This example focuses on AP Chemistry Curriculum Framework
Learning Objectives 1.5−1.8 and highlights how photoelectron
spectroscopy (PES) data can be used to build a model of
atomic subshells by exploring the energies of all of the electrons
in an atom. More detail about how to use this approach in an
AP class can be found in Concept Development Studies in
Chemistry 2013.19 In a typical lesson on atomic structure,
students are introduced to electron shells and the patterns of
electron configuration for elements by being told that electron
subshells correspond to orbitals of specific shapes and that
these shapes are named s, p, d, and f. They are often then told
that subshells and orbitals depend on what period and group
the element is located in on the periodic table. This causes
misconceptions as it is actually the opposite that is true: the
location of an element in the periodic table is determined by its
electron configuration, not vice versa. They are also given an
order by which orbitals are “filled” by following an increase in
energy sequence but without any justification for why this is the
case or how this order was determined.20−22
A typical lesson may include a mnemonic device, card sort or
some hands-on activity that helps students learn the “rules” for
“writing” the electron configuration for an element.23−25
Students can memorize this pattern but it does not provide a
mechanism for students to gain a conceptual understanding of
what these subshells are and why there is a limit on the number
of electrons in each shell.
In the Data First approach, the students are not told about
electron configuration but rather students explore the PES data
in Table 2 and identify trends by scanning the data line-by-line
and looking for patterns. The PES data used are all possible f irst
ionization energies of gaseous atoms. We can ask students what
they observe for hydrogen and helium. How many thresholds
are there? It is clear to the students why there is only one for
hydrogen. But why is there also only one for helium, when we
know it has two electrons? The students can conclude from the
data that both electrons must require an identical amount of
energy to be removed from the atom. Therefore, they must
have identical energies. Why are there two energies for lithium
when it has three electrons? Two electrons must have identical
energies while one has a different energy. Because the two
electrons in helium were also allowed to have identical energies,
perhaps this initial energy “level” can only accommodate two
electrons? If so, is the two-electron maximum true for all energy
levels? Beryllium, boron, and carbon seem to support this idea,
but nitrogen has only three distinct energies even though it
contains seven electrons, indicating that at least three electrons
must occupy the same energy level. In fact, an additional energy
level is not gained until after neon, indicating that six electrons are accommodated at this particular level. Analysis of the
photoelectron spectrum of argon provided in Figure 2
strengthens this interpretation and helps make yet another
pattern in the table of data more obvious.
This example focuses on AP Chemistry Curriculum Framework
Learning Objectives 1.5−1.8 and highlights how photoelectron
spectroscopy (PES) data can be used to build a model of
atomic subshells by exploring the energies of all of the electrons
in an atom. More detail about how to use this approach in an
AP class can be found in Concept Development Studies in
Chemistry 2013.19 In a typical lesson on atomic structure,
students are introduced to electron shells and the patterns of
electron configuration for elements by being told that electron
subshells correspond to orbitals of specific shapes and that
these shapes are named s, p, d, and f. They are often then told
that subshells and orbitals depend on what period and group
the element is located in on the periodic table. This causes
misconceptions as it is actually the opposite that is true: the
location of an element in the periodic table is determined by its
electron configuration, not vice versa. They are also given an
order by which orbitals are “filled” by following an increase in
energy sequence but without any justification for why this is the
case or how this order was determined.20−22
A typical lesson may include a mnemonic device, card sort or
some hands-on activity that helps students learn the “rules” for
“writing” the electron configuration for an element.23−25
Students can memorize this pattern but it does not provide a
mechanism for students to gain a conceptual understanding of
what these subshells are and why there is a limit on the number
of electrons in each shell.
In the Data First approach, the students are not told about
electron configuration but rather students explore the PES data
in Table 2 and identify trends by scanning the data line-by-line
and looking for patterns. The PES data used are all possible f irst
ionization energies of gaseous atoms. We can ask students what
they observe for hydrogen and helium. How many thresholds
are there? It is clear to the students why there is only one for
hydrogen. But why is there also only one for helium, when we
know it has two electrons? The students can conclude from the
data that both electrons must require an identical amount of
energy to be removed from the atom. Therefore, they must
have identical energies. Why are there two energies for lithium
when it has three electrons? Two electrons must have identical
energies while one has a different energy. Because the two
electrons in helium were also allowed to have identical energies,
perhaps this initial energy “level” can only accommodate two
electrons? If so, is the two-electron maximum true for all energy
levels? Beryllium, boron, and carbon seem to support this idea,
but nitrogen has only three distinct energies even though it
contains seven electrons, indicating that at least three electrons
must occupy the same energy level. In fact, an additional energy
level is not gained until after neon, indicating that six electrons are accommodated at this particular level. Analysis of the
photoelectron spectrum of argon provided in Figure 2
strengthens this interpretation and helps make yet another
pattern in the table of data more obvious.
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