1 question, one for each idea where we can. Answer them, then see which ideas to fix.
Question 1 of 1
The complete photoelectron spectrum of an element is shown. According to the spectrum, how many electrons does an atom of the element have in the subshell from which an electron is most easily removed?
Answer and reasoning
A1 A student who thinks each electron gives its own peak picks this, taking the right-hand peak to be a single electron. The height of a peak shows how many electrons share that binding energy; the right-hand peak has height 3.
B2 A student who reads the left-hand peak as the electrons easiest to remove picks this, reading the height of the peak at about 208 MJ/mol. That peak is at the highest binding energy, the 1s electrons, which are the hardest to remove.
C3Correct The binding-energy axis increases to the left, so the least tightly held electrons give the right-hand peak, at about 1.01 MJ/mol. Its relative height is 3, so that subshell (3p in this element, phosphorus, 1s² 2s² 2p⁶ 3s² 3p³) contains three electrons.
D6 A student who thinks a p-subshell peak always stands for six electrons picks this. The right-hand peak belongs to the 3p subshell, but its height, 3, shows that the subshell contains only three electrons; six is its capacity.
Working Binding energy increases to the left, so the most easily removed electrons give the right-hand peak, at about 1.01 MJ/mol. Its relative height is 3, so that subshell contains 3 electrons. (Reading all five peaks, 2 : 2 : 6 : 2 : 3, gives 1s² 2s² 2p⁶ 3s² 3p³, phosphorus; the 3p subshell holds the most easily removed electrons.)
In preparation: 0 of 1 sections compiled and reviewed. The rest show key terms and common mistakes from our question bank until they are.
1.6.A.1 Photoelectron spectroscopy (PES) Fix
Photoelectron spectroscopy (PES)
An experimental technique in which gaseous atoms or ions are struck by high-energy radiation that ejects electrons. Measuring the energies of the ejected electrons gives the energy that was needed to remove electrons from each subshell, and the results are plotted as a photoelectron spectrum.
Binding energy (in PES)
The energy required to remove an electron from a particular subshell of an atom or ion, usually given in MJ/mol. Photoelectron spectra are usually plotted with binding energy increasing to the left.
Peak position
Where a peak sits on the binding-energy axis. It is related to the energy required to remove an electron from the corresponding subshell: a peak at a higher binding energy belongs to electrons that are attracted more strongly to the nucleus.
Relative peak height
The height of a peak compared with the other peaks in the same spectrum. Ideally it is proportional to the number of electrons in that subshell, so peaks for 2, 6 and 3 electrons have heights in the ratio 2 : 6 : 3.
Subshells in a photoelectron spectrum
Each occupied subshell (1s, 2s, 2p, 3s, 3p, …) gives its own peak, so electrons in the same shell but different subshells, such as 2s and 2p, give separate peaks. The number of peaks and their relative heights give the ground-state electron configuration.
Coulombic attraction and binding energy
The energy needed to remove an electron increases with the attraction between the electron and the nucleus. By Coulomb's law that attraction is greater for a larger nuclear charge and a shorter distance, and it is reduced when other electrons lie between the electron and the nucleus (shielding).
Students often think All of the electrons in one shell have the same energy, so the electrons of a shell give a single peak. In fact Two. The 2s and 2p electrons are in the same shell but in different subshells, and different energies are needed to remove them, so each subshell gives its own peak. In an atom that has electrons in both, the 2s peak lies at a higher binding energy than the 2p peak.
Students often think A taller peak shows electrons that need more energy to remove. In fact The relative number of electrons in that subshell; ideally the height is proportional to that number. The energy needed to remove those electrons is shown by the position of the peak on the binding-energy axis, not by its height.
4 more questions. Every wrong answer here is a real mistake students make, and you see why it is wrong as soon as you answer.
Question 1 of 4
A student draws the four photoelectron spectra shown, numbered 1 to 4, as possible spectra of gaseous magnesium atoms in their ground state. Which spectrum is consistent with the ground-state electron configuration of magnesium and with what the position and height of a peak represent?
Answer and reasoning
ASpectrum 1 A student who thinks all of the electrons in a shell have the same energy picks this spectrum, which has a single peak of height 8 for the whole n = 2 shell. The 2s and 2p electrons need different energies to remove, so they give two peaks, of heights 2 and 6.
BSpectrum 2Correct Mg is 1s² 2s² 2p⁶ 3s². Each of the four occupied subshells gives one peak; the heights are in the ratio 2 : 2 : 6 : 2, matching the numbers of electrons; and the binding energy falls from the 1s peak at the left to the 3s peak at the right, because the 3s electrons are farthest from the nucleus and most shielded from it.
CSpectrum 3 A student who thinks a taller peak shows electrons that need more energy to remove picks this spectrum, whose peaks get taller as the binding energy increases. Peak heights show the numbers of electrons, so they should be in the ratio 2 : 2 : 6 : 2.
