HOW COME? Air is one confounding compound

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Besides oxygen, what gases make up Earth's air, and how small are the gas molecules? asks a reader.If you could grab a single, random air molecule, odds are you'd find a squirming nitrogen molecule in your palm. OK, maybe not squirming. But even as you read this, about 78 percent of the air surrounding your head -- and the rest of you -- is made of nitrogen.

Leaving aside air's varying water content, good old oxygen makes up about 21 percent more, or about one of every five air molecules. Result: By volume, nitrogen and oxygen make up 99 percent of dry air.

Most of the remaining 1 percent is a gas called argon. Next comes carbon dioxide, at about 4/100ths of 1 percent. Even fewer and farther between are molecules of neon (as in old-style sign lighting), helium (think floaty birthday balloons), methane (swamp gas) krypton, hydrogen, nitrous oxide (so-called laughing gas), xenon, ozone and more, including iodine and ammonia.

Gas molecules aren't lazily floating, like dandelion puffballs. Instead, they zip around us, at speeds from 700 to more than 3,000 mph. While air molecules come in different varieties, they have one thing in common: All are impossibly tiny.

A nitrogen molecule, for example, is made up of just two nitrogen atoms. Likewise, an oxygen molecule contains two oxygen atoms. Each individual air molecule measures from a few hundred-millionths down to a billionth of an inch across.

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Look at the markings on a ruler, each inch divided into 16 parts. Now imagine dividing an inch into a million, or a billion (a thousand million) eensy parts. No wonder we can't see air molecules stream in and out through our nose.

But while we can't see them with our naked eyes, we can visualize how small the molecules are, in our mind's eye. Scientists Gerald Feinberg and Robert Shapiro, in their book "Life Beyond Earth," suggested comparing an air molecule to a salt crystal.

Spill a little salt on a table, and separate out one tiny grain. Now, like Alice in Wonderland, imagine yourself shrinking down, down, down. As you shrink, the salt crystal grows in front of your eyes, first to the size of a baby's block and then to the size of a house.

Keep shrinking until the salt crystal towers above you, stretching into the air like a transparent skyscraper. As you get smaller and smaller, watch the top disappear from view.

Shrinking accomplished, glance around. A gumball-sized object suddenly zooms near your head. Reach out and grab it. Rattling around in the palm of your hand: a stray air molecule. While above you looms the salt crystal -- a hundred Empire State Buildings high.

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