Scientific definition: An atom is the smallest particle of an element that has the properties of the element.
An atom consists of a positively charged nucleus which contains at least one positively charged proton and usually one or more neutrons. The rest of the atom consists of an electron cloud surrounding the nucleus, which contains the electrons in specific energy levels, sublevels, and orbitals. The nucleus of an atom is very dense and contains most of the mass of the electron and the electron cloud contains most of the volume.
If the nucleus of an atom is the size of a golf ball, the diameter of the atom would be two kilometers.