Understanding Double Bonds

* Double bonds involve the sharing of two pairs of electrons between two atoms.

* Carbon can form four bonds. When a carbon is part of a double bond, it only has two bonds available for other atoms.

Hydrogen Attachment

* Adjacent to a double bond means the carbon atom is directly connected to a carbon atom that is part of the double bond.

* Hydrogen atoms typically form single bonds.

The Answer

Since a carbon with a double bond already has two bonds used, it can only accommodate one hydrogen atom.

Example:

Consider the molecule ethene (C2H4):

```

H

\

C=C

/

H

```

Each carbon in the double bond has one hydrogen atom attached to it.

In summary, each carbon adjacent to a double bond will have one hydrogen atom attached.