Which formula name pair is incorrect?
Four binary molecular compounds (nonmetal + nonmetal) appear below, each matched with a name using Greek prefixes. Exactly one pair violates standard covalent compound nomenclature.
| Option | Formula | Proposed name |
|---|---|---|
| A | SO2 | monosulfur dioxide |
| B | CO | carbon monoxide |
| C | N2O5 | dinitrogen pentoxide |
| D | PCl3 | phosphorus trichloride |
Direct result
Option A is incorrect: SO2 is named sulfur dioxide, not “monosulfur dioxide.”
Binary molecular naming conventions
Element order follows the written formula for a binary molecular compound, with the more electropositive nonmetal typically written first. The first element keeps its elemental name (sulfur, carbon, nitrogen, phosphorus).
Second-element ending uses the anion-style suffix -ide (oxygen → oxide, chlorine → chloride).
Prefix system indicates the number of atoms: mono-, di-, tri-, tetra-, penta-, hexa-, hepta-, octa-, nona-, deca-. The second element almost always carries a prefix because its atom count varies widely across compounds.
One-atom convention omits mono- on the first element in standard systematic naming. The first element receives a prefix only when its subscript is 2 or larger.
Prefix behavior for the “mono-” case
The formula SO2 contains one sulfur atom and two oxygen atoms. The systematic name reflects:
| Part | Count | Naming piece | Resulting word |
|---|---|---|---|
| First element | 1 | prefix omitted + elemental name | sulfur |
| Second element | 2 | di- + oxide | dioxide |
The phrase “monosulfur dioxide” incorrectly inserts a first-element mono- prefix that systematic binary molecular naming omits.
Oxygen spelling contraction
Oxygen compounds often show a contracted form where the final vowel is dropped for smoother pronunciation: “penta- + oxide” becomes “pentoxide” in N2O5, giving dinitrogen pentoxide.
Prefix reference for common counts
| Count | Prefix | Example fragment |
|---|---|---|
| 1 | mono- | monoxide (CO) |
| 2 | di- | dioxide (SO2) |
| 3 | tri- | trichloride (PCl3) |
| 4 | tetra- | tetrafluoride (CF4) |
| 5 | penta- | pentoxide (N2O5) |
| 6 | hexa- | hexafluoride (SF6) |
| 7 | hepta- | heptoxide (I2O7) |
| 8 | octa- | octachloride (hypothetical) |
| 9 | nona- | nonahydride (hypothetical) |
| 10 | deca- | decaoxide → decoxide (P4O10) |
Conclusion
The incorrect formula–name pair is SO2 — monosulfur dioxide; the systematic name is sulfur dioxide.