The Atom is the smallest unit of 'ordinary matter' that has the properties of a chemical element. Every solid, liquid, gas and plasma is made up of neutral or ionized atoms. Atoms are very small with typical sizes around 1-ten billionth of a meter. However, counter to most historical portrayals, atoms are not little balls of particles. Atoms do not have well defined boundaries. There is a 'spherical wave' of electrons that surrounds the nucleus. Classical physics is most accurate for observables that occur on a scale larger than atoms. To better model the behavior of atoms, science has to use the principles of quantum mechanics. Every atom is composed of a nucleus and one or more electrons that are 'bound' to the nucleus. The nucleus of the atom is comprised of one or more protons and typically a similar number of neutrons (other than Hydrogen - which is the only element with no neutrons). The protons and neutrons are called nucleons. Over 99.94% of the atoms mass is in the nucleus. This book focuses on the atom as 'the building block of matter'.