No prefix before hydrogen
Hydrogen first, and common names.
Two hydrogens, but no di-?
H₂S has two hydrogen atoms and one sulfur atom. By the prefix rule you might expect dihydrogen sulfide.
What do you think the compound H₂S is called?
What this lesson covers
The idea
No prefix is added before hydrogen when it is the first element in a formula, whatever the number of its atoms, and a few binary compounds are known only by their common names.
Two hydrogens, but no di-?
H₂S has two hydrogen atoms and one sulfur atom. By the prefix rule you might expect dihydrogen sulfide.
What do you think the compound H₂S is called?
- Hydrogen sulfide
- Dihydrogen sulfide
- Hydrogen monosulfide
Match formula and name
Match each formula with the name used for it.
Two exceptions
When hydrogen is the first element in a formula, no prefix is added before it. A few binary compounds are known only by their common names.
No prefix is added before hydrogen when it is the first element in a formula, whatever the number of its atoms. A few binary compounds are known only by their common names.
- Formula | Name used | Name by the prefix system
- H₂S | hydrogen sulfide | not dihydrogen sulfide
- H₂O | water | hydrogen monoxide
- NH₃ | ammonia | nitrogen trihydride
Notes
No prefix before hydrogen when it is first in the formula. A few binary compounds, like H₂O (water) and NH₃ (ammonia), keep their common names.
Check yourself
What is H₂S named?
Why is there no prefix before hydrogen in the name hydrogen sulfide?
What is NH₃ commonly known as?
What would the prefix system call H₂O?
- Dihydrogen sulfide. No prefix is added before hydrogen when it is the first element, whatever the number of its atoms.
- Hydrogen sulfide — correct. Yes!
- Hydrogen disulfide. There is one sulfur atom, so there is no di- on sulfide.
- Hydrogen is a gas. The rule depends on hydrogen being the first element in the formula, not on its state.
- The formula has only one atom of hydrogen. H₂S has two hydrogen atoms. The rule applies whatever the number of hydrogen atoms.
- Hydrogen is the first element in the formula — correct. Yes!
- Ammonia — correct. Yes!
- Hydrogen nitride. NH₃ is known by its common name, ammonia. Its prefix name is nitrogen trihydride.
- Water. Water is the common name of H₂O, not of NH₃.
- Dihydrogen monoxide. No prefix is added before hydrogen when it is the first element.
- Hydrogen monoxide — correct. Yes!
- Hydrogen dioxide. There is one oxygen atom, which is mono-, not di-.