What is the name of the covalent compound PCl5? I typed in phosphorous pentachloride but on Mastering Chemistry it says that is wrong.
The correct answer and explanation is:
The correct name of the covalent compound PCl₅ is phosphorus pentachloride, not “phosphorous pentachloride.”
The reason “phosphorus” is correct and “phosphorous” is wrong lies in the difference between elemental names and oxidation state naming conventions.
🔹 Explanation:
In naming binary covalent compounds (those made of two nonmetals), we follow the IUPAC system:
- The first element keeps its elemental name (like carbon, nitrogen, sulfur, or in this case, phosphorus).
- The second element is named as if it were an anion: root name + -ide (e.g., chloride for Cl).
- Prefixes (mono-, di-, tri-, tetra-, penta-, etc.) are used to show how many atoms of each element are present.
So for PCl₅:
- The first element is phosphorus (no prefix because it’s only one atom and it’s the first element).
- The second is chlorine, but since there are 5 atoms of chlorine, it becomes pentachloride.
Putting it together:
✅ Phosphorus pentachloride
⚠️ Why “Phosphorous” Is Wrong:
“Phosphorous” (ending in -ous) is used in older naming systems (Stock system) to refer to a lower oxidation state of phosphorus (usually +3), as in phosphorous acid (H₃PO₃).
On the other hand, “phosphoric” acid (H₃PO₄) refers to the +5 oxidation state.
In PCl₅, phosphorus has an oxidation state of +5, not +3. But even then, for binary covalent compounds, we do not use “phosphorous” or “phosphoric” — we stick to the elemental name.
✅ Summary:
- Correct name: Phosphorus pentachloride
- Incorrect: Phosphorous pentachloride (because “phosphorous” implies a different naming system and oxidation state)
Always use the element name and prefixes for binary covalent compounds!