What is an Atom?     The atom is the fundamental building block of all matter. Stars, planets, objects, as well as ourselves, are all made out of atoms. All atoms are composed of three primary particles known as protons, neutrons, and electrons. The protons and neutrons are densely packed in what is known as a nucleus while the electrons float around the nucleus in what is known as an electron cloud. The number of protons within the nucleus determines the element and properties of the atoms while the number of neutrons can vary. Various neutron counts are different isotopes and the combination of neutrons and protons determines the atomic mass number. Typically an atom will have an equal amount of protons and electrons, but if the number is unequal, it means that the atom is either positively or negatively charged.