3.10 Electron Configurations

Quantum numbers help to describe the location of an electron within an atom, but when the arrangement of electrons in an atom is desired, electron configurations are used.

When writing electron configurations, a shorthand notation is used, where the principal quantum number, the subshell, and number of electrons within each subshell are written. The principal quantum number is written first, with the orbital shape listed next, and the count of electrons within that subshell is listed as a superscript on the subshell shape. For example, a notation of 4d3, this indicates that you are in shell 4 (principal quantum number = 4), you are in the d subshell, and there are three electrons within that subshell.

There are some general rules for writing electron configurations:

  1. Electrons will reside in the available orbitals of the lowest possible energy.
  2. Each orbital can accommodate a maximum of two electrons.
  3. Electrons will not pair in degenerate orbitals if an empty orbital is available.
  4. Orbitals will fill in the order indicated by the periodic table.
    1. There are blocks for each angular momentum quantum number (l)—there are blocks for each orbital shape. Each block is indicated in the image below.
    2. Each time an electron configuration is started, you begin with hydrogen and move from left to right, top to bottom.
    3. As you leave a row or block (s, p, d, f), you must write down where you are moving (write down the principal quantum number—what row you are in, and the angular momentum quantum number—which block are in. You will leave a block by filling it with electrons.
      1. s Block: There is only 1 s orbital per principal quantum shell, so 2 electrons will be needed to move out of the s block.
      2. p Block: There are 3 p orbitals (one described by each magnetic quantum number—l = 1, ml = -1, 0, or +1). Six electrons are required to move out of the p block.
      3. d Block: There are 5 d orbitals per principal quantum shell (l = 2, ml = -2, -1, 0, +1, +2), which requires 10 electrons to move out of the d block.
      4. f Block: There are 7 f orbitals per principal quantum shell (l = 3, ml= -3, -2, -1, 0, +1, +2, +3), which requires 14 electrons to move out of the f block.

To see how to write electron configurations, please review Section 3.4 in the textbook.

Practice – Electron Configuration

Which of the following pairs is isoelectronic?

  1. C4–, Cl–
  2. O2–, Na+
  3. K+, Na+
  4. O2–, S2–
Solution

Answer: B

Practice – Electron Configuration

Which of the following is the electron configuration for Mn?

  1. 1s22s22p63s23p63d7
  2. 1s22s22p63s23p64s23d5
  3. 4s23d5
  4. 1s22s22p63s23p64s13d6
Solution

Answer: B

Practice – Electron Configuration

Which of the following is the electron configuration for Cu+?

  1. 1s22s22p63s23p63d10
  2. 1s22s22p63s23p64s13d9
  3. 1s22s22p63s23p64s23d9
  4. 1s22s22p63s23p64s13d10
Solution

Answer: A

Practice – Electron Configuration

Which of the following is the complete electron configuration for O2–?

  1. [He]2s22p5
  2. 2s22p6
  3. 1s22s12p6
  4. 1s22s22p6
Solution

Answer: D

Practice – Electron Configuration

Which of the following is the correct electron configuration for Au?

  1. [Kr]6s24f145d9
  2. [Xe]6s14f145d10
  3. 1s22s22p63s23p64s23d104p65s24d105p66s24f145d9
  4. [Xe]6s25f145d9
Solution

Answer: B

Practice – Magnetism

Which of the following elements are diamagnetic?

  1. lead
  2. ruthenium
  3. lithium
  4. tin
Solution

Answer: A and D

Practice – Magnetism

Which of the following elements are paramagnetic?

  1. Na+
  2. Ca2+
  3. Cr3+
  4. O2–
Solution

Answer: C

Practice – Quantum Numbers

Choose the set of quantum numbers (ordered as n, l, ml, ms) that is not possible.

  1. 3, 1, 0, –1/2
  2. 4, 0, 0, +1/2
  3. 2, 3, 1, +1/2
  4. 4, 3, –2, –1/2
Solution

Answer: C

Practice – Quantum Numbers

Choose the only set of quantum numbers (ordered as n, l, ml, ms) that is possible.

  1. 6, 1, –1, +1/2
  2. 3, 0, 1, –1/2
  3. 2, 2, –1, +1/2
  4. 1, 1, 0, –1/2
Solution

Answer: A

Practice – Quantum Numbers

Which of the following is a possible set of quantum numbers (ordered as n, l, ml, ms) for an electron found in the third period of a d orbital?

  1. 3, 1, –1, +1/2
  2. 4, 2, –1, –1/2
  3. 3, 3, 0, –1/2
  4. 3, 2, –2, +1/2
Solution

Answer: D

Practice – Electron Configuration

What is the number of valence electrons for Co?

  1. 9
  2. 27
  3. 7
  4. 2
Solution

Answer: A



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