Atoms, Molecules and States of Matter

Everything you can touch, drink, or breathe is made of matter. Zoom in far enough and every piece of matter is built from incredibly small particles called atoms. Atoms can join with other atoms to form molecules, and the way those particles move and stick together decides whether the substance behaves like a solid, a liquid, or a gas.

What Atoms Are

• An atom is the smallest unit of an element that still has the properties of that element.
• Atoms are far too small to see — millions of them fit across the width of a human hair.
• There are about 118 known elements, and each one is made of a different kind of atom (hydrogen, oxygen, carbon, iron, gold, and so on).
• Atoms cannot be created or destroyed by ordinary changes — they only get rearranged.

From Atoms to Molecules

When two or more atoms link together, the new particle is called a molecule. A molecule of water (H2O) is two hydrogen atoms bonded to one oxygen atom. A molecule of carbon dioxide (CO2) is one carbon atom bonded to two oxygen atoms. The atoms in a molecule are held together by chemical bonds, and the molecule has different properties from the atoms alone.

• Element — a substance made of only one kind of atom (oxygen gas, gold, copper).
• Compound — a substance made of two or more different kinds of atoms bonded together (water, salt, sugar).
• Molecule — the smallest particle of a compound (or of some elements like O2) that still has the properties of that substance.

The Three Common States of Matter

The same substance can exist in different states depending on how much energy its particles have. In a SOLID, particles are packed tightly and only vibrate in place — the substance keeps its own shape and volume. In a LIQUID, particles are still close but can slide past each other — the substance keeps its volume but takes the shape of its container. In a GAS, particles are far apart and zoom around freely — the substance has no fixed shape and no fixed volume; it fills whatever container it is in.

• Solid — fixed shape, fixed volume, particles vibrating in place. Examples: ice, wood, iron.
• Liquid — no fixed shape, fixed volume, particles sliding past each other. Examples: water, milk, oil.
• Gas — no fixed shape, no fixed volume, particles flying apart. Examples: water vapor, air, oxygen.

Changing State

Adding heat gives particles more energy and pushes a substance from solid to liquid (melting), and from liquid to gas (boiling or evaporating). Removing heat takes energy away and reverses the change: gas to liquid (condensing), and liquid to solid (freezing). The atoms and molecules themselves do not change — only how they are arranged and how fast they move.