Electrons do not travel around
the nucleus in neat orbits, as
Bohr’s model proposed.

Electrons exist in certain regions within the
atom called orbitals as described by the
Quantum Mechanical Model of the atom.

An orbital is a three-dimensional
region around the nucleus that
indicates the probable location
of an electron.

Energy increases
further from the

nucleus.

Each Principal Energy Level has sublevels.

n = Principal
Energy Level (PEL)
n = 1-7

Each type of sublevel is
represented by a letter: s orbital

p orbital

And, each sublevel has a

particular shaped

orbital.

s, p, d, & f

There are different
numbers and types of
orbitals in each sublevel.
s sublevel = 1 orbital

sphere shape

p sublevel = 3 orbitals

dumbbell shape

There are different
numbers and types of
orbitals in each sublevel.

d sublevel = 5 orbitals
4 cloverleaf shapes & 1 donut shaped f sublevel =

7 orbitals, difficult
to portray in 2-d.

There can be only two
electrons in each orbital!

s

1

2

p
d
f

3
5
7

6
10
14

The arrangement of electrons in
an atom is known as the atom’s

Three rules govern electron configuration:

Aufbua Principle

Pauli’s Exclusion Principle

Hund’s Rule

Electrons occupy

positions of the lowest energy

Electrons are added one at a time to the lowest energy orbital available until all of the electrons of the atom have been accounted for. *Aufbau is German for “building up”

Electrons

occupy

positions of
the lowest

energy

6d

5f

Start here

5d
4d
3d

4f

6p
5p
4p
3p
2p

7p

2s
1s

Filling Order
for Electrons

No two electrons are alike. Two

electrons in the same orbital

must have opposite spins.

The opposite spin of the two electrons

forms a magnetic field within the orbital.

S

N

-

-

Wolfgang

Pauli

Each orbital can hold TWO electrons with
opposite spins. These are represented with
an up half-arrow and a down half-arrow.

This is called

Orbital Notation

Electrons in the same sublevel

occupy empty orbitals first

before pairing up. And, unpaired
electrons have the same spin.

Within a sublevel, place one electron

per orbital before pairing them.
Orbital Notation: half

arrows represent electrons

within orbitals.

He

H

1s

Li

Be

2s

C

B

1s

N

2s
2p

Ne

F

1s

Na

2s
2p
3s

O

Cl

1s

2s

2p

3s

3p

This time use dashes instead of boxes.

Be sure to use a bracket to show

sublevels together. Ex: 2p