The Hydrogen emission series
The hydrogen emission spectrum
The electron in the ground state energy level of the hydrogen atom receives
energy in the form of heat or electricity and is promoted to a higher energy
level.
It cannot remain at a higher level (excited state) for very long, and falls
back to a lower level.
When the electron falls back down (relax) it must lose the energy difference
between the two energy levels. This loss of energy is performed by releasing
electromagnetic energy in the form of infrared, visible light or ultraviolet
radiation.
Movement of electrons between the shells is called electron transitions.
The Balmer series
When electron transitions take place the energy emitted can be detected and
its wavelength measured. This provides information about the relative energies
of the shells.
In the hydrogen atom (the simplest case with only one electron to 'jump'
between shells) the energy emitted appears in several series of lines, each
series corresponding to electrons falling back to different levels. This is
shown in the diagram below.
The Lyman series corresponds to transitions between the higher shells and
the lowest shell (ground state). The energy of these transitions produces
radiation in the ultra-violet region of the spectrum
The energy shells are usually given a letter 'n' to describe the specific
energy level. The lowest level is n=1, the second level is n=2 etc.
Transitions from higher shells (n>2) to n=2 produce radiation in the visible
region of the spectrum. It can be seen by splitting the light using a prism
or diffraction grating and projecting it onto a screen.
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Worked examples
Q133-01 The lines in the series
that appears in the visible region of the hydrogen spectrum are caused by transitions
to which energy level?
Answer
|
The series are, in order of decreasing energy:
- The Lyman series - appears in the ultra violet region (transitions
down to n=1)
- The Balmer series - appears in the visible region (transitions
down to n=2)
- The Paschen series appears in the infra-red region (transitions
down to n=3)
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Q133-02 Which phrase correctly
completes the sentence. The lines in a series in the hydrogen spectrum...
- Converge towards the high energy side
- Diverge towards the high energy side
- Are evenly spaced
- Converge towards the IR region
Answer
|
The easiest way to remember this is to think about the ionization
as the highest energy transition, from n=1 to n=∞. Convergence
occurs towards this transition and so it is towards the highest energy
end of each series.
Correct response - A
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Q133-03 An electron can move
from one orbital of a hydrogen atom to another. In which movement will the photon
of highest energy be emitted?
- 2p ---> 1s
- 1s ---> 2p
- 3s ---> 2s
- 2s ---> 3s
Answer
|
The largest transitions occur in the ultraviolet series down to n=1
(the ground state).
Correct response - A
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Q133-04 In which electronic
state would a hydrogen atom be able to absorb a photon of electromagnetic radiation
but not be able to emit a photon? (HL only)
- 1s1
- 2s1
- 3p1
- 4d1
Answer
|
Photons of energy cannot be emitted from the ground state as there
is no lower state for the electron to go to .
1s1 represents the lowest orbital of hydrogen therefore this is the
correct response.
Correct response - A
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Q133-05 The emission spectrum
of the hydrogen atom
- is caused by the removal (ionization) of the electron.
- is continuous because the electron can emit any frequency of light during
a transition.
- is caused by the absorption of light at characteristic frequencies the
electron to be excited into higher energy levels.
- is a result of the excited electron undergoing transitions to lower energy
levels and emitting photons of light at specific frequencies.
Answer
|
Transitions of electrons to lower levels emit the extra energy in
the form of electromagnetic radiation.
Correct response - D
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Q133-06 Which transition is
associated with the largest change in energy in the hydrogen atom?
- n = 5 to n = 3
- n = 2 to n = 1
- n = 3 to n = 2
- n = 4 to n = 2
Answer
|
The largest energy transitions are down to the n=1 level.
Correct response - B
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Q133-07 According to the evidence
supplied by the hydrogen spectrum, which transition involves the greatest energy
difference, n=2 to n=1 or n=∞ to n=2?
Answer
|
The Lyman series (transitions to n=1) and the Balmer series (transitions
to n=2) do not overlap. This means that the energy involved in each
one of the transitions down to n=1 is greater than any of the transitions
to n=2.
The greatest energy difference wil be due to the transition n=2
to n=1
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Q133-08 What name is given
to an electron that is in a higher energy level than it should be?
- transition electron
- excited electron
- relaxed electron
- ground state electron
Answer
|
The term 'excited' is used to indicate that the electron has more
energy than it should have. It is in an excited state.
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Q133-09 Which transition corresponds
to the ionization energy of the hydrogen atom?
|
For ionization to occur the electron must leave the (gaseous) atom.
M(g) → M(g)+
+ 1e
The ionization energy is always measured from the ground state, so
the transition is from n = 1 to n = infinity. This is expressed as:
n = 0 → n = ∞
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Q133-10 The term 'relaxation'
refers to which of the following in the context of the hydrogen spectrum:
- The dark gap between the different series of the hydrogen spectrum.
- The difference in energy between the lines in the Balmer and Lyman series
- The movement of an electron from a higher to a lower energy level
- The movement of an electron from a lower to a higher energy level
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To relax, in the context of spectra, means to move from a higher
energy level to a lower energy level.
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