ELECTRONS
IN ATOMS
THE SODIUM SPECTRUM AND SUBSHELLS
OBJECTIVES
Bohr's model of the atom used the idea of quantized electronic energy
levels (shells) to explain the sequence of lines in the emission spectrum
of hydrogen. He labelled the shells n = 1, n = 2, etc. in order of
increasing energy. The Bohr model explains the emission spectrum of the
hydrogen atom in terms of electronic transitions between shells.
However, the emission spectrum of sodium is a good deal more complex.
To explain this spectrum, there must be subdivisions of the Bohr shells,
called subsheZZs. The structure of the atom must be more complicated
than Bohr thought.
• Shells
• s, p, and d subsheIls
I
I
I
Wavelength
I
A wave of wavelength À, has a longer
wavelength than a wave of
wavelength À2 if the numerical value
of À, is greater than the value of À2'
i.e. the yellow sodium emission at
589 nm has a longer wavelength than
the green emission at 569 nm.
~---__,[
f subsheIls
[1------
For the shells n = 4 and higher, there
are four subsheIls:
s. p, d, and f. The f
subshell will be ignored for the
moment.
The sodium spectrum
The hydrogen spectrum is simple throughout most of the visible region
(corresponding to the Balmer series for electronic transitions to energy
level n = 2). There is a single red line at 656 nm and then no further line
until 486 nm. This red line originates from electronic transitions from
energy level n = 3 to n = 2.
The sodium emission spectrum is more complex than the very simple
hydrogen emission spectrum. If we look in particular at the region where
wavelengths are greater than 550 nm, we find that, whereas there is a
single line for hydrogen, there are three lines fairly close together for
sodium, i.e.
• a green line at 569 nm;
• a yellow line at 589 nm;
• an orange line at 616 nm.
This observation suggests that there are more energy levels available in
sodium than in hydrogen. In the sodium atom, the shells must be
composed of subshells, each of which is at a different energy. It turns
out that:
• For the shell n = 1, there is only one subshell, labelled Is.
• For the shell n = 2, there are two subsheIls, labelled 2s and 2p. The 2p
subshell is at a higher energy than the 2s subshell.
• For the shell n = 3, there are three subsheIls, labelled 3s, 3p, and 3d.
These subsheIls increase in energy in the order 3s < 3p < 3d.
569 nm-,
5891nm
,--616 nm
Interpreting the sodi~m spec~rum. .
We can explain the .sodium a~om~c emISSIOn
spectrum by assig~l~g each Ime m the spect:um to
an electronic transition on an energy level diagram.
The diagram shows the energy levels corresponding
o the subsheIls in the sodium atom. It is called a
~rotrian diagram, after its originator, Walter
Grotrian.
The most intense line, the yellow emission at
589 nm, is caused by an electronic transition from
the 3p energy level down to the 3s energy level.
Expressed more simply, the yellow line is the result
of a 3p to 3s transition.
The orange line at 616 nm is the result of a Ss to
3p transition. This line is at a slightly longer
wavelength than the yellow line. The frequency is
therefore lower, corresponding to the smaller energy
gap seen on the Grotrian diagram.
The green line at 569 nm is the result of a 4d to 3p
transition. This line is at a slightly shorter
wavelength than the yellow line. The frequency is
therefore higher, corresponding to the larger energy
gap seen on the Grotrian diagram.
At even shorter wavelength are emissions at
515nm (ós to 3p), 498 nm (5d to 3p), and so on. At
even longer wavelength, the 3d to 3p transition
causes an intense line at 819 nm, which is in the
infrared.
The Bohr model of the atom assumes that
electrons are particles moving in orbits around the
nucleus. This model became inadequate when it was
understood in the 1920s that the electron can also
behave as a wave. This startling discovery is
explained in the next spread.
The ideas introduced in the next spread will not
he tested in exams but are essential to explain why
we now focus on the idea of electrón density.
-----5d
6s----5p-----
-----4d
5s-----
4s-----
589 nm
3s----A Grotrian diagram (drawn accurately to scale) showing some of
the electronic transitions responsible for the emission spectrum
of sodium. Notice how the energies of the subsheIls get closer to
each other as the principal quantum number increases.
SUMMARY
Absorption
If continuous radiation
(electromagnetic radiation of all
wavelengths) passes through the
vapour of an element, lines of certain
wavelengths will be absorbed by the
atoms and removed from the
radiation. Looking through a
spectrometer, you would see a series
of black lines where wavelengths
have been absorbed, against the
background of continuous radiation.
This is an absorption spectrum. The
wavelengths of these lines correspond
to the quantized energy taken in by
the atoms to promote electrons from
lower to higher energy levels. For
example, excited hydrogen atoms in
the photosphere of the Sun cause a
dark line at 656 nm in the solar
spectrum.
42
;
spectra
400
I I I '
Wàvelength
3, etc. in .order of
• SubsheIls are labelled s, p, d, and f.
700
600
500
• Shells are labelled 1,2,
. increasing energy.
I nm
The visible region of the sodium emission spectrum. Note the green, yellow, and
orange lines referred to in thè text. There are numerous other lines at wavelengths +
below 550 nm. Lines at wavelengths greater than 616nm are due to sodium tons Na
and not to sodium atoms themselves.
s, p, and d subsheIls - history
The letters used to label the subsheIls came from the descriptions of the
series observed in the sodium spectrum. The series of lines had been given
names that reflected their character in some way, e.g. 'sharp' because they
were sharp. Each series arose from transitions in which the electron fell
from a particular subshell. So it was natural to name the subsheIls after
the associated series. The 'sharp series' arose from transitions in which the
electron fell from an s subshell. The 'principal series' (in which the
electron fell from a p subshell) was so named because these lines also
occurred in the absorption spectrum of sodium. The 'diffuse series' (in
which the electron fell from a d subshell) was so named because of the
characteristic visible difference from the sharp series.
advanced
CHEMISTRY
The test for sodium in the laboratory makes use of the bright
yel/ow line in its spectrum: when heated in a flame, sodium
colours the flame yellow.
PRACTICE
1 What subsheIls can occur in each of the
following shells?
a n", 1
h n", 2
c 11.",3
d n", 4.
2 Look at the Grotrian diagram for sodium, and
arrange the following subsheIls of the sodium
atom in order of increasing energy:
3s,2p, Is, 2s, 3d, 4s,4p, 3p.
3 Look at the Grotrian diagram for sodium, and
estimate the wavelength of the emission
resulting from a 5d to 4p transition.
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advanced CHEMISTRY
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