Ask Dr. Knowledge

What's the difference between silicon and silicone?

January 18, 2010

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What’s the difference between silicon (used in electronics) and silicone (used in breast implants)? Also, what’s the deal with silicon-based life on other planets?

Silicon is a chemical element, in many ways similar to carbon and usually described as a metalloid, meaning it has properties similar to those of a metal without actually being one. In its pure form, silicon forms gray crystals with a metallic luster.

It’s the second-most-abundant element, after oxygen, making up nearly 26 percent of the earth’s crust. From its pure form, one can engineer all sorts of interesting devices by the carefully controlled addition of particular impurities.

You’ve probably seen silicon most often in a combination of two oxygen atoms to one silicon atom: silicon dioxide, also known as quartz! You can think of silicon atoms as having four “hooks’’ that can link to other atoms, and oxygen atoms as having two hooks, with all the links used up to join two oxygen atoms and a silicon one to make silicon dioxide. Another possibility is to form a chain of silicon-oxygen-silicon-oxygen-etc., which ties up the hooks of the oxygen atoms but leaves each silicon atom with two hooks free.

Silicones are what you get if you attach some carbon-containing molecules onto the free hooks of the silicons in the chain. Lots of choices of chain length and molecules to tack on means there are lots of silicones possible.

Silicones, unlike silicon itself, or quartz, are typically rubbery substances, which find all sorts of applications where rubber itself would be damaged or react with chemicals or living tissues. They are generally very good electrical insulators, can tolerate temperature extremes well, and are water-repellent.

Carbon has a number of similarities to silicon, perhaps most notably that the carbon atom also carries four “hooks,’’ which has led many science fiction authors to postulate that silicon-based life might exist on other planets, with silicon taking the place of carbon, the basis of life on earth. The existence of rubbery materials like silicones is sometimes used to bolster that point of view, but there’s a big problem when it comes to metabolisms based on oxygen: While carbon dioxide is an easily exhaled gas, it would be difficult to imagine organisms that had to exhale silicon dioxide - quartz!

Ask Dr. Knowledge is written by Northeastern University physicist John Swain. E-mail questions to or write to Dr. Knowledge, c/o The Boston Globe, PO Box 55819, Boston, MA 02205-5819.