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why does ionisation energy to remove electrons from the same subshell increases?

Electrons in the same subshell possesses the same energy. So why does ionisation energy to remove electrons from the same subshell increases?

For example, $\mathrm{2s^2}$ has two electrons. Since both of them are in the same subshell shouldn't ionisation energy to remove each of them stay the same?

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    $\begingroup$ "Electrons in the same subshell possesses the same energy." That's not true between different atoms. $\endgroup$ – Zhe Mar 25 '19 at 13:54
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    $\begingroup$ Beside the comment above, which is related, you already see the same with a classical distribution of charges. .The remaining electron is more bound after removal of its companion. $\endgroup$ – Alchimista Mar 25 '19 at 14:00
  • $\begingroup$ By related, we mean the following: an atom without an electron has the same electronic structure as the atom with atomic number $Z - 1$. $\endgroup$ – Zhe Mar 25 '19 at 14:07
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The ionisation energies would be same if there was no shielding effect but it exists. The electrons due to mutual repulsion prevent the outer electrons from the nuclear pull. Thus the force with which the nucleus attracts the electrons decreases as we move away from the nucleus both because of the increasing distance and also due to the increasing shielding effect of the inner electrons . Now in the same shell when one electron is removed the shielding effect decreases . The shielding effect due to the electrons of the same shell is the greatest. Thus the successive ionisation energies of any atom increases due to decreasing shielding effect.

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