Electron Configurations
Quantum Mechanical Model of the Atom
• The Quantum Mechanical Model is the current
description of electrons in atoms.
- does not describe the electron’s path around the nucleus
• The Quantum Mechanical Model is based on several
ideas including:
–Schrodinger wave equation (1926) treats electrons as waves.
–Heisenberg uncertainty principle (1927) states that it is
impossible to know both the velocity and position of a particle
at the same time.
Where do electrons “live”?
A. Principal Energy Levels
1. Principal energy levels n =1 to 7.
(Row # on the periodic table)
•
The electron’s principal energy
level is based on its location around
the nucleus.
•
Electrons closer to the nucleus are
at a lower energy level and have
lower energy than those farther
away from the nucleus
Energy levels are like rungs of a
ladder. You cannot be in between a
rung
Energy levels in an atom’s electron
are unequally spaced. The higher
energy levels are closer together.
B. Atomic Orbitals
• An atomic orbital is a region of space in which
there is a high probability of finding an electron
• assigned letters s, p, d or f (smart people do
fine)
• Energy sublevels correspond to a shape where
the electron is likely to be found.
s orbital (1) - spherical
p orbitals (3) – dumb-bell shaped
d orbitals (5)
f-orbitals (7)
Energy Levels, Sublevels, and Orbitals
1. Principal energy levels – n, assigned values 1-7
(Like floors in a hotel)
2. Energy sublevels- s, p, d, f (Type of suite in a hotel)




s sublevel – 1 orbital
p sublevel – 3 orbitals
d sublevel – 5 orbitals
f sublevel – 7 orbitals
(Orbitals are like the
number of rooms in a
suite)
3. Orbitals – Two electrons per orbital (Two people
per room)
Electron Configurations
• Electron configuration – the arrangement of
electrons in an atom.
• Example Sodium (Na) – 1s22s22p63s1
• Three rules determine electron configurations
– the Aufbau Principle,
– the Pauli Exclusion Principle
– Hund’s rule
The Aufbau Principle
• Each electron occupies the lowest energy orbital
available
• Like filling the hotel from the bottom up
Aufbau Diagram
Which has a lower energy level 4s or 3d orbitals?
Filling the Orbitals in the correct order
Orbitals in the Periodic Table
Pauli Exclusion Principle
• A maximum of two electrons may occupy a
single orbital
• Like only two people sharing one hotel
room
Hund’s Rule
• If two or more orbitals of equal energy are
available, electrons will occupy them singly
with the same spin, before filling them in pairs
with opposite spins
• A spin is denoted with an up or down 
arrow to fill orbitals
• This is like trying to find your own room in the
same suite before having to share a room with
someone else
Writing Electron Configurations
Aufbau diagram for sodium (Na) which has 11 electrons
Na electron configuration1s22s22p63s1
Exceptions to Electron Configurations
• Copper and chromium are exceptions to the
Aufbau principle.
Element
Copper
Should be
1s22s22p63s23p63d44s2
Chromium 1s22s22p63s23p63d94s2
Actually is
1s22s22p63s23p63d54s1
1s22s22p63s23p63d104s1
• Some configurations violate the Aufbau
Principle because half-filled sublevels are not as
stable as filled sublevels, but they are more
stable than other configurations
Valence electrons
• Valence electrons are electrons in the outermost orbital
• For A group elements the group number corresponds to
number of valence electrons.
• Electron-dot structures – Element’s symbol surrounded
by dots representing the valence electrons
8A