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Everything consists of tiny atoms, which are made up of even smaller particles. Particle physics is the study of these smaller particles, which form the most basic building blocks of all matter in the universe. The study of particles enables scientists to learn more about the universe and the nature of all matter. Scientists generally agree that the universe started with a huge explosion in space, which they call the «big bang».    

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Matter is anything that occupies a space, including everything on the Earth and throughout the universe. Drops of water and specks of dust are kinds of matter, and so are plants, animals, and even planets. All matter has a mass, which is a measure of the particles (atoms and molecules) that is consisted of. Particles can group together in many different ways, varying the structure of matter. That is why so many different kinds of matter exist. All kinds can be classified as solid, liquid, or gas.

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Many substances can exist in more than one state – as a solid, liquid, or gas. Which state they are in depends on their temperature and pressure. At certain temperatures, under normal atmospheric pressure, some substances change state. Liquids, for example, and gases may condense into liquids. Elements change state, under normal pressure, at specific temperatures, known as melting (or freezing) and boiling (or condensation) points.

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All substances are made up of tiny particles called atoms; they are the building blocks of everything on Earth. Atoms are so small that a pinhead contains about 60 billion of them. The ancient Greeks first guessed that matter might be made up of small particles. Atoms contain even smaller particles called protons, neutrons, and electrons.     

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An element is a substance that contains only one kind of atom. Atoms cannot be broken down by chemical means, so it is impossible to break up an element into simpler substances by chemical changes. So far 109 elements have been discovered; 92 occur naturally, the other 17 are artificial elements that can be made only in the laboratory. There are two main kinds of elements – metals and nonmetals. Some elements chemically combine with other elements to form substances called compounds.

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Compound is a substance in which the atoms of two or more elements are joined together by chemical bonds. Because of these bonds, it is very difficult to split a compound into its individual elements. The type and number of bonds formed depend on the electrons (particles with a negative electrical charge) in each atom. The properties of a compound can be very different from those of the elements it contains. For example, the salt we use in our food is a harmless compound, yet it is made up of two dangerous elements – sodium, which reacts violently with air; and chlorine, which is poisonous in large quantities.

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A molecule is a group of atoms joined together by chemical bonds. The molecules of an element are identical and contain atoms of one type only. Oxygen gas, for example, has molecules made up of two oxygen atoms. The molecules of a compound are also identical, but in this case each molecule contains atoms of more than one type. In water, for example, each molecule is made up of two hydrogen atoms and one oxygen atom. All gases, most liquids, and many nonmetallic solids consist of molecules.