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'.This book is designed to be a general overview of the topic and provide you with the structured knowledge to familiarize yourself with the topic at the most affordable price possible. The level of discussion is that of Wikipedia. The accuracy and knowledge is of an international viewpoint as the edited articles represent the inputs of many knowledgeable individuals and some of the most currently available general knowledge on the topic, based on the date of publication.