In this topic you will learn about what substances are made of.

Elements are the basic building blocks of all matter. An element is a pure substance that cannot be broken down into anything simpler. Each element is given a special symbol of one or two letters. The first letter is always capital. The second letter, if there is one, is never capital. Sometimes the letters match the English name. In other cases the symbol comes from an ancient name.

Any substance that is formed by the chemical combination of two or more elements is called a compound. All compounds are single substances that can only be broken apart into simpler substances by chemical reactions. Compounds have different properties than the elements that make them up.

Matter is made of tiny particles called atoms. An atom is the smallest unit of an element that still has the properties of the element.

Atoms are mostly empty space. Atoms contain a dense center part called the nucleus. The nucleus contains particles called protons and neutrons. A proton has a positive charge. A neutron is uncharged particle. Moving around the nucleus is a particle with a negative charge called an electron.

All atoms have equal numbers of electrons and protons, so they have no overall electric charge. The number of protons in an atom determines what element it is.

Elements have many different properties, such as how they react chemically, melting temperatures, boiling temperatures, whether they are metals and nonmetals, etc. There are patterns in these properties that help to group elements into the periodic table.

Some elements are made up of single atoms that do not attach to any partners. Other elements have atoms that attach to one or more additional atoms. Particles that contain more than one atom joined together that act like a single particle are called molecules.

Scientists have learned how to prepare compounds that are very useful in our lives. Most of the things in the world around you, including plants and animals, are compounds.