules and two O2 molecules, predict the initial counts of H and O atoms before inspecting the output.

Your prediction and reason

Experiment

Compare initial inventories (4,2) and (5,2). Capture both. Count the atoms in the water and in everything left over.

Rearrange atoms without losing the leftovers

Worked teaching model · synthetic values · Ideal whole-molecule bookkeeping for 2 H2 + O2 → 2 H2O. Each allowed reaction group uses two H2 and one O2. This is not a molecular trajectory, rate, equilibrium calculation or physical mixing procedure.

Initial hydrogen molecules H2: 5 molecules Initial oxygen molecules O2: 2 molecules

Reset keeps your prediction, captures and practice history.

The boxes connect composition to the calculated result. They show counts, not particle size, bond geometry or a reaction movie.

Calculated values on the same fixed scale
Row	Quantity	Value (count)
1	Initial H atoms	10
2	Final H atoms in all species	10
3	Initial O atoms	4
4	Final O atoms in all species	4
5	Water molecules formed	4
6	Hydrogen molecules left	1
7	Oxygen molecules left	0

4 H2O molecules formed; 1 H2 and 0 O2 remain.

Allowed reaction groups = min(floor(H2/2), O2). Count atoms in products AND unreacted material. Total molecule count can change while each element count is conserved. A single leftover H2 cannot complete this stipulated whole reaction group.

Observation to attach to a capture

Captures preserve recipes and replay their calculated results. They are synthetic model evidence, not physical measurements.

Understand

Read the substance before multiplying

A water molecule contains two hydrogen atoms and one oxygen atom. Its formula H2O records that composition. The subscript two belongs to hydrogen; oxygen has an implicit subscript one. In the displayed formulas, digits following an element symbol are subscripts written in plain text.

A coefficient multiplies the whole formula. 2 H2O means two water molecules in a particle drawing: four H atoms and two O atoms in total. H2O2 names a different composition, hydrogen peroxide. Changing a subscript to make an equation balance changes the substance. Change coefficients when balancing an equation, while preserving the formulas of the specified substances.

Elements, compounds and mixtures

O2 contains two oxygen atoms bonded together. It is a molecule of an elemental substance because it contains only one element. Water is a molecular compound because its molecules contain different elements in a definite ratio. A mixture can contain both water and unreacted oxygen; describing the total oxygen inventory does not turn that mixture into a single compound.

Not all substances consist of discrete molecules. Sodium chloride is commonly represented by a formula unit NaCl describing the ion ratio in a lattice, rather than isolated NaCl molecules throughout a crystal. The hydrogen/oxygen/water model here uses molecular species; do not generalize its drawing to every solid.

Count across species

Five H2 molecules contain ten H atoms. Two O2 molecules contain four O atoms. A possible final inventory is four H2O plus one H2: the water contains eight H atoms and four O atoms, and the leftover H2 contains two more H atoms. The atom totals agree even though the species and molecule totals differ.

In the experiment, change the starting H2 count from four to five while keeping O2 at two. The added hydrogen is not hidden or destroyed. It appears in the final inventory. Explain why counting only atoms inside the water would give a misleading conservation result.

Independent practice: a container holds three CO2 and two CO molecules. Count carbon and oxygen atoms separately. Then explain why the combined totals do not identify one new molecular formula for the mixture.

Check the mixed carbon-oxide inventory

There are 3 + 2 = 5 carbon atoms and 3 × 2 + 2 × 1 = 8 oxygen atoms. Five carbon atoms and eight oxygen atoms are totals across five molecules; they do not imply that a C5O8 molecule exists in the container.

Connect representations carefully

A formula gives composition, a particle diagram shows selected entities, and an equation states a stoichiometric relationship. None alone supplies temperature, phase, rate or molecular geometry. A particle drawing of a few molecules is a conceptual model; a laboratory sample contains enormous numbers of particles. Later lessons will use the mole to connect particle ratios to measurable amounts. For now, label the species, preserve every atom, and state exactly what each coefficient counts.

Further explanation in OpenStax Chemistry 2e.

Apply

Public practice checks a specific claim. It does not establish independent mastery.

Explain the difference between 2 H2O and H2O2, and why an oxygen molecule is an elemental substance rather than a compound.

Your explanation and limitations

Continue

Completion requires a saved prediction, at least one capture and a reflection. Presence checks do not grade your reasoning.

Skipping retains experiments and awards no mastery.

Previous: Distinguish atoms, isotopes and ions Next: Balance atoms and identify what remains

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