3. Introduction: Learning Objectives
1. Describe the structure of energy levels
2. Define s, p and d orbitals, and describe their
shapes
3. Describe the rules for assigning electrons to
subshells
4. Deduce the electron configuration of an atom
from its atomic number
5. Compare electron configurations of atom and ion
4. Electrons exist in layers called shells. (The shells can also be called energy
levels).
The arrangement of electrons in these shells is often called the electron
configuration.
1st shell
2nd shell
3rd shell
Arrangement of electrons in an atom
5. Each energy level has a maximum number of electrons that it
can hold. Electrons will fill the level nearest the nucleus first.
1st level holds
a maximum of
2 electrons
2nd level holds
a maximum of
8 electrons
3rd level holds
a maximum of
8 electrons
This electron arrangement is written as 2,8,8.
Arrangement of electrons in an atom
6. • Each sublevel (s, p, d, f) contains orbitals.
• Orbitals are electron-clouds that hold the
electrons 90% of the time.
• Each orbital can hold TWO electrons.
Atomic orbitals
Maximum Number of Electrons In Each Sublevel
Maximum Number
Sublevel Number of Orbitals of Electrons
s 1 2
p 3 6
d 5 10
f 7 14
8. 4f
4d
4p
4s
n = 4
3d
3p
3s
n = 3
2p
2s
n = 2
1s
n = 1
Energy
Sublevels
s
s
s
s
p
p
p
d
d f
1s22s22p63s23p64s23d104p6…
Electron configuration
9. • Hund’s Rule
• Within a sublevel, place one electron per orbital
before pairing them.
• “Empty Bus Seat Rule”
• Pauli Exclusion Principle
• Each orbital can hold TWO electrons with opposite spins.
WRONG RIGHT
RIGHT
WRONG
Filling rules for electron orbitals
• Aufbau Principle
• Electrons fill the lowest energy orbitals first.
• “Lazy Tenant Rule”
3d
5s
5p
6p
2s
3s
4s
5s
6s
7s
1s
2p
3p
4p
5p
6p
3d
4d
5d
6d
4f
5f
1s
2s
2p
3s
3p
4s
4p
4d
6s
7s
6d
4f
5f
5d
Energy