Chemistry · Lesson
Chemical formulas, subscripts and charge notation
Read coefficients, subscripts and parentheses as different instructions. Identify what a formula tells you and when connectivity needs another representation.
Back to the Chemistry mapBy the end, you can
- Count atoms represented by a formula and a coefficient.
- Distinguish empirical and molecular formulas.
- Explain why the same molecular formula can leave structure unresolved.
Read each symbol with its subscript
A molecular formula specifies how many atoms of each element occur in one molecule. H₂O says two hydrogen atoms and one oxygen atom. The omitted subscript after O means one. CO₂ says one carbon atom and two oxygen atoms. Changing a subscript changes the composition being named: H₂O₂ is not another way to write water.
Start by identifying complete element symbols. Na contains one symbol, sodium; it is not nitrogen followed by a second element “a”. Letter case separates symbols correctly. Then read each lower-right number with the symbol or parenthesized group immediately before it. The formula itself does not state a sample’s amount, temperature, concentration or purity.
A subscript and coefficient multiply different things
In Ca(OH)₂, the 2 applies to the entire OH group. The composition has one calcium, two oxygen and two hydrogen atoms per formula unit. In 3 Ca(OH)₂, the leading coefficient 3 multiplies the whole formula: three calcium, six oxygen and six hydrogen atoms are represented. The coefficient does not turn Ca into a different element or change the formula-unit composition.
Similarly, Ca₃(PO₄)₂ has three calcium, two phosphorus and eight oxygen atoms per formula unit. Expand the parentheses before adding counts. These formulas retain polyatomic-ion grouping. Do not apply a blind “reduce every subscript” rule to an ionic formula: preserving a recognized ion, such as oxalate in Na₂C₂O₄, matters to what the formula conveys.
| Written expression | Ca count | O count | H count | How the multiplier acts |
|---|---|---|---|---|
| Ca(OH)₂ | 1 | 2 | 2 | 2 multiplies OH |
| 3 Ca(OH)₂ | 3 | 6 | 6 | 3 multiplies the whole formula |
Composition counts in the formulas shown. The last row uses a coefficient of three; it does not change the identity of a formula unit.
Empirical and molecular formulas answer different questions
An empirical formula gives the simplest whole-number atom ratio. A molecular formula gives the actual atom counts in one molecule. For a substance with molecular formula C₆H₁₂O₆, the empirical formula is CH₂O because all three counts share a factor of six. The empirical formula alone does not tell you that the molecule contains six carbon atoms.
Different substances can share an empirical formula. A measured composition may support CH₂O as the ratio while leaving the molecular size and structure unresolved. Additional evidence, including molar mass, can identify an appropriate integer multiple. For extended solids, a formula may describe composition without defining a separate molecule at all.
Composition is not the whole structure
A structural representation shows which atoms are connected and may show bond types or spatial arrangement. A molecular formula leaves that information largely unstated. Ethanol and dimethyl ether both have formula C₂H₆O. In ethanol, the oxygen is connected to a carbon and a hydrogen; in dimethyl ether, it is connected between two carbons. The different connectivity helps explain their different properties.
A sketch still has limits. A line in a structural formula is a bond convention, not a photographed rod, and a flat drawing may hide three-dimensional information. Choose the representation for the question: composition for atom counts, connectivity for which atoms join, and a spatial model when geometry matters. Later organic lessons develop the distinctions in detail.
Worked examples
Expand a grouped formula
Count Ca, P and O in Ca₃(PO₄)₂.
- Ca₃ contributes three calcium atoms.
- Two PO₄ groups contribute 2 × 1 = 2 phosphorus atoms.
- Those groups also contribute 2 × 4 = 8 oxygen atoms.
Ca:P:O = 3:2:8. For the ionic solid this is a formula-unit composition, not a claim of an isolated molecule.
Try it yourself
How many H and O atoms are represented by 2 H₂O?
Show the answer and reasoning
4 H and 2 O.
The coefficient multiplies the entire molecular formula.
Give the empirical formula corresponding to molecular formula C₂H₆.
Show the answer and reasoning
CH₃.
Divide both atom counts by their common factor of two.
Can you change H₂O to H₂O₂ merely to balance an equation?
Show the answer and reasoning
No.
Changing a subscript changes the substance. Equation balancing changes coefficients.
Does C₂H₆O by itself distinguish ethanol from dimethyl ether?
Show the answer and reasoning
No.
The shared atom counts leave their different connectivity unstated.
What to carry forward
- A molecular formula counts atoms in one molecule.
- Parentheses group a subscript’s scope; a coefficient multiplies the whole formula.
- Empirical formulas express the simplest atom ratio.
- Composition alone often leaves connectivity and geometry unresolved.
References
Teaching resources and reference material supporting this lesson’s explanations and examples.