3.5: ATOMIC STRUCTURE AND THE PERIODIC TABLE
DISTRIBUTION OF ELECTRONS IN ORBITALS
Rules: 1. The Aufbau Principle
Electrons occupy the lowest energy orbital of the lowest energy level
first before going to a higher one.
2. Hund’s Rule
No pairing takes place until each orbital contains one electron. No orbital can contain more
than two electrons. Also, the electrons in those half-filled orbitals must have the same spin.
CREATING ENERGY-LEVEL DIAGRAMS AND ELECTRON
CONFIGURATION FOR ATOMS
Class practice
Draw energy level diagram & electron configuration for Potassium (K), Sulfur (S), Copper (Cu),
Chromium (Cr) and Cerium (Ce) into your notebooks.
What pattern do you observe?
Creating Energy-level diagrams for anions (negative charge ions)
Done using the same method as for atoms.
Add the extra electrons corresponding to the ionic charge to that of the atom.
Draw energy level diagram for the following anions: e.g. Cl-, P-3, S-2.
Creating Energy level diagram for cations (positive charged ions)
This is different from anions.
Draw the energy level diagram for the neutral atom first, and then remove the number of
electrons corresponding to the charge from the orbitals with the highest principal quantum
number, n.
Note: This may not be the highest-energy electron
ELECTRON CONFIGURATION
Done in order of increasing energy level.
1s 2s 2p 3s 3p 4s 3d 4p 5s 4d 5p……
↑
(4s has less energy than 3d.)
Shorthand method (This uses noble gases for inner configuration)
Lithium Li [He] 2s1
Sodium Na [Ne] 3s1
1
Potassium  k [Ar] 4s1
Practice:
Draw electron configuration for the following elements
Sodium and calcium (s-block), oxygen and aluminum (p- block), titanium and manganese
(d-block),promethium and thulium (f-block)
Electron Configuration in Transition metals (varied oxidation number or
charge)
Electron configuration for Zinc atom (Zn) is [Ar] 4s2 3d10. This shows 12 outer electrons.
The formation of ionic charges can be explained with the following:
Zinc ion:
In forming the ion of Zn2+, the two electrons are removed from the 4s rather than the 3d10 which
is more stable because of the electron clouds.
Then, a relatively stable state, like atom with filled sub-shell is: Zn2+= [Ar] 3d10
ANOMALOUS ELECTRON CONFIGURATIONS
The rules for electron Configuration as described above may not work
for all of the elements.
Following the rules, we would expect the following configurations:
Cr: [Ar] 4s2 3d4
Cu: [Ar] 4s2 3d9
However, the actual electron configurations, determined experimentally, are:
Cr: [Ar] 4s1 3d5
Cu: [Ar] 4s1 3d10
Corresponding diagrams would be:
Cr: [Ar]
Predicted
(↑↓) (↑) (↓) (↑) (↓) ( )
Cu: [Ar} (↑↓) (↑↓) (↑↓) (↑↓) (↑↓) (↑)
4s
3d
Cr: [Ar]
Actual
(↑) (↑) (↑) (↑) (↑) (↑)
Cu: [Ar]
(↑)
4s
(↑↓) (↑↓) (↑↓) (↑↓) (↑↓)
3d
Practice:
Give the predicted and actual electron configuration for silver and gold.
2