# Why does ionisation energy to remove electrons from the same subshell increases? [closed]

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?

• "Electrons in the same subshell possesses the same energy." That's not true between different atoms. – Zhe Mar 25 '19 at 13:54
• 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. – Alchimista Mar 25 '19 at 14:00
• By related, we mean the following: an atom without an electron has the same electronic structure as the atom with atomic number $Z - 1$. – Zhe Mar 25 '19 at 14:07