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 onlyEnergy level 2 – s, p sublevel onlyEnergy level 3 – s, p, d sublevel onlyEnergy 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 ConfigurationsFirst 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 electronsFourth 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

Incr

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1s

2s

3s

4s

5s6s7s

2p

3p

4p5p6p

3d

4d5d

7p 6d

4f5f

Energy Level Diagram

Try it!

• Let’s determine the electron configuration for Phosphorus

• Need to account for 15 electrons

• The first to electrons go into the 1s orbital

• Notice the opposite spins

• only 13 moreIncr

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nerg

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1s

2s

3s

4s

5s6s7s

2p

3p

4p5p6p

3d

4d5d

7p 6d

4f5f

• The next electrons go into the 2s orbital

• only 11 more

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nerg

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1s

2s

3s

4s

5s6s7s

2p

3p

4p5p6p

3d

4d5d

7p 6d

4f5f

• The next electrons go into the 2p orbital

• only 5 more

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nerg

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1s

2s

3s

4s

5s6s7s

2p

3p

4p5p6p

3d

4d5d

7p 6d

4f5f

• The next electrons go into the 3s orbital

• only 3 more

Incr

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nerg

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1s

2s

3s

4s

5s6s7s

2p

3p

4p5p6p

3d

4d5d

7p 6d

4f5f

Incr

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ng e

nerg

y

1s

2s

3s

4s

5s6s7s

2p

3p

4p5p6p

3d

4d5d

7p 6d

4f5f

• 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