Electron Configurations
• Where the electrons are in the energy levels
and orbitals.
• The configuration that requires the least
energy is the most stable - called groundstate electron configuration.
• 3 specific rules are used to find an atom’s
electron configuration:
– Aufbau principle (German for build up)
– Pauli exclusion principle
– Hund’s rule
Aufbau Principle
• Says each electron occupies the lowest
energy orbital available.
• An Aufbau diagram shows the orbitals in
order from lowest to highest energy required
• Hints:
– All orbitals within a sublevel have equal energy.
(All 3 p sublevels are equal energy at any level.)
– Energy sublevels within a principle energy level
have different energies (s<p<d<f).
– Generally energy increases with n but there is
some overlap between sublevels.
Pauli Exclusion Principle
• Each electron in an atom is spinning.
• There are two possible directions for an
electron to spin.
• An orbital can only contain two electrons if
they have opposite spins.
• The two spins are represented by an
arrow pointing up and an arrow pointing
down.
Hund’s Rule
• Because electrons repel each other, each
orbital within an energy level will have one
electron before any orbital has a second
electron in it.
• Can be thought of as the chemistry “bus”
rule – all of the double seats on a bus fill
up with single riders before anyone
doubles up.
Orbital Diagrams
• One way to represent an atom’s electron
configuration.
• Includes a box for each of the atom’s orbitals.
• Each box can contain an up arrow and a
down arrow.
• Each box is labeled with the principle
quantum number (energy level) and sublevel.
Electron Configuration Notation
• Shows each energy level and sublevel
followed by a superscript indicating the
number of electrons in it.
• Does not show the orbital distribution of
electrons like an orbital diagram does.
• For example, carbon in its ground state
would be represented:
2
2
2
1s 2s 2p
Valence Electrons
• The electrons in the outermost energy
level – includes all orbitals.
• These electrons determine an atom’s
chemical behavior and properties.
• These electrons form chemical bonds with
other atoms.
Noble Gases
• Noble gases always have a full s and a full
p orbital for their valence electrons
• So the electron configuration for a noble
gas can be represented by:
[Ne]
Instead of 1s22s22p6
Noble Gas Configuration
• Used to shorten any element’s electron
configuration
• You find the noble gas from the previous
period then write the configuration for the
outer energy level:
Na = 1s22s22p63s1 = [Ne]3s1
• The noble gas shows what’s in the
inner energy levels and the rest is the
valence electrons
Try It
1. Write the electron configuration for
bromine then rewrite it in noble gas
configuration:
2. Repeat #1 for rubidium:
Electron Dot Configuration
• Even quicker than noble gas
configurations.
• Shows only the elements symbol and
a dot for each valence electron it has
• Examples of Rb and Br: