Searching for water on Mars and looking for other distant planets that may be Earth-like recalls that it is not only water, but also carbon that is necessary for life.

Science fiction abounds with stories of silicon-based life, so what is it that makes us think that extraterrestrial life needs to be based on carbon and water?

Carbon and silicon atoms are similar. The two elements both reside in period IV in the periodic table. In general, two elements that lie in the same column have the same number of valence electrons.

Quantum mechanics has redefined the old description of valance electrons. Physicists and chemists now realize that it is unpaired electrons that characterize chemical activity.

Carbon and silicon atoms both have four unpaired electrons, which means that they can each form four chemical bonds.

Carbon can easily form multiple bonds. For example, in carbon dioxide two unpaired carbon electrons partner with two unpaired oxygen electrons. Carbon dioxide thus has two double bonds, one with each oxygen atom for a total of four.

As much as carbon likes oxygen, it likes hydrogen even better. Hydrogen atoms have a single unpaired electron that easily mates with an unpaired electron on a carbon atom.

It is not surprising that the most basic molecules of life, formed during photosynthesis, result from breaking up carbon dioxide and water molecules, the latter composed solely of hydrogen and oxygen atoms.

Carbon has another fascinating and fundamental feature. Carbon atoms easily form chemical bonds with other carbon atoms. In fact, carbon is the only chemical element that can form long chain polymers that leave unpaired electrons free to combine with other atoms.

Most of the atoms in millions of known carbon compounds are hydrogen and oxygen. Atoms of nitrogen, sulfur and phosphorus find their way into increasingly complex carbon chains and rings ultimately to form the most complex molecules that we know as DNA.

Considering the similarities between carbon and silicon, we wonder why there is no silicon-based life.

The answer is surprising and has mostly to do with the size of the atoms. Silicon atoms are more than 50% larger than carbon atoms. Their size allows silicon atoms to fit perfectly between four large oxygen atoms.

Although this arrangement creates a molecular complex with an unbalanced charge, the packing is so energetically favorable that it overcomes the tendency for molecules or chemically bonded groups of atoms to have a balanced electrical charge.

This molecular complex is called a silicate ion.

Instead of bonding with other silicon atoms to form long polymers, or with hydrogen atoms like its sister carbon atoms, silicate ions form a sticky, viscous mass that crystallizes at several hundred degrees Fahrenheit into the mineral that we know as quartz.

In nature silicate ions are found mixed with other chemical elements. We call it magma while underground and lava if it erupts to the surface to form the most primitive type of rock.

Rather than forming the molecules of life, silicate ions instead form the building blocks of solid rock, or when manipulated by technology become the various and beautiful materials we know as glass.

Richard Brill is a retired professor of science at Honolulu Community College. His column runs on the first and third Fridays of the month. Email questions and comments to brill@hawaii.edu.