The atomic orbitals are more complicated than this description implies, but this offers a glimpse at their arrangement. All atoms follow the basic arrangement, yet atoms of different substances are different. Scientists currently know of approximately 118 different atoms. Each is unique chemically and physically. Elements are identified by the number of protons in the nucleus, which is given as the atomic number on the periodic table.
Neutral atoms contain an equal number of electrons around the nucleus as protons in the nucleus.
The electrons are arranged in energy levels as described in the quantum model. As one moves across the periodic table of elements, one finds a single proton and a single electron are added to the preceding atom. The number of neutrons is not always as straightforward as the number of protons. In a single atom, the number of neutrons is found by subtracting the atomic number from the atomic mass. Usually, atomic masses on the periodic table are not integral values because the reported values are weighted averages of all known isotopes or forms of the atom.