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

1s22s22p1

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 Lithium1s22s1

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 argonCondensed 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 [Ar]4s1

Ca [Ar]4s2

Sc [Ar]4s23d1

Ti [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 orbitalsie.

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)