An atom consists of nucleus (protons and neutrons), and electrons moving in space around the nucleus. Matter is made up of atoms
Any substance which is made up of one kind of atom is known as an element. Because atoms cannot be created or destroyed in a chemical reaction, elements such as phosphorus (P4) or sulfur (S8) cannot be broken down into simpler substances by these reactions.
Water decomposes into a mixture of hydrogen and oxygen when an electric current is passed through the liquid. Hydrogen and oxygen, on the other hand, cannot be decomposed into simpler substances. They are therefore the elementary, or simplest, chemical substances - elements.
Elements can be categorized as, metals, non-metals and semi-metals.
Two or more different types of elements combine to form chemical compound, which can be classified as ionic compounds and covalent compounds.
Compounds can be broken down into a simpler type of matter (elements) by chemical means (but not by physical means). A compound has properties that are different than its component elements, and always contain the same ratio of component elements.
A mixture consists of two or more different elements or compounds which are physically intermingled. The components of a mixture can be separated by physical means. A mixture often retains many of the properties of its components.