Sub-module 5A, chem lesson, page 2

The stick rule says each carbon must have 4 sticks, but it does not forbid the carbons to attach to each other:

Hydrocarbons with more than one bond between at least one pair of carbons are called unsaturated. The proper name for hydrocarbons with one or more double bond in the molecule are alkenes, the trade name for them is olefins.

From here things start to get complicated.

Start by substituting a chlorine atom for one of the hydrogen atoms. For chlorine (and fluorine and bromine) there is likewise only one stick. Oxygen take two sticks.

Here is a common contaminant:

This is known as trichloroethylene or TCE. It is a solvent, toxic in relevant concentrations, and persistent in the environment.

Unsaturated compounds can also be cyclic:

How many hydrogen atoms does benzene have

Because the benzene molecule is so common, there is a common abbreviation for it:

Benzene is a common component of fuels, such as gasoline. As chronic toxicant, it causes leukemia and possibly other cancers. It is also acutely toxic, as are many of the lighter hydrocarbons in high doses.

Benzene and similar unsaturated rings are called aromatics. They can be linked together edge to edge into polycyclic aromatics. Many polycyclics are quite toxic, some are not.

The simplest is naphthalene

Here is the chemical that has killed more Americans than all our wars combined:

It is called benzo[a]pyrene. Millions of Americans are exposed to the chemical each day. Why hasn't the government done any thing about it. .

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