An atom is a basic unit of matter consisting of a dense, central nucleus surrounded by a cloud of negatively charged ions. The nucleus itself is a mixture of positively charged protons amd electrically neutral neutrons. Different groups of elements have respective atomic numbers. You can use the periodic table as a tool to draw atoms of elements. The periodic table is organized into periods, groups and families. This video is a tutorial that reviews the subatomic particles found in an atom. It is designed to focus on the needs of middle school students, but you may find it a good refresher as well. With knowledge of an element's atomic number and atomic mass (AMU), you will be able to determine the number of protons, neutrons and electrons.