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

An in-depth understanding of electronic configurations in atoms through two different methods: spdf notation and orbital diagrams. the principles of filling electrons, the first three rows of the periodic table, rules for writing electron configurations, exceptions, and solved problems. Key concepts include Pauli's exclusion principle, Hund's rule, and Aufbau principle.

What you will learn

  • What is Hund's rule?
  • What are the exceptions when filling out shells?
  • How do you write electron configurations according to rules?
  • What is the Pauli exclusion principle?
  • What are the two methods for filling electrons in atoms?

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2021/2022

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Chapter 8
Electronic Configurations
Dr. Sapna Gupta
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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