# Are there atoms with different numbers of protons, but the same number of electrons, with different orbital occupations?

Imagine there is an electron-stripped nucleus with $Z$ protons, in high vacuum and isolated. Then you add electrons, one by one, and check which orbitals are filled. Will this depend on $Z$ at all, or does it only depend on the number of electrons that are added?

In other words, will an ion have different orbits filled than the corresponding neutral atom with the same number of electrons?

In other words: are there atoms with numbers of protons $Z_1$ and $Z_2$, both with $n\leq\operatorname{min}(Z_1, Z_2)$ electrons that have different orbital fillings?

Yes, the number of protons in the nucleus does affect how the electrons are filled into orbitals. There are a number of real-world examples that usually stem from transition metals since they do not only have a rather rich oxidation state chemistry but also two energy levels (3d and 4s) which are rather close by each other.

To give you a concrete example, take $\ce{Fe^2+}$ and $\ce{Cr^{\pm 0}}$. They both have the same number of electrons. However, chromium’s ground state electronic configuration is given as $\mathrm{3d^5 4s^1}$, while iron(II) typically has $\mathrm{3d^6 4s^0}$.

The general explanation is that the higher nuclear charge in the iron nucleus attracts the electrons better than the lower nuclear charge in chromium. This difference causes the orbitals to contract. The d-orbitals which are closer to the nucleus, are contracted more and thus experience more stabilisation than the s-orbitals of the next shell.

Since electrons prefer to maintain their lowest energy states, electrons orbiting a stripped nucleus will occupy the lowest available energy levels before filling higher levels (1s before 3p, for example). This is called the Aufbau Principle.

Your question about a striped nucleus is a little confusing. Obviously an electron filling a hydrogen ion will have different energy states compared to an electron rushing toward a helium ion.

Check out https://en.wikipedia.org/wiki/Aufbau_principle as a starting point to your research.

Hope this helps!

• My next few minutes will be absorbed by analyzing the link given by Mithoron. I have learned about the Aufbau Principle quite recently, and that is exactly what made me ask this question: is the Aufbau Principle influenced or independent of the number of protons in the nucleus? Will the sequence in which new electrons are "assigned" to the shells be independent of Z? – Gyro Gearloose Jan 1 '16 at 21:32
• @ Ruggiero, I don't see any wrong in your answer, and I thank you for trying to help. It was not me that downvoted it and I really can't see any reason for anyone to do so. – Gyro Gearloose Jan 1 '16 at 22:05