Section: 3.3 - Electron Configurations
and Periodic Trends
• Electron Configuration – a shorthand notation
that shows:
– The number of electrons
– The arrangement of electrons in orbitals
• An atom’s ground state electron configuration
determines most chemical properties
• Therefore we usually write electron
configurations for atoms in ground state
Fig. 3.19 - Atomic Orbitals and Relative
Energies
Writing Electron Configurations
• Using Boron as an example
# of electrons in each sublevel
2
2
1
1s 2s 2p
Letters represent orbital shape
Principal Quantum Number
(Energy Level)
We do not need spin numbers...they must be
opposite if they are in the same sublevel
Orbital Diagrams/Energy Level
Diagrams
• Use boxes or circles to represent each orbital
= empty orbital
= orbital with one electron (+1/2 spin)
= orbital with one electron (-1/2 spin)
= orbital with 2 electrons...with opposite spin
Completing Orbital Diagrams and
Writing Electron Configurations
• For Lithium
1s22s1
1s 2s
• For Nitrogen
1s22s2p3
1s 2s
2p
• For Oxygen
1s22s23p4
1s 2s
2p
* The text uses circles and places orbitals vertically from lowest to highest
Condensed Electron Configurations
• For atoms with a large number of electrons
• The configuration is very large
• For Potassium (Atomic # = 19)
1s22s22p63s23p64s1
• Condensed Form - Element symbol for previous
Noble Gas represents the orbitals up to that
point. Additional orbitals are added to it.
• For Potassium – previous Noble Gas is argon
Condensed Electron Configuraton
[Ar]4s1
Electron Configurations for Period 4
• Notice that the 4s orbital has a lower energy
level than 3d orbitals
• Therefore 4s fills before 3d (Aufbau Principle)
K
Ca
Sc
Ti
[Ar]4s1
[Ar]4s2
[Ar]4s23d1
[Ar]4s23d2
Fig. 3.19 - Atomic Orbitals and Relative
Energies
Exceptions to the Aufbau Principle
• Some elements are more stable when they do
not follow the Aufbau Principle
• Eg. Cr is most stable when it have only 1
electron in 4s and 1 in each of the 3d orbitals
ie.
4s
3d
4s
3d
Patterns in Electron Configuration and
Periodic Table Location
Patterns in Electron Configuration and
Periodic Table Location
• For Main Group Elements – last number in the
group number = # of valance electrons
Eg. O is group # 16 – has 6 valance electrons
• The n value of the highest occupied energy
level is the period number
Eg. For Li – 1s22s1
Li is in period 2
For K – [Ar]4s1
K is in period 4
Patterns in Electron Configuration and
Periodic Table Location
• n2 = the total number of orbitals in that
energy level
For n = 2, there are n2 = 4 orbitals
(one 2s orbital and three 2p orbitals)
• 2n2 = the maximum number of electrons in an
energy level
For n = 2, there are 2n2 = 8 electrons max
Examining the Periodic Table
• Elements with similar properties in the
periodic table are filling the same subshell
• Look at the electron configuration for lead
– What does it look like?
– How does the electron configuration explain why
this transition metal can form both Pb2+ and Pb4+
ions?
• Magnetism
– Look at configuration for iron. What do you notice
about the d-orbital electrons?
Exceptions to the rules...
• Chromium
– Predicted
[Ar]4s23d4
– Actual
[Ar]4s13d5
(an s-orbital e- is promoted)
• Copper
– Predicted
[Ar]4s23d9
– Actual
[Ar]4s13d10 (an s-orbital e- is promoted)