# Why is the first ionisation energy of a sodium cation bigger than that of a neon atom?

I understand that the 1st ionization energy gets bigger for elements along a period from left to right and along a group from down to up.

But why is the 1st ionization energy of $\ce{Na+}$ bigger than $\ce{Ne}$?

I would have guessed the opposite.

• $\ce{Na+}$ has more protons than $\ce{Ne}$, so there would be a greater pull on the electrons towards the nucleus, meaning that you would have to supply more energy to remove the electron. Remember, $\ce{Na+}$ has a noble-gas configuration and is already very stable! – DHMO Jan 17 '17 at 15:20
• $\ce{Na+}$ is the same as $\ce{Ne}$, only with more protons. Guess what does that mean for electrons. – Ivan Neretin Jan 17 '17 at 15:21

A sodium cation $\ce{Na+}$ and a neutral neon atom $\ce{Ne}$ are isoelectronic species; meaning they have the same number of electrons and also the same electronic configurations.
It may now become obvious why the ionisation enthalpy of $\ce{Na+}$ is much larger than that of neon. Different ways to express this all boil down to the fact that it is harder to remove a negatively charged electron from a positively charged species than it is to remove one from a neutral species.
$\ce{Na+}$ and $\ce{Ne}$ are isoelectronic species and size of cationic species is smaller among isoelectronic species. Ionization energy increases with decrease in size.