Criss-cross for covalent compounds
Writing a formula quickly.
A shortcut for formulae
Hydrogen has valency 1 and sulfur has valency 2. The formula of hydrogen sulfide has a small number in it.
Which formula do you think is right for hydrogen sulfide?
What this lesson covers
The idea
Write the symbols of the elements, write their valencies, then cross over the valencies as subscripts after the symbols; a subscript of one is not written.
A shortcut for formulae
Hydrogen has valency 1 and sulfur has valency 2. The formula of hydrogen sulfide has a small number in it.
Which formula do you think is right for hydrogen sulfide?
- H₂S
- HS₂
- H₂S₂
Cross over the valencies
Open every tab. In each, the valencies cross over and become subscripts.
Three steps
Write the symbols, then the valencies, then cross the valencies over so each becomes the subscript of the other symbol. A subscript of 1 is left out.
Write the symbols of the elements, write their valencies, then cross over the valencies as subscripts after the symbols. A subscript of one is not written.
Notes
Write symbols, write valencies, cross over the valencies as subscripts. A subscript of one is not written: HCl, H₂S, CCl₄.
Check yourself
Hydrogen has valency 1 and sulfur has valency 2. What is the formula of hydrogen sulfide?
Carbon has valency 4 and chlorine has valency 1. In CCl₄, what is the subscript of chlorine?
Answer: 4
The valency of carbon, 4, crosses over to become the subscript of chlorine.
Why is HCl not written as H₁Cl₁?
What is the correct order of the steps?
- HS₂. The valency of sulfur, 2, crosses over to become the subscript of hydrogen, not of sulfur.
- H₂S — correct. Yes!
- H₂S₂. After crossing over, sulfur gets the subscript 1, which is not written.
- Hydrogen and chlorine have no valency. Hydrogen and chlorine each have valency 1, which becomes a subscript of 1.
- Subscripts are used only for three or more atoms. A subscript of one is simply not written, and subscripts such as 2 are written.
- A subscript of one is not written — correct. Yes!
- Write symbols, write valencies, cross over as subscripts — correct. Yes!
- Cross over, write symbols, write valencies. The symbols and valencies must be written before they can be crossed over.
- Write valencies, cross over, write symbols. The symbols come first, then the valencies, then the cross over.