Electron Filling in Atoms: Spdf Notation & Orbital Diagrams, Lecture notes of Chemistry

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Chapter 8

Electronic Configurations

Dr. Sapna Gupta

Structure of Atom

• We have learned that electrons exist in atoms in specific locations and are always in motion – in the orbit and within the orbital (magnetic spin – ms ).
• In this chapter we will learn how to fill these electrons in atoms in two different ways: - Electronic configuration (spdf notation): here we fill electrons in the various sub shells according to set rules. - Orbital diagram (box configuration): in this case we show electrons as arrows and subshells as boxes and then fill out the electrons.

The First Three Rows

H 1 s 1 He 1s^2 Li 1s^2 2s^1 Be 1s^2 2s^2 B 1s 2 2s 2 2p 1 C 1s 2 2s 2 2p 2 N 1s^2 2s^2 2p^3 O 1s^2 2s^2 2p^4 F 1s^2 2s^2 2p^5 Ne 1s 2 2s 2 2p 6 Na 1s^2 2s^2 2p^6 3 s^1 Mg 1s 2 2s 2 2p 6 3 s 2 Al 1s 2 2s 2 2p 6 3 s 2 3 p 1 Si 1s^2 2s^2 2p^6 3 s^2 3 p^2 P 1s^2 2s^2 2p^6 3 s^2 3 p^3 S 1s^2 2s^2 2p^6 3 s^2 3 p^4 Cl 1s 2 2s 2 2p 6 3 s 2 3 p 5 Ar 1s 2 2s 2 2p 6 3 s 2 3 p 6 Row 1 Row 2 Row 3

Rules for Writing Electron Configurations

• Electrons reside in orbitals of lowest possible energy
• Maximum of 2 electrons per orbital
• Electrons do not pair in degenerate orbitals (same energy orbitals) if an empty orbital is available
• Orbitals fill in the following order: 1s 2 s 2 p 3 s 3 p 4 s 3 d 4 p 5 s 4 d 5 p 6 s or as shown on the right. ( Follow the arrow )

Some More Examples

• Note: these configurations are filled with full arrows, unlike the previous slides – you can fill them either way.
• Z = 20 (Ca) 1 s 2

2 s

2

2 p

6

3 s

2

3 p

6

4 s

2

• Z = 35 (Br) 1 s 2

2 s

2

2 p

6

3 s

2

3 p

6

4 s

2

3d

10

4p

5

• Z = 26 (Fe) 1 s 2

2 s

2

2 p

6

3 s

2

3 p

6

4 s

2

3d

6

Exceptions

• There are a things to watch out for when filling out shells.
  • Fill s in the higher n number before starting d:
    • Fill the 4s before 3d because of energy consideration. Same goes for 5s before 4d.
  • Fill s completely before d for two columns
    • Chromium ( th column in transition metals) and elements below: Should be 4s^2 , 3d^4 ; But is 4s^1 , 3d^5 (for Mo: 5s^1 , 4d^5 ) This is to make the d configuration more stable.
    • Copper and the elements below it are filled as: Should be 4s^2 3d^9 ; But is 4s^1 , 3d^10 (for Ag: 5s^2 , 4d^10 )

Electronic Configuration of Periodic Table

Filling out electrons in Iodine

I 53e-

53 (I) 1 s

2

2 s

2

2 p

6

3 s

2

3 p

6

4 s

2

3d

10

4 p

6

5 s

2

4d

10

5p

5

1 s

2

2 s

2

2 p

6

3 s

2

3 p

6

4 s

2

3d

10

4 p

6

5 s

2

4d

10

5p

5 1s 2 2s 2 3 s 2 2p 6 4 s 2 5 s 2 3d 10 4 d 10 3 p 6 4 p 6 5p 5

Valence and Core Electrons

• Silicon has 4 valence electrons (those in the n = 3 principal shell) and 10 core electrons.
• Selenium has 6 valence electrons (those in the n = 4 principal shell). All other electrons, including those in the 3 d orbitals, are core electrons.

Noble Gas Configurations

• This helps to shorten the electronic configurations so we don’t have to write long notations.
• Take the noble gas of the previous period and continue on filling with the rest of the electrons.
• E.g. Bromine configuration is: 1 s 2 2 s 2 2 p 6 3 s 2 3 p 6 3 d 10 4 s 2 4 p 5
• But the noble gas configuration is: [Ar]3 d 10 4 s 2 4 p 5 ( It is important to remember that the “d” electrons are core so the only the s and p electrons are valence electrons )

Paramagnetism and Diamagnetism

A paramagnetic substance is one that is weakly attracted by a magnetic field, usually as the result of unpaired electrons. A diamagnetic substance is not attracted by a magnetic field generally because it has only paired electrons. Visit UC Davis ChemWiki for more information.

Key Words

• Spdf and box configurations
• Noble gas configuration
• Pauli exclusion principle
• Hund’s rule
• Aufbau principle
• Paramagnetism
• Diamagnetism