MadSci Network: Chemistry
Query:

Re: can the electron configuration of ruthenium (Ru) be written as 4d6 5s2 instead of 4d7 5s1?

Date: Mon Jul 4 11:01:54 2005
Posted By: Dan Berger, Faculty Chemistry/Science, Bluffton University
Area of science: Chemistry
ID: 1119804091.Ch
Message:

Short answer: NO. Not unless you want to write the wrong answer.

Long answer: The ground spectroscopic state of monatomic, gaseous ruthenium is 5F5; if you want to know what that means, look in a P-chem textbook! We get the electronic configurations of elements by fitting experimental spectroscopic data to a hydrogen-like atomic orbital model.

When we do this, we arrive at the accepted electron configuration of ruthenium, which is [Kr]4d75s1. [Kr] means that the "core orbitals" have the same electronic configuration as krypton does; the extra eight electrons are distributed, seven in the 4d orbitals and 1 in the 5s orbital.

The hydrogen-like orbital model takes both orbital energies and overall electron-interaction energies into account; and in the case of ruthenium, having the 5s orbital singly-occupied, with seven electrons in the 4d subshell, has a lower overall energy than having the 5s doubly-occupied with six electrons in the 4d. This is counterintuitive since we normally think of the 5s as being lower in energy than the 4d -- it's occupied first! -- but the energy difference is rather small, and can be overridden by "electron-interaction factors." In many ways this result is empirical; I know of no good way to explain the result of the modeling calculation short of doing the calculation itself.

If it's any comfort to you, the electronic configuration is not a purely experimental result; only the 5F5 spectroscopic state can be directly measured. The electronic configuration is an artifact of the atomic orbital model. Nevertheless, within the constraints of that model [Kr]4d75s1 is correct and [Kr]4d65s2 is incorrect. If we ignore the atomic orbital model, neither one of them is correct! So there's no philosophical wiggle room: however many qualifiers we add, one of 'em is wrong and the other one is right and that's that.

My source for the electronic-configuration and spectroscopic information in this answer is WebElements, the standard WWW periodic table.

Dan Berger
Bluffton University






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