Read photoelectron spectra: peak position gives binding energy (subshell identity), peak height gives relative electron count.
Photoelectron spectroscopy (PES) gives chemists a direct experimental window into electron configuration — rather than inferring configuration purely from periodic table position, PES measures it, and correctly reading a PES spectrum is an AP Chemistry Unit 1 skill that tests real data interpretation rather than memorized rules.
In PES, high-energy photons eject electrons from an atom, and the kinetic energy of each ejected electron is measured; subtracting from the known photon energy gives the binding energy of the subshell that electron came from. Because different subshells hold their electrons with characteristically different binding energies, a PES spectrum — plotted as peak position (binding energy) versus peak height (relative electron count) — is essentially a fingerprint of an atom's full electron configuration.
Peak position tells you which subshell a given peak corresponds to: core electrons, held very tightly by the nucleus with little shielding, produce peaks at very high binding energy, while valence electrons, more shielded and farther from the nucleus, produce peaks at much lower binding energy. Peak height directly reflects how many electrons occupy that subshell — a 2s peak is always shorter than a 2p peak's neighboring peaks in an element with a full 2p⁶ subshell, since 2s holds only 2 electrons versus 2p's 6.
Reading a complete electron configuration from a PES spectrum works in reverse from the usual Aufbau-based approach: rather than filling orbitals theoretically, you identify each peak's binding energy, assign it to the correct subshell based on that binding energy scale, and read off the relative electron count from peak height directly — reconstructing the actual measured configuration rather than a predicted one.
Mystery element identification is the natural culminating exercise: given only a PES spectrum with no other information, correctly counting subshells and electrons from peak position and height identifies the unknown element unambiguously, testing whether the underlying logic is genuinely understood rather than pattern-matched from familiar elements.
This reader lets you explore real PES spectra for known elements or attempt mystery-element identification, distinguishing core from valence electrons purely from spectral data.