These diagrams show how the order of atomic energy levels
corresponds to the arrangement of elements in the periodic table.
With each increment in atomic number one electron is added to the
energy diagram. Electrons are added from the lowest level. Each
level can accommodate at most an electron pair at which point that
level is considered “full” (Pauli exclusion principle). Once a level
(or a set of degenerate levels) is ﬁlled, a jump in energy is required to
reach the energy of the next available level. This jump corresponds to
movement to a new section of the periodic table (an exception is the
ﬁlling of the 1s level, in which case there is a jump from the upper left
corner to the upper right corner). !
The most signiﬁcant point
is this: the degeneracy in
the energy levels matches
the number of elements in
each section of the periodic
table (note the color coding
between the two diagrams).!