Worksheet 24: Orbital Filling Rules

Chapter 5: Orbital filling rules The rules that you have been applying in order to determine the electronic configuration of an atom are summarized below. A. THE AUFBAU PRINCIPLE. Lowest energy orbitals are filled first. 1s2s2p3s3p4s3d4p5s4d5p6s 4f5d6p7s5f6d7p

B. THE PAULI EXCLUSION PRINCIPLE. Orbitals can only contain a maximum of two electrons and when two electrons enter the same orbital they must have opposite spins (+ ½ or – ½). (In the “electrons in boxes” diagram they must be drawn  NOT  OR ).

C. HUNDS RULE. When orbitals of identical energy are available, electrons enter these orbitals singly before any spin pairing takes place. In the p, d, and f sublevels (that contain more than one orbital) no orbital may contain a pair of electrons before all the orbitals of that sublevel contain one electron. See detailed explanation below.

D. EXCEPTIONS TO THE AUFBAU PRINCIPLE. There are some notable exceptions. For example Cr and Cu achieve extra stability by forming a half-filled or completely filled d sub-shell by using one of their 4s electrons.

Cr: 1s2 2s2 2p6 3s2 3p6 4s1 3d5

D:\Docs\2018-04-13\01ea2d149821da2ca0fe185c31f60fc1.doc Cu: 1s2 2s2 2p6 3s2 3p6 4s1 3d10

Detailed Explanation of Hund’s Rule:

D:\Docs\2018-04-13\01ea2d149821da2ca0fe185c31f60fc1.doc Consider each of the elements listed and the INCORRECT electronic configuration associated with each one. In each case identify which of the above rules or principles (A, B, C or D) is violated and insert the correct electronic configuration. Then add a possible set of quantum numbers for the outer most electron. An example is completed for you.

POSSIBLE SET OF INCORRECT CORRECT ELEMENT VIOLATION QUANTUM NUMBERS FOR CONFIGURATION CONFIGURATION OUTERMOST ELECTRON N 1s2 2s2 2px2 2py1 C 1s2 2s2 2px1 2py1 2pz1 2 1 -1 +½ Al 1s2 2s2 2p6 3p3 B 1s2 2s3 P 1s2 2s2 2p6 3p5 Cu [Ar] 4s2 3d9 Mg [Ne]  C 1s2 2s1 2px1 2py1 2pz1 C 1s2 2s2 2px2 Ag [Kr] 5s2 4d9 Mn [Ar] 4s1 3d6

2 2 2 2 2 0 Ni [Ar] 4s 3dxy 3dxz 3dyz 3dz2 3dx2-y2 Cl [Ne]     Sc [Ar] 3d3 B 1s2 2s1 2px1 2py1 Na 1s1 2s2 2p6 3s2 S [Ne] 3s2 3px2 3py2 V [Ar] 3d5 P [Ne] 3s2 3px2 3py1 Kr [Ar] 4s2 3d16 Cr [Ar] 4s2 3d4

D:\Docs\2018-04-13\01ea2d149821da2ca0fe185c31f60fc1.doc