1.2 Size and Weight

Each element is a substance, and an atom is the smallest possible piece of that substance. (True, atoms can be split, but splitting an atom changes it into things that aren't that element.) Atoms are very very tiny; their sizes are measured in angstroms (abbreviated Å), a unit so small that it takes 10 thousand million angstroms to equal one meter. A single atom of carbon is only 3.4Å across, meaning almost 75 million carbon atoms could fit side by side across just one inch of space.

Each element has an atomic weight, which represents how many grams a mole of that element weighs, and a mole is a little more than 600 thousand million million million atoms. (This is actually Avogadro's number, equal to about 6.022 x 10²³.) Hydrogen is the lightest element, with an atomic weight of just over 1, while the heaviest atoms weigh a few hundred times that.

Substances made of more than one element are called compounds, and a molecule is the smallest possible piece of a compound. Molecules are made up of atoms, and the molecular weight of a molecule is the sum of its atom's weights. A mole of water, all 600 thousand million million million molecules, weighs just over 18 grams or a little less than ²/₃ of an ounce.

Hydrogen is the lightest element, and its atoms can sometimes be treated almost as an afterthought. Atoms that are not hydrogen are called heavy atoms. Most organic molecules are made of a skeleton of heavy atoms with hydrogen atoms bonded to them.

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