Molecule

A molecule is a group of two or more atoms that are joined together. The atoms are held by connections called chemical bonds. Molecules are the building blocks of almost everything around you. Air, water, sugar, plastic, and your own body are all made of molecules.

Atoms are the tiny pieces that make up all matter. When atoms stick together, they form a molecule. Some molecules are made of just one kind of atom. The oxygen you breathe is two oxygen atoms bonded together. Other molecules are made of different kinds of atoms. Water is one oxygen atom bonded to two hydrogen atoms. Chemists write water as H2O, which is a short way of saying "two hydrogens and one oxygen."

Molecules come in very different sizes. The smallest have just two atoms. The largest can have millions. A single strand of DNA, the molecule that carries the instructions for building living things, can contain billions of atoms linked in a long twisted ladder. Proteins in your body are also huge molecules, folded into complicated shapes that let them do their jobs.

Molecules are far too small to see, even with a regular microscope. If you lined up 10 million water molecules in a row, the row would be about as wide as a human hair. Scientists need special tools, like electron microscopes, to study them.

The shape of a molecule matters a lot. Two molecules with the same atoms in different arrangements can act very differently. For example, the sugar in fruit and the sugar in milk have almost the same atoms but different shapes, so your body treats them in different ways. Smells work this way too. Your nose recognizes certain molecule shapes, and your brain reads those shapes as flavors like mint, lemon, or smoke.

Molecules are always moving. In a solid, like ice, they jiggle in place. In a liquid, like water, they slide past each other. In a gas, like steam, they fly around quickly and spread out. Heat speeds them up, and cold slows them down. That is why water freezes when it gets cold and turns to steam when it boils.

When molecules meet, they sometimes swap atoms and form new molecules. This is called a chemical reaction. Every time wood burns, food digests, or a plant grows, molecules are being broken apart and built into new ones.