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An x ray diffraction image of a protein. The image shows an array of small black dots, arranged in slightly curved rows, on a white background. A white arm extends from the top left to the center of the image, where there is a small white disk. This white disk is the shadow of the beam block, which blocks the part of the incident x ray beam that was not diffracted by the crystal.
X-ray diffraction from the crystal of a protein (hen egg lysozyme) produced this interference pattern. Analysis of the pattern yields information about the structure of the protein. (credit: “Del45”/Wikimedia Commons)


  • Radiation is absorbed and emitted by atomic energy-level transitions.
  • Quantum numbers can be used to estimate the energy, frequency, and wavelength of photons produced by atomic transitions.
  • Atomic fluorescence occurs when an electron in an atom is excited several steps above the ground state by the absorption of a high-energy ultraviolet (UV) photon.
  • X-ray photons are produced when a vacancy in an inner shell of an atom is filled by an electron from the outer shell of the atom.
  • The frequency of X-ray radiation is related to the atomic number Z of an atom.

Conceptual questions

Atomic and molecular spectra are discrete. What does discrete mean, and how are discrete spectra related to the quantization of energy and electron orbits in atoms and molecules?

Atomic and molecular spectra are said to be “discrete,” because only certain spectral lines are observed. In contrast, spectra from a white light source (consisting of many photon frequencies) are continuous because a continuous “rainbow” of colors is observed.

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Discuss the process of the absorption of light by matter in terms of the atomic structure of the absorbing medium.

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NGC1763 is an emission nebula in the Large Magellanic Cloud just outside our Milky Way Galaxy. Ultraviolet light from hot stars ionize the hydrogen atoms in the nebula. As protons and electrons recombine, light in the visible range is emitted. Compare the energies of the photons involved in these two transitions.

UV light consists of relatively high frequency (short wavelength) photons. So the energy of the absorbed photon and the energy transition ( Δ E ) in the atom is relatively large. In comparison, visible light consists of relatively lower-frequency photons. Therefore, the energy transition in the atom and the energy of the emitted photon is relatively small.

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Why are X-rays emitted only for electron transitions to inner shells? What type of photon is emitted for transitions between outer shells?

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How do the allowed orbits for electrons in atoms differ from the allowed orbits for planets around the sun?

For macroscopic systems, the quantum numbers are very large, so the energy difference ( Δ E ) between adjacent energy levels (orbits) is very small. The energy released in transitions between these closely space energy levels is much too small to be detected.

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What is the minimum frequency of a photon required to ionize: (a) a He + ion in its ground state? (b) A Li 2+ ion in its first excited state?

For He + , one electron “orbits” a nucleus with two protons and two neutrons ( Z = 2 ). Ionization energy refers to the energy required to remove the electron from the atom. The energy needed to remove the electron in the ground state of He + ion to infinity is negative the value of the ground state energy, written:
E = −54.4 eV .
Thus, the energy to ionize the electron is +54.4 eV.
Similarly, the energy needed to remove an electron in the first excited state of Li 2+ ion to infinity is negative the value of the first excited state energy, written:
E = −30.6 eV .
The energy to ionize the electron is 30.6 eV.

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Questions & Answers

how does colour appear in thin films
Nwjwr Reply
in the wave equation y=Asin(kx-wt+¢) what does k and w stand for.
Kimani Reply
derivation of lateral shieft
James Reply
how are you?
I'm fine
total binding energy of ionic crystal at equilibrium is
All Reply
How does, ray of light coming form focus, behaves in concave mirror after refraction?
Bishesh Reply
Refraction does not occur in concave mirror. If refraction occurs then I don't know about this.
What is motion
Izevbogie Reply
Anything which changes itself with respect to time or surrounding
and what's time? is time everywhere same
how can u say that
do u know about black hole
Not so more
Radioactive substance
These substance create harmful radiation like alpha particle radiation, beta particle radiation, gamma particle radiation
But ask anything changes itself with respect to time or surrounding A Not any harmful radiation
explain cavendish experiment to determine the value of gravitational concept.
Celine Reply
For the question about the scuba instructor's head above the pool, how did you arrive at this answer? What is the process?
Evan Reply
as a free falling object increases speed what is happening to the acceleration
Success Reply
of course g is constant
acceleration also inc
which paper will be subjective and which one objective
normal distributiin of errors report
normal distribution of errors
photo electrons doesn't emmit when electrons are free to move on surface of metal why?
Rafi Reply
What would be the minimum work function of a metal have to be for visible light(400-700)nm to ejected photoelectrons?
Mohammed Reply
give any fix value to wave length
40 cm into change mm
Arhaan Reply
40cm=40.0×10^-2m =400.0×10^-3m =400mm. that cap(^) I have used above is to the power.
i.e. 10to the power -2 in the first line and 10 to the power -3 in the the second line.
there is mistake in my first msg correction is 40cm=40.0×10^-2m =400.0×10^-3m =400mm. sorry for the mistake friends.
40cm=40.0×10^-2m =400.0×10^-3m =400mm.
this msg is out of mistake. sorry friends​.
what is physics?
sisay Reply
why we have physics
Anil Reply
because is the study of mater and natural world
because physics is nature. it explains the laws of nature. some laws already discovered. some laws yet to be discovered.
physics is the study of non living things if we added it with biology it becomes biophysics and bio is the study of living things tell me please what is this?
physics is the study of matter,energy and their interactions
all living things are matter
why rolling friction is less than sliding friction
thanks buvanas
is this a physics forum
Physics Reply
Practice Key Terms 5

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Source:  OpenStax, University physics volume 3. OpenStax CNX. Nov 04, 2016 Download for free at http://cnx.org/content/col12067/1.4
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