What are electron configurations?
• The way electrons are arranged in atoms.
• Used to indicate which orbitals (energy
levels) are occupied by electrons for a
particular atom
• The letters s, p, d and f are used to identify
sublevels; they represent shapes
Remember!
• Each sublevel orbital holds only 2
electrons but sublevels contain different
number of orbitals
s – 1 orbital (maximum 2 e-)
p – 3 orbitals (maximum 6 e-)
d – 5 orbitals (maximum 10 e-)
f – 7orbitals (maximum 14 e-)
Energy Levels
• Different energy levels contain only certain
sublevels
Energy level 1 – s sublevel only
Energy level 2 – s, p sublevel only
Energy level 3 – s, p, d sublevel only
Energy level 4 – s, p, d, f sublevel only
Some rules!
• Aufbau Principle - each electrons is added
to the lowest orbital available
• This causes difficulties because of the
overlap of orbitals of different energies.
• Pauli Exclusion Principle - at most 2
electrons per orbital - different spins
• Hund’s Rule - When electrons occupy
orbitals of equal energy they don’t pair up
until they have to .
Electron Configurations
First Energy Level
• only s sublevel (1 s orbital)
• only 2 electrons
• 1s2
Second Energy Level
• s and p sublevels (s and p orbitals are available)
• 2 in s, 6 in p
• 2s22p6
• 8 total electrons
Third energy level
• s, p, and d orbitals
• 2 in s, 6 in p, and 10 in d
• 3s23p63d10
• 18 total electrons
Fourth energy level
• s,p,d, and f orbitals
• 2 in s, 6 in p, 10 in d, and 14 in f
• 4s24p64d104f14
• 32 total electrons
So…..
• The electrons fill in the following order:
1s 2s 2p 3s 3p 4s 3d 4p 5s 4d 5p 6s 4f 5d
6p 7s 5f 6d 7p
Increasing energy
7s
6s
5s
7p
6p
5p
4p
4s
6d
5d
4d
3d
3p
3s
2p
2s
1s
Energy Level
Diagram
5f
4f
Try it!
• Let’s determine the electron configuration
for Phosphorus
• Need to account for 15 electrons
Increasing energy
7s
6s
5s
4s
3s
2s
1s
7p
6p
5p
4p
6d
5d
4d
5f
4f
3d
3p • The first to electrons go
into the 1s orbital
2p
• Notice the opposite
spins
• only 13 more
Increasing energy
7s
6s
5s
4s
3s
2s
1s
7p
6p
5p
4p
6d
5d
4d
5f
4f
3d
3p • The next electrons go
into the 2s orbital
2p
• only 11 more
Increasing energy
7s
6s
5s
4s
3s
2s
1s
7p
6p
5p
4p
6d
5d
4d
3d
3p • The next electrons go
into the 2p orbital
2p
• only 5 more
5f
4f
Increasing energy
7s
6s
5s
4s
3s
2s
1s
7p
6p
5p
4p
6d
5d
4d
3d
3p • The next electrons go
into the 3s orbital
2p
• only 3 more
5f
4f
Increasing energy
7s
6s
5s
4s
7p
6p
6d
5d
5p
4d
4p
3p •
3s
2s
1s
2p •
•
•
5f
4f
3d
The last three electrons
go into the 3p orbitals.
They each go into
separate shapes
3 unpaired electrons
1s22s22p63s23p3
Orbitals fill in order
• Lowest energy to higher energy.
• Adding electrons can change the energy
of the orbital.
• Half filled orbitals have a lower energy,
which makes them more stable.
• Changes the filling order
Write these electron
configurations
• Magnesium – 12 electrons
•
•
•
•
•
1s22s22p63s2
Titanium - 22 electrons
1s22s22p63s23p64s23d2
Vanadium - 23 electrons
1s22s22p63s23p64s23d3
Transition Metals are special!
• Draw an energy level diagram and the
electronic configuration for:
• Chromium - 24 electrons
• 1s22s22p63s23p64s23d4 is expected
• But this is wrong!!
Chromium is actually
•
•
•
•
•
1s22s22p63s23p64s13d5
Why?
This gives us two half filled orbitals.
Slightly lower in energy.
The same principal applies to copper.
Try it!
• Copper has 29 electrons
•
•
•
•
•
Copper’s electron
configuration
Copper has 29 electrons so we expect
1s22s22p63s23p64s23d9
But the actual configuration is
1s22s22p63s23p64s13d10
This gives one filled orbital and one half
filled orbital.
• Remember these exceptions