Drawing structures of carbon compounds
Link the carbons, then fill with hydrogen.
Fill up the valencies
Carbon has four valencies and hydrogen has one. Two carbon atoms are linked by a single bond. Hydrogen atoms will fill all the valencies that are left.
How many hydrogen atoms are needed in all?
What this lesson covers
The idea
To draw a carbon compound's structure, first link the carbon atoms with single bonds, then use hydrogen atoms to satisfy the remaining valencies; any valency still unsatisfied is met by a double or triple bond between carbons.
Fill up the valencies
Carbon has four valencies and hydrogen has one. Two carbon atoms are linked by a single bond. Hydrogen atoms will fill all the valencies that are left.
How many hydrogen atoms are needed in all?
- Four
- Six
- Eight
Build ethane and ethene
Press Next step. A dashed line is a valency that is still unsatisfied.
Three steps
Link the carbons, add hydrogen, and if a valency is still left, use a double or triple bond between carbons.
To draw the structure of a carbon compound, first link the carbon atoms with single bonds, then use hydrogen atoms to satisfy the remaining valencies. Any valency still unsatisfied is met by a double or triple bond between carbons.
Notes
First link the carbon atoms with single bonds, then use hydrogen atoms to satisfy the remaining valencies. A valency still unsatisfied is met by a double or triple bond between carbons.
Check yourself
Two carbon atoms are linked by a single bond. How many valencies does each carbon atom have left?
Answer: 3
4 − 1 = 3 valencies left on each carbon atom.
Three carbon atoms are linked in a chain by single bonds, C-C-C, and hydrogen satisfies every other valency. How many hydrogen atoms are needed?
Answer: 8
Three carbon atoms have 12 valencies. The two C-C bonds use 4 of them. 12 − 4 = 8 hydrogen atoms, which is propane, C₃H₈.
In ethene, C₂H₄, one valency per carbon atom is left after four hydrogen atoms are added. How is it satisfied?
What is the first step in drawing the structure of a carbon compound?
- By one more hydrogen atom on each carbon. That would give C₂H₆, ethane, with only single bonds. Ethene is C₂H₄.
- By a double bond between the two carbons — correct. Yes!
- By a triple bond between the two carbons. A triple bond is found in ethyne, C₂H₂, where each carbon has only one hydrogen atom.
- Place all the hydrogen atoms in a row. Hydrogen atoms are used after the carbon atoms are linked, to satisfy the remaining valencies.
- Draw a double bond between the carbons. A double bond is used only when a valency is still unsatisfied after the hydrogen atoms are added.
- Link the carbon atoms together with single bonds — correct. Yes!