The smallest component of matter which retains its chemical properties. An atom consists of a nucleus composed of at least one proton, some number of neutrons, and at least one electron.
The atomic number of an atom corresponds to the number of protons present in the nucleus of an atom. This determines its elemental identity. The number of neutrons determines the isotope of the atom.
Figure:
A conventional depiction of an atom. The nucleus consistest of
positively chared protons (blue) and neutral neutrons (black). The
nucleus is surrounded by negatively charged electrons (red) in
specific orbits. This atom corresponds to Boron (atomic number of
5). If there are also 5 neutrons, the atom is
isotope Boron-10 (symbol 10B).