the long form of periodic table is the present-day periodic table and is based upon the modern periodic law which establishes the atomic number of the elements as the basis for periodicity.
The significance of atomic number in the arrangement of elements in the modern periodic table lies in the fact that as electronic configuration is based upon atomic number, therefore the arrangement of elements in accordance with increasing atomic number displays the periodicity (repetition of properties after regular intervals) in the electronic configuration of the elements leading to periodicity in their properties.
The atom consists of protons, neutrons and electrons. The protons and neutrons reside within the nucleus whereas electrons revolve around the nucleus in fixed orbits or shells. These shells are complex regions further consisting of subshells and orbitals.
Electrons are arranged in shells (energy levels) around the nucleus.
Energy levels are represented by the symbol n and are numbered according to how far they are from the nucleus.
The lowest energy level n=1 is closest to the nucleus, the energy level n=2 is further out and so on.
The different energy levels are designated by the alphabets K, L, M and so on.