Atomic Orbital Filling
Order
and Electron
Configurations
There are 3 main rules for filling
atomic orbitals
1. Aufbau principle - Electrons get filled in
order from lowest to highest energy levels.
 Each atom has seven possible energy levels
(cleverly labeled 1 through 7). One is the
lowest, seven is the highest. They go by
period (rows) in the periodic table.
 The possible number of electrons for an
energy level is found by using the equation
2n2 (where n = an energy level, 1-7).

Each energy level is further divided into
sublevels. There are four basic types of
sublevels which are arranged by groups.

s -p -d -f --



The “s” sublevel has one orbital.
The “p” sublevel has three orbitals.
The “d” has 5 orbitals.
The “f” sublevel has 7 orbitals.
Increasing energy
7s
6s
5s
7p
6p
5p
4p
4s
3p
3s
6d
5d
4d
5f
4f
3d
Orbital diagram
2s
Aufbau is German for “building up”
We must follow this orbital energy
diagram!
1s
Notice that the 4s orbital will fill
before the 3d because it is lower in
energy!
2p
2. Pauli exclusion principle
- Only two electrons can fit in each orbital,
one with an “up” spin, the other with a
“down” spin
 -This means that each “s” has 2 electrons,
“p” has 6 (3 orbitals with 2 e- each), “d”
has 10, and “f” has 14.

•To show the different
direction of spin, a pair in the
same orbital is written as:
3. Hund’s rule

When filling a sublevel with multiple
orbitals (p, d, or f), each orbital gets one
electron (the “up” spin) before any get
two.
Orbital Diagram form Sodium
Energy
Sodium has 11
electrons
4s
3p
3s
2p
2s
1s
Electron Configuration

An atom’s electron configuration is the
position of its electrons around the nucleus. It
reflects the orbital filling diagram.
 The electron configuration of Sodium is
1s22s22p63s1
-- This goes in order on the periodic table from
the first level to last electron the atom has.
 The number in front of the letter tells the
current energy level (1-7).
 The letter tells you the sublevel. The superscript
after the letter tells you how many electrons
have been filled in that sublevel.

Writing Electron Configuration of
Sodium
Energy
4s
3p
3s
2p
2s
1s
1s 2s2 2p6 3s
2
1
Electron Configurations - Ions
If you know how many electrons an
element or ion has, you can write its
electron configuration or draw its orbital
filling diagram.
 Even for an ion!
 Example - Na has 11 electrons, Na+1
(the ion) has 10 electrons.

 Ions
will form so that it ends
with a completely filled main
energy level.
Writing Electron Configuration of
Sodium
Ion
Energy
Na+
4s
Means it lost 1
electron
3p
3s
2p
2s
1s 2s2 2p6 3s
2
1s
1
Na+ electron configuration 1s22s22p6
Adding Electrons = (-) Ions

If it needs one to three more electrons to
complete the main energy level, then it
will tend to gain those electrons and
become a (-) charged ion.
– If it needs to gain 1 electron, it is a (-1)
charged ion.
– If it needs to gain 2 electrons, then it will
become a (-2) charge ion. Etc……
Removing Electrons = + Ions

If the atom has 3 electrons or less in a
main energy level, then it will lose those
electrons to become a (+) charge ion.
– If it loses 1 electron  +1 ion,
– if it loses 2 electrons  +2 ion, etc.