Chemical meaning of “no subscripts”
The phrase what chemical formula compoubd would contain no subcripts corresponds to formulas in which every element has an implied subscript of 1, so no numbers are written. Standard chemical notation omits the subscript 1.
A written formula such as NaCl is interpreted as \( \mathrm{Na_1Cl_1} \). Each subscript equals 1, and the atom (or ion) ratio is \(1:1\).
A formula like H2O contains subscripts because at least one element appears in a count different from 1.
When a compound can be written with no subscripts
In general chemistry, a chemical formula is written with no subscripts when the simplest whole-number composition has one atom of each element per formula unit or per molecule. In symbols, if a compound contains elements \(A, B, C, \ldots\), then “no subscripts” means
\[ \mathrm{A_1B_1C_1\cdots} \;\;\text{is written as}\;\; \mathrm{ABC\cdots}. \]
Common contexts where this occurs
- Binary ionic compounds with charge balance achieved in a \(1:1\) ratio (examples: NaCl, KBr, MgO, CaS).
- Binary molecular compounds with a \(1:1\) atom ratio (examples: CO, NO, HCl, HF).
- Some interhalogen molecules with \(1:1\) composition (examples: ICl, BrCl).
Coefficient versus subscript
Subscripts belong to the formula itself and describe composition. Coefficients multiply an entire formula in a chemical equation and do not change the formula’s internal atom ratio. For example,
\[ 2\,\mathrm{H_2O} \;\; \text{represents two molecules of } \mathrm{H_2O}, \text{ not a different formula.} \]
Examples of formulas with no subscripts
| Formula (no subscripts written) | Implied subscripts | Type | Why no subscripts appear |
|---|---|---|---|
| NaCl | \(\mathrm{Na_1Cl_1}\) | Ionic (formula unit) | \(\mathrm{Na^+}\) and \(\mathrm{Cl^-}\) balance in a \(1:1\) ratio. |
| MgO | \(\mathrm{Mg_1O_1}\) | Ionic (formula unit) | \(\mathrm{Mg^{2+}}\) and \(\mathrm{O^{2-}}\) balance in a \(1:1\) ratio. |
| CO | \(\mathrm{C_1O_1}\) | Molecular (molecule) | One carbon atom and one oxygen atom per molecule. |
| HCl | \(\mathrm{H_1Cl_1}\) | Molecular (molecule) | One hydrogen atom and one chlorine atom per molecule. |
| ICl | \(\mathrm{I_1Cl_1}\) | Molecular (molecule) | Interhalogen with one atom of each element. |
Visualization of implied subscript 1
Common pitfalls in reading formulas
- Element symbols alone (Na, Fe, O) describe elements, not compounds; “no subscripts” in a compound still requires at least two different element symbols.
- Empirical versus molecular formulas: a molecular formula may contain subscripts even when the empirical formula does not; for example, an empirical \( \mathrm{CH} \) contrasts with a molecular \( \mathrm{C_6H_6} \).
- Parentheses usually signal subscripts outside the parentheses; examples such as \( \mathrm{Ca(OH)_2} \) inherently contain written subscripts.