The smallest unit of matter is called the atom. Atoms are made of smaller units called protons, electrons, and neutrons. The electrons orbit the nucleus of the atom. Each orbital shell of an atom can hold a certain number of electrons. Different atoms share electrons and join to form molecules.
All matter is made of elements or compounds, and is found in the form of solids, liquids or gases.
Substances with only one type of atom are called elements. Elements are arranged on the Periodic Table of the Elements according to their chemical properties. The atom is the smallest unit of an element. Substances with more that one type of atom are called compounds. The molecule is the smallest unit of a compound.