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Hello! A few students have written to ask why carbon to carbon bonds just don't fulfill the octet rule?

Here's a quick explanation to help you connect the dots.

Two carbon atoms can form single, double or triple bonds to share their electrons.

carbon to carbon single bond forming covalent bond

The picture above shows two carbon atoms sharing an electron each to form a single bond. However, a single covalent bond with a second carbon atom will not fulfill either atom’s valence shell, each carbon only have up to five valence electrons. In order to fulfill the octet rule, each carbon atom must bond with other atoms. Hydrocarbons are very common organic compounds and carbon easily bonds with hydrogen atoms to have eight valence electrons.

Example below is an ethane molecule. Count the number of valence electrons for each atom. There should be eight around the carbon atom and two around the hydrogen atom. Hydrogen can only hold up to two valence electrons.

carbon to carbon single bond - ethane molecule

The next step is to think about how carbon forms double and triple bonds and reducing the number of spaces to form bonds with other atoms.

Example:

C2H6 - single bond

c2h6 - single carbon to carbon bond

C2H4 - double bond

c2h4 carbon to carbon double bond

C3H2 - triple bond

c2h2 carbon to carbon triple bonds

(Notice the number of hydrogen atoms decrease as carbon to carbon bonds increase)


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