Atoms—the particles that make up all matter—vary in their subatomic composition. The number and arrangement of an atom's particles (neutrons, protons, and electrons) determines the properties of the atom. For example, the number of protons in an atom's nucleus determines what type of element it is.

There are more than 100 known elements that are the basic building blocks of matter. Pure elements are substances that are made of only one type of atom. However, atoms can join together to form molecules. If two or more atoms of different elements join together, the resulting molecule is called a compound. For example, water is a common example of a compound—water molecules are each composed of two hydrogen atoms and one oxygen atom.

The behavior and interactions of atoms and molecules within a substance determine its physical state. There are three main physical states of matter—solid, liquid, and gas. (A fourth state of matter, called plasma, is a special case where electrons are stripped away from the atoms in a gas.) Solids are composed of atoms or molecules that are closely packed together, usually in an orderly way. The particles cannot move around very much but can jiggle in place. Since the particles are unable to slide past each other, solids hold their shape and volume. In a liquid, the molecules are somewhat close together, but remain free to move. Liquids flow and take on the shape of their container. In a gas, molecules are far apart from one another, with much empty space between them. The particles in gases are free to bounce and move around, which allows gases to change in volume to fill a container of any shape or size.

All substances can exist in any of the three states of matter; transitions between the states are called phase changes. For a given substance, the particles from which it is made maintain their properties in each state. For example, whether water is solid, liquid, or gas, it maintains the same molecular structure (H₂O). The difference is in the energy of the molecules and the way they interact with each other.

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