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The DNA molecule, made up millions of atomsThe DNA molecule, made up millions of atoms Atoms make up all the objects and substances in our world. But they rarely exist as single atoms, alone or unattached. They are usually attached or joined to other atoms. For example, oxygen, a gaseous element that makes up one fifth of the air on Earth, does not float about as single atoms of oxygen, O (its chemical symbol). It is in the form of oxygen atoms joined together in pairs, written as O2. Two or more atoms linked or joined together make a molecule. O2 is a molecule of oxygen. Atoms of different elements often link together as molecules, too. These molecules are known as compounds.

Hydrogen and oxygen atoms link to form waterHydrogen and oxygen atoms link to form water


If atoms of one chemical element join or combine with atoms from other elements, in a process called a chemical reaction, a compound is formed. O2 is a molecule of oxygen but not a compound: the two atoms are of the same element. Two atoms of hydrogen and one of oxygen form a very familiar substance: water, written as H2O. This is both a molecule and a compound.

Some compounds, like minerals in rocks, may have, say, between 50 and 100 atoms from 15 or 20 different elements in each molecule. Other compounds, like certain plastics, have millions of atoms in each molecule, but these are usually from only a few elements—mainly carbon, hydrogen, oxygen and nitrogen.

There are as many molecules in a teaspoon of water as there are teaspoons of water in the Atlantic Ocean.

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