The energy required to remove electron from a gaseous atom or ion is what is called an ionization energy. As we remove electrons continually in a gaseous atom or ion, the ionization energy increases which are know as the first ionization energy, the second ionization energy, third ionization energy and so on.
Looking at the electronic configuration of Silicon, Si; Ne 3s2 3p2. We can can see that the first four ionization energies are from the removal of the 3p2 and 3s2 electrons and the fifth ionization energy, which is the highest ionization energy of 14800 kJ/mol is the the electron removed from the core shell.
Answer: The number of energy levels increases as you move down a group as the number of electrons increases. Each subsequent energy level is further from the nucleus than the last. Therefore, the atomic radius increases as the group and energy levels increase.
