AP Chemistry · Lesson 2

Electron configurations and photoelectron spectra

Know Know it

Electrons fill subshells in order of rising energy: 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p. An s subshell holds 2 electrons, p holds 6 and d holds 10. The shorthand starts from the previous noble gas: sulfur is [Ne] 3s² 3p⁴.

The valence electrons are those in the highest shell (largest n). For a main-group atom, that is the s and p electrons of the outer shell; filled d electrons are not counted.

For a cation, remove electrons from the highest shell first. Transition metals lose their 4s electrons before any 3d electrons. For an anion, add electrons to the next open subshell.

A photoelectron spectrum plots the number of electrons ejected against binding energy. Each peak is one subshell. A peak's position shows how tightly those electrons are held, and its height is proportional to how many electrons are in that subshell. Many spectra put high binding energy on the left, so read the axis before you read the peaks.

Worked example Easy

Which is the ground-state electron configuration of a silicon atom, which has 14 electrons?

  1. 1s² 2s² 2p⁶ 3s² 3d²
  2. 1s² 2s² 2p⁶ 3p⁴
  3. 1s² 2s² 2p⁸ 3s²
  4. 1s² 2s² 2p⁶ 3s² 3p²Answer

How to solve it

  1. Fill in order of energy: 1s, 2s, 2p, 3s, 3p.
  2. 1s² 2s² 2p⁶ uses 10 electrons; 3s² brings the count to 12.
  3. The last 2 go into 3p: 1s² 2s² 2p⁶ 3s² 3p², or [Ne] 3s² 3p².

Why each choice is right or wrong

  • A. Incorrect. 1s² 2s² 2p⁶ 3s² 3d² puts the last two electrons in 3d, but 3p is lower in energy and fills well before 3d.
  • B. Incorrect. 1s² 2s² 2p⁶ 3p⁴ skips 3s, which is lower in energy than 3p and must fill first.
  • C. Incorrect. 1s² 2s² 2p⁸ 3s² puts 8 electrons in 2p, but a p subshell holds at most 6.
  • D. Correct. Filling in order gives 2 + 2 + 6 = 10 electrons through 2p, then 3s² and 3p² for 14 in all.

Trap Transition metals lose 4s first

The 4s subshell fills before 3d, but it also empties first. An iron atom is [Ar] 4s² 3d⁶, and the Fe²⁺ ion is [Ar] 3d⁶, not [Ar] 4s² 3d⁴. Remove electrons from the shell with the highest n.

Explain Explain it

Task verbs: explain, interpret

To read a spectrum, count the peaks (one per occupied subshell) and read the relative heights (electrons per subshell). Adding the heights gives the number of electrons, which for a neutral atom is the atomic number.

To explain a peak's position, compare the electrons in that subshell with others on two counts: how far they are from the nucleus (which shell) and how many electrons shield them. Then use Coulomb's law to say which are held more strongly. Comparing one subshell across two elements, look at the number of protons too.

Worked example Medium

Photoelectron spectrum of an unknown element

PeakBinding energy (MJ/mol)Relative height
11042
26.842
33.676
40.501

Which element produced this spectrum?

  1. Neon, Ne
  2. Sodium, NaAnswer
  3. Magnesium, Mg
  4. Potassium, K

How to solve it

  1. Four peaks means four occupied subshells: 1s, 2s, 2p and 3s.
  2. The heights 2, 2, 6 and 1 give 1s² 2s² 2p⁶ 3s¹.
  3. 2 + 2 + 6 + 1 = 11 electrons, so a neutral atom with 11 protons: sodium.

Why each choice is right or wrong

  • A. Incorrect. Neon would show only the first three peaks, 1s² 2s² 2p⁶, with no fourth peak at low binding energy.
  • B. Correct. The heights give 1s² 2s² 2p⁶ 3s¹, which is 11 electrons, so the element is sodium.
  • C. Incorrect. Magnesium is 1s² 2s² 2p⁶ 3s², so its lowest-energy peak would be twice as tall as the one in the table.
  • D. Incorrect. Potassium has 19 electrons and would add peaks for 3s, 3p and 4s, six peaks in all, not four.

Explain Your turn

The photoelectron spectrum of a fluorine atom has three peaks, for 1s, 2s and 2p. In a photoelectron spectrum, the height of a peak is proportional to the number of electrons in that subshell. Explain why the 1s peak is at a much higher binding energy than the 2s peak. Then give the relative heights of the 2s and 2p peaks.

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A full answer does this

  • Explains that the 1s electrons are in the first shell, closer to the nucleus than the 2s electrons, so they are attracted more strongly and need more energy to remove
  • Gives the 2p peak a height of 5 and the 2s peak a height of 2 (a ratio of 5 : 2)

Model answer

The 1s electrons are in the first shell, much closer to the nucleus than the 2s electrons in the second shell. By Coulomb's law, a smaller distance means a stronger attraction, so far more energy is needed to remove a 1s electron. Fluorine is 1s² 2s² 2p⁵, so the 2p peak has a height of 5 and the 2s peak a height of 2, a ratio of 5 : 2.

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