DSpectrum 4 A student who reads the left-hand peak as the electrons easiest to remove picks this spectrum, the correct spectrum drawn in reverse: it places the two 3s electrons at the highest binding energy and the 1s electrons at the lowest. The 1s electrons are closest to the nucleus and are the hardest to remove, so their peak belongs at the left.
A student will record the photoelectron spectra of gaseous potassium atoms and gaseous calcium atoms under identical conditions. Which prediction about the 1s peak in the spectrum of calcium, compared with the 1s peak in the spectrum of potassium, is correct?
Answer and reasoning
AIt will be at a higher binding energy, because a calcium nucleus has one more proton to attract the 1s electrons.Correct Calcium (Z = 20) has one more proton than potassium (Z = 19). Its extra electron is in the 4s subshell, far outside the 1s electrons, so it hardly shields them. The 1s electrons of calcium are therefore attracted more strongly and need more energy to remove: the 1s peak of Ca lies at a higher binding energy (about 390 MJ/mol, compared with about 347 MJ/mol for K).
BIt will be at a lower binding energy, because calcium's additional electron will shield the 1s electrons from the nucleus. A student who thinks every added electron shields all of the other electrons, including the inner ones, picks this. The extra 4s electron of calcium is, on average, far outside the 1s electrons and barely shields them, while the extra proton increases their attraction to the nucleus.
CIt will be at the same binding energy, because 1s electrons have the same energy in the atoms of every element. A student who thinks a subshell has the same energy in every atom picks this. The energy needed to remove an electron depends on the nucleus that attracts it; with one more proton, calcium holds its 1s electrons more strongly than potassium does.
DIt will be at a higher binding energy, because calcium's filled 4s subshell will make all of its electrons more stable. A student who thinks a filled subshell makes an atom hold all of its electrons more tightly picks this. The prediction of a higher binding energy is correct, but the reason is the greater nuclear charge acting on the 1s electrons; the filling of the 4s subshell is not what sets how strongly the 1s electrons are held.
In the photoelectron spectrum of lithium, the 1s peak is at 6.26 MJ/mol and the 2s peak is at 0.52 MJ/mol. Which statement best supports the claim that a lithium atom holds its 1s electrons much more strongly than its 2s electron?
Answer and reasoning
AThe 1s peak is twice as tall as the 2s peak, so it shows electrons that need more energy to remove. A student who thinks a taller peak shows electrons that need more energy picks this. The 1s peak is twice as tall because the 1s subshell holds two electrons and the 2s subshell one; the energy needed is shown by the position of the peak, 6.26 MJ/mol compared with 0.52 MJ/mol.
BThe 1s subshell is filled, so its electrons are more stable than the single electron in the 2s subshell. A student who thinks a filled subshell holds its electrons more tightly because it is filled picks this. How strongly an electron is held depends on its Coulombic attraction to the nucleus, which is far greater for the 1s electrons because they are much closer to it.
CThe 1s electrons are removed after the 2s electron, so each needs more energy than the electron before it. A student who thinks each electron needs more energy than the one removed before it picks this. In photoelectron spectroscopy each electron is ejected from a separate neutral atom, so the 1s peak shows the energy needed to remove a 1s electron from a lithium atom, not from an ion that has already lost its 2s electron.
DThe 1s electrons are much closer to the nucleus, so the nucleus attracts them much more strongly.Correct By Coulomb's law the attraction between the nucleus and an electron increases as the distance between them decreases. The 1s electrons are, on average, much closer to the nucleus than the 2s electron, and no electrons lie between them and the nucleus, so they need far more energy to remove.
Which description of the complete photoelectron spectrum of a ground-state silicon atom is correct?
Answer and reasoning
AThree peaks, one for each of the occupied shells A student who thinks all of the electrons in a shell have the same energy picks this, expecting one peak for each of the three occupied shells. Electrons in different subshells of the same shell need different energies to remove, so the 2s and 2p electrons, and the 3s and 3p electrons, give separate peaks.
BFive peaks, with the 2p peak tallest of allCorrect Si is 1s² 2s² 2p⁶ 3s² 3p². Each of the five occupied subshells gives one peak, and the heights are proportional to the numbers of electrons, 2 : 2 : 6 : 2 : 2, so the 2p peak, for six electrons, is the tallest.
CFourteen peaks, each of the same height A student who thinks each electron gives its own peak picks this, expecting one peak for each of the 14 electrons. All of the electrons in one subshell share one binding energy and are counted in a single peak, so there are five peaks of different heights.
DFive peaks, with the 1s peak the tallest A student who thinks a taller peak shows electrons that need more energy to remove picks this, because the 1s electrons are the hardest to remove. The 1s peak is at the highest binding energy, but its height corresponds to two electrons; the 2p peak, for six electrons, is the tallest.
Compiled from the AP Chemistry Course and Exam Description (effective Fall 2024) and our question bank · Specialist review in progress. How these pages are made · Free, no account
