How to read a photoelectron spectrum
Each peak is one subshell. Its position is the energy needed to remove an electron from that subshell, and its height is in proportion to the number of electrons in it. On these graphs, as in the AP course and exam description, binding energy falls from left to right on a log scale, so the 1s peak is on the far left. Add up the electrons to name a neutral atom, then read the configuration peak by peak.
The binding energies are free-atom values from NIST data. A textbook table may give somewhat different values, so always work from the numbers printed with each spectrum; no answer here depends on those differences.
Example: silicon
In set PSN372, five peaks sit at 179, 15.8, 10.5, 1.30 and 0.787 MJ/mol with heights 2, 2, 6, 2 and 2. That is 14 electrons, so the atom is silicon: 1s² 2s² 2p⁶ 3s² 3p². Open set PSN372
Questions
How do you read a PES graph?
Each peak is one subshell. Its position shows the energy needed to remove an electron from that subshell, and its height shows how many electrons are in it. Read the peaks from the left, 1s first, and the heights give the configuration: 2, 2, 6 is 1s² 2s² 2p⁶.
Why does binding energy decrease from left to right?
The AP course draws PES graphs with binding energy falling from left to right, on a log scale so that 1s peaks near 100 MJ/mol and valence peaks below 1 MJ/mol fit on one axis. Always check the axis labels on a graph you are given.
Why is the 1s peak at the highest binding energy?
The 1s electrons are closest to the nucleus and have almost no electrons between them and it. Coulomb's law says the attraction grows as the distance shrinks, so they take the most energy to remove.
Why does the same peak move left for the next element?
One more proton raises the nuclear charge. The 1s electrons of sodium, for example, sit at 104 MJ/mol and those of magnesium at 127 MJ/mol: the same subshell is pulled harder, so it takes more energy to remove.
How do you find the element from a photoelectron spectrum?
Add up the electrons in all the peaks, using the 1s peak (2 electrons) as a ruler for the heights. In a neutral atom that total is the atomic number. Five peaks of 2, 2, 6, 2 and 2 make 14 electrons: silicon.
Where do the binding energies come from?
They are the free-atom values in MJ/mol, worked out from NIST atomic data and rounded to 3 significant figures; the table below names the source of each. AP questions give their own numbers, so always use the ones printed on the graph.
Binding energies used, hydrogen to calcium (MJ/mol)
| Atom | 1s | 2s | 2p | 3s | 3p | 4s |
|---|---|---|---|---|---|---|
| H | 1.31 | |||||
| He | 2.37 | |||||
| Li | 6.21 | 0.520 | ||||
| Be | 11.9 | 0.900 | ||||
| B | 19.4 | 1.25 | 0.801 | |||
| C | 28.9 | 1.60 | 1.09 | |||
| N | 39.5‡ | 1.96 | 1.40 | |||
| O | 52.4‡ | 2.75 | 1.31 | |||
| F | 67.2‡ | 3.65 | 1.68 | |||
| Ne | 84.0† | 4.68 | 2.08 | |||
| Na | 104† | 6.85 | 3.67 | 0.496 | ||
| Mg | 127† | 9.72† | 5.55 | 0.738 | ||
| Al | 151† | 12.6† | 7.91† | 1.02 | 0.578 | |
| Si | 179† | 15.8† | 10.5† | 1.30 | 0.787 | |
| P | 208† | 19.3† | 13.5† | 1.56 | 1.01 | |
| S | 239† | 23.1† | 16.7† | 1.95 | 1.00 | |
| Cl | 273† | 27.2† | 20.2† | 2.37 | 1.25 | |
| Ar | 309† | 31.6† | 24.1† | 2.82 | 1.52 | |
| K | 349† | 37.3† | 29.1† | 3.36‡ | 2.36 | 0.419 |
| Ca | 391† | 43.5† | 34.6† | 4.27‡ | 3.31 | 0.590 |
Free atoms, from the NIST Atomic Spectra Database (ionization energy plus the energy of the ion left with that hole) unless marked: † NIST X-ray Transition Energies Database, theoretical free-atom edges; ‡ X-ray Data Booklet, measured on N₂, O₂ and F₂ gas or on the metal. 1 eV per atom = 0.0965 MJ/mol.
Sources, read October 27, 2026: College Board AP Chemistry course and exam description, topic 1.6 and sample free-response question 1; NIST Atomic Spectra Database ionization energies and levels; NIST X-ray Transition Energies Database; electron binding energies table (X-ray Data Booklet values).