Chemistry 11
Quantum Mechanical Model
Electron Configurations
♦ It is the nature of things to seek the lowest possible energy.
♦ High energy systems are unstable and tend to lose energy in order to become more stable.
♦ In terms of the atom, electrons and the nucleus interact to form the most stable arrangement
possible.
Electron Configuration: The ways in which electrons are arranged around the nuclei of atoms.
Three rules govern the filling of atomic orbitals by electrons within the principle energy levels.
1. Aufbau Principle
2. Pauli Exclusion Principle
3. Hund’s Rule
1. Aufbau Principle
♦ Electrons enter orbitals of lowest energy first
♦ The various orbitals within a sublevel of principle energy level are always equal.
♦ Within a principle energy level, the “s” orbital is always the lowest energy level.
♦ The range of energy levels within a principle energy level can overlap the energy levels of
an adjacent principle energy level. See diagram below.
♦ 4s orbital is lower in energy than 3d.
♦ 4f orbital is lower in energy than 5d.
1
As you can see from the diagram above, the filling order is as follows:
1s 2s 2p 3s 3p 4s 3d 4p 5s 4d 5p 6s
2. Pauli Exclusion Principle
♦ An atomic orbital contains a maximum of 2 electrons
♦ In order to occupy the same orbital, these electrons must have opposite spins (clockwise
and counter clockwise)
♦ Vertical arrows are used to show directions of spins
♦ Paired electrons
3. Hund’s Rule
♦ When electrons occupy orbitals of equal energy, one electron enters each orbital until all
orbitals contain one electron with parallel spins.
Px
Py
Pz
♦ Second electrons then add themselves to the orbital to “pair” their spins.
Practice Problem
Arrange the following sublevels in order of decreasing energy:
2p 4s 3d
3p
Distribution of electrons in the 2p sublevel
Number of Electrons
in the 2p sublevel
2 Px
2 Py
2 Pz
1
2
3
4
5
6
2
Chemistry 11
Unit 1: Atomic Theory
Drawing Energy Level Diagrams
6d
7s
6p
6s
5d
Increasing Energy
5p
5s
4d
4p
4s
3d
3p
3s
2p
2s
1s
Practice Problems
Using the diagram above as a guide, draw energy level diagrams for the following:
a. iron
26Fe
b. calcium
d. nickel
k. lithium 3Li
20Ca
l. chlorine
28Ni
m. sulphur
e. carbon 6C
h. arsenic
i. neon
16S
n. chromium
f. phosphorous
g. strontium
17Cl
15P
o. gallium
24Cr
31Ga
l
38Sr
33As
10Ne
j. potassium
19K
3