The Ground State Electronic Structure for Atoms and Monatomic Ions R. J. Tykodl Southeastern Massachusetts University, North Dartmouth, MA 02747
In presenting the ground state electronic structure of atoms and monatomic ions at the general chemistry level, the ions is a vexing one: the problem of what to do about configurations do not follow the aufbau rule (which works reasonably well for neutral atoms and monatomic anions), and talk about screening effects, which orbitals are more penetrating, etc., strikes the student as a lot of arcane twaddle. I have found i t healthier to state at the very beginning of the discussion of ground state structures that there are two rules to follow: one for neutral atoms and monatomic anions--the aufbau (build up) rule; and one for monatomic cations-the abbau (tear down) rule. Both rules only establish general trends and are subject to exception (the aufbau rule is correct only about 80%of the time). The Aufbau (Bulld Up) Rule for Neutral Atoms and Monatomic Anlons
Set up a sequence o f atomic orbitals in order of increasing n I value (order of increasing energy); for orbitals of the 1 value, use the order of increasing n. Take the same n necessarvnumber of orbital electrons and assign them t o the orbitals in order of increasing energy: fill u p a lower energy level before moving on to a higher one, each orbital holding a maximum of 2 (spin-paired) electrons (Pauli principle); for an energy level that has a cluster (21 1)of orbitals, put the electrons in the orbitals so as to avoid pairing as long as possible (Hund's rule). The resulting configuration will be right about 80% of the time. There are several very nice diagrammatic ways of showing the sequence of energy levels according to the aufbau rule.'-3
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The Abbau (Tear Down) Rule for Monatomic Catlons
(2) After the orbitals of greatest n value have been emptied, remove electrons from partially filled d levels, then from partially filled f levels. (3) If more electrons must be removed, return to rule 1)and continue accordingly. Examples of the Tear Down Rule In Actlon
Let the notation K, L, M, etc., indicate that all the orhitals with n = 1,n = 2, n = 3, etc., are fully occupied. The ground state configuration for Cu is KL3~~3p~3d~~4s'. Rule 1) gives K L 3 ~ ~ 3 p ~for 3 dCuZ+. '~ The ground state configuratian for Ti is KL3sZ3p63dZ4se. Rule 11 gives KL3s23p63dZfor Ti2+,and rules 1and 2 give KL3s23p6forTi4+. T h e ground state eonfiguraGon f o r U is KLMN5s25p65d1%f36s26p66d'7sZ.Rules 1 and 2 give KLMN5s25p65d'05P6s26p6 for U3+and KLMN5s25p65dl05F6s26p6 for U" (there is some evidence indicating a 6d2rather than a 50 structure for U4+-the tern down rule is not infallible; indeed Ce2+is known to have a 40 structure rather than the expected 4P5d1structure). The tear down rule does not always give exactly right ground state electronic configurations for cations; i t does, however, give the right configuration most of the time and is, therefore, worth presenting. Present Two, Co-Equal Rules
To avoid a lot of unproductive "hassle" in presenting ground state electronic structures of atoms and monatomic ions to general chemistry students, give those students two, co-equal rules to follow: the aufbau (build up) rule for neutral atoms and monatomic anions, and the abbau (tear down) rule for monatomic cations.
Stnrt with the rorrect ground statr electronir configuration for thr neutral atom, thrn rrmow rhe required number of electrons according to the following scheme: (1) Remove electrons from the orbital of greatest n value; if there are several orbitals of greatest n value, start with the one with
the greatest 1 yalue.
'Carpenter, A. K. J. Chem. Educ. 1983, 60,562. Howland. A. K . J. Chem. Educ. 19813,63.607. Strong, J. A. J. Chem. Educ. 1986, 63.834.
Volume 64
Number 11
November 1987
943
