13 • Electron Configurations
Electron Energy Levels
(1 of 4)
The electrons in an atom exist in various energy levels.
When an electron moves from a lower energy level to a
higher energy level, energy is absorbed by the atom.
When an electron moves from a higher to a lower energy
level, energy is released (often as light).
Neils Bohr was able to determine the energy levels of
hydrogen by the visible light energy that is released when
the electron drops from 3 → 2 (red light), 4 → 2 (blue-
green), 5 → 2 (blue-violet) and 6 → 2 (violet).
Transitions to level n = 1 are too high energy to see (UV).
Transitions to level n = 3 are too low energy to see (IR).
13 • Electron Configurations
Showing Electron Arrangments:
Orbital Diagrams and Electron Configurations
(2 of 4)
1s
2s
2p
3s
3p
4s
4p
3d
A shorthand notation is the electron configuration:
1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6, etc.
13 • Electron Configurations
Filling Orbitals
(3 of 4)
Three rules define how the orbitals fill:
The Pauli Exclusion Principle
Each orbital can be occupied by no more than two
electrons.
The Aufbau Principle
The electrons occupy the lowest energy orbitals
available. The “Ground State” for an atom is when
every electron is in its lowest energy orbital.
Hund’s Rule
When more than one orbital exists of the same energy
(p, d, and f orbitals), place one electron in each orbital
before doubling them up.
13 • Electron Configurations
Valence Electrons
(4 of 4)
The valence electrons are the outermost electrons… those
farthest from the nucleus. They have the largest principal
quantum number, n.
These electrons occupy the s and p orbitals in the highest
energy level. These orbitals are called the valence orbitals.
The columns of the periodic table are labeled, I, II, III, IV,
V, VI, VII and VIII (ignoring the transition and rare earth
elements). This label tell you the number of valence
electrons of every element in that column (except He.)
The valence electrons are important in how atoms bond.
Note that filled energy levels match up with the noble