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Evidence for the Bohr model: line spectra
- Emission spectra of atoms emissions of light from atoms that
have been provided withenergy such as heat, light or
electricity
- If atoms are subjected to large amounts of energy from eat,
light or electricity, electrons can
change energy levels- Electrons jump to energy levels further
from the nucleus than they would usually occupy =
excited state- When electrons return to the ground state this
extra energy is releases in the form of light- Electrons make
specific jumps depending on the energy levels involved, therefore
the light
released has a specific wavelength- The emitted light, a line
spectrum, looks like a series of coloured lines on a black
background
as some of the emissions may be radiation of a wavelength that
is not visible to the nakedeye
- Model enabled him to predict correctly an emission line that
had previously not beendetected
- 1885 Johann Balmer was able to calculate the wavelengths of
the four lines in the hydrogenemission spectrum
- Energy of these lines corresponds to the difference in
energies between outer electronshells and the second electron shell
of hydrogen group of lines became Balmer series
- Ultraviolet region of the spectrum transitions from higher
energy levels to the 1 st shell- Infrared region of the spectrum
transitions from the higher energy levels to the 3 rd shell- Each
line in the hydrogen emission spectrum corresponds to a transition
between two
energy levels of the hydrogen atom- Lines become closer to each
other as the wavelength decreases- Bohrs model explained the
increasing closeness of the emission lines in terms of the
decreasing difference between the energies of the shells as
their distance from the nucleusincreased
- Lines becoming closer together as their energy increases
because the energy of the shells isincreasing by diminishing
amounts
- Shell 4 is closer to shell 3 than shell 3 is to shell 2 and
shell 2 is to shell 1- At the outermost edge of the atom, energies
of the electron shells are so close they are
indistinguishable from each other highest energy of each series
of lines in the emissionspectrum, they merge into a continuum =
convergence
Section 1.3 Exercises
1. Outline the model of electron movement around the nucleus
proposed by Bohr. Niel Bohrproposed that electrons moved around the
nucleus in fixed energy levels called shells.
2. Identify the electron that will have the greater energy: an
electron in shell 1 or one in shell2. Explain your answer. An
electron in shell 2 will have the greatest energy as the
shellsclose to the nucleus are of low energy.
3. Draw a Bohr diagram for a magnesium atom, indicating the
number and position of eachsubatomic particle.
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4. A) Determine the electron arrangement for each of the
following elements.31
15P 2, 8, 519
9F 2, 740
18Ar 2, 8, 839
19K 2, 8, 8, 1
B) Determine the electron arrangement for each of the following
ions.24
12Mg2+ 2, 8
199F
- 2, 832
16S2- 2, 8, 8
3919K
+ 2, 8, 85. A) State how many electrons there are in the valence
shell of each of the following atoms.
126C 4
2713Al 3
199F 7
B) State how many electrons there are in the valence shell of
each of the following ions?27
13Al3+
83115P
3- 835
17Cl- 8
6. Compared to the visible region of the electromagnetic
spectrum, state where you wouldfind: A) The ultraviolet region
contains waves with shorter wavelengths than visible light and
therefore left of the visible light region on the emission
spectrumB) The infrared region contains waves with longer
wavelengths than visible light and
therefore right of the visible light region of the emission
spectrum7. Consider the emission spectrum of hydrogen. Identify the
electron shell to which electrons
are falling for the following series. A) The Balmer series 2B)
The Lyman series 1C) The Paschen series 3
8. Draw a labelled flowchart to describe how an emission
spectrum is produced for anelement such as hydrogen.
9. Explain how each of the four lines in the visible region of
the hydrogen emission spectrumis related to an energy level in
hydrogen. Each line in the hydrogen emission spectrumcorresponds to
a transition between two energy levels of the hydrogen atom as
afterelectrons reach an excited state through being heated, jumping
to an energy level furtherfrom the nucleus than they would usually
occupy the energy is released in a form of light.The electrons make
specific jumps depending on the energy levels involved therefore
thelight released has a specific wavelength.
10. Predict which is larger: the energy released by an electron
transition between shell 6 andshell 5 or the energy released by an
electron transition between shell 4 and shell 3. Explainyour
answer. More energy will be released by an electron transition
between shell 6 andshell 5 as the lines become closer together
increasing their energy because the energy of theshells is
increasing by diminishing amounts
11. The term convergence describes the decreasing distance
between the lines in an emissionspectrum as the energy of a set of
spectral lines increases. Explain why this occurs. Eachline in the
hydrogen emission spectrum corresponds to a transition between two
energy
levels of the hydrogen atom therefor the lines become closer to
each other as thewavelength decreases
