3.11 Spectroscopy and the Electromagnetic Spectrum

1. AArrow 1
   A student who thinks infrared radiation changes the rotational level of a molecule picks this arrow, between two rotational levels. Transitions that change only the rotational level are associated with microwave radiation.
2. BArrow 2
   A student who thinks every absorption moves an electron to a higher level picks this arrow, between the two electronic levels. Electronic transitions are associated with ultraviolet/visible radiation.
3. CArrow 3 Correct
   Infrared radiation is associated with transitions between vibrational levels. This arrow points up, as it must for absorption, from one vibrational level to the next within the same electronic level.
4. DArrow 4
   A student who confuses the directions of absorption and emission picks this arrow, which is between vibrational levels but points down. A molecule that absorbs a photon moves to a higher level.

CED 3.11.A.1 · Read this in Fix

1. AThe bonds vibrate with more energy than they did before.
   A student who thinks molecules respond to light of any kind only by vibrating picks this. The photon is in the ultraviolet region, which is associated with electronic transitions; vibrational transitions are associated with infrared radiation, at much longer wavelengths on the scale.
2. BAn electron moves to a higher electronic energy level. Correct
   The wavelength 2.0 × 10⁻⁷ m lies in the ultraviolet region of the diagram. Ultraviolet/visible radiation is associated with transitions in electronic energy levels; in absorption the electron moves to a higher level and remains in the molecule.
3. CThe molecule rotates with more energy than before.
   A student who thinks turning a whole molecule takes the highest-energy radiation picks this. Rotational transitions are associated with microwave radiation, at the long-wavelength end of the scale, not with ultraviolet radiation.
4. DAn electron is removed completely from the molecule.
   A student who carries over the picture of photoelectron spectroscopy picks this. Ultraviolet/visible absorption is associated with an electron moving to a higher electronic energy level within the molecule, not leaving it.

Working On the scale, 2.0 × 10⁻⁷ m lies between 10⁻⁷ m and 4 × 10⁻⁷ m, inside the ultraviolet region. Ultraviolet/visible radiation is associated with transitions in electronic energy levels, so an electron moves to a higher electronic energy level.

CED 3.11.A.1.iii · Read this in Fix

1. A1.5 × 10⁻⁷ m
   A student who thinks turning a whole molecule takes the highest-energy radiation picks the shortest wavelength, in the ultraviolet region. Ultraviolet radiation is associated with electronic transitions.
2. B5.5 × 10⁻⁷ m
   A student who thinks visible light is the only radiation that molecules absorb in spectroscopy picks the wavelength in the visible region. Visible radiation is associated with electronic transitions.
3. C4.7 × 10⁻⁶ m
   A student who thinks infrared radiation changes the rotational level of a molecule picks the wavelength in the infrared region. Infrared radiation is associated with vibrational transitions.
4. D2.6 × 10⁻³ m Correct
   Transitions between rotational levels are associated with microwave radiation, and 2.6 × 10⁻³ m (2.6 mm) is the only choice that lies in the microwave region of the diagram.

Working A transition between rotational levels is associated with microwave radiation. On the scale the microwave region begins at 10⁻³ m, so the wavelength must be longer than 10⁻³ m. Of the choices, only 2.6 × 10⁻³ m lies in the microwave region; 4.7 × 10⁻⁶ m is infrared, 5.5 × 10⁻⁷ m is visible and 1.5 × 10⁻⁷ m is ultraviolet.

CED 3.11.A.1.i · Read this in Fix

1. AA transition between two vibrational energy levels of the molecules Correct
   The absorption is at about 3.5 μm (3.5 × 10⁻⁶ m), in the infrared region. Infrared radiation is associated with transitions in molecular vibrational levels: the H–Cl bond stretches and compresses with more energy.
2. BA transition between two rotational levels, with no change in vibration
   A student who thinks infrared radiation changes only the rotational level of a molecule picks this. Transitions that change only the rotational level are associated with microwave radiation, at wavelengths hundreds of times longer than 3.5 μm.
3. CA transition of an electron to a higher electronic energy level
   A student who thinks every absorption moves an electron to a higher level picks this. Electronic transitions are associated with ultraviolet/visible radiation, at wavelengths below about 0.7 μm.
4. DAn increase in the speed at which the molecules travel, with no transition
   A student who thinks absorbed infrared 'heat' directly makes molecules travel faster picks this. The photon is absorbed in a transition between vibrational levels; the gas warms only afterward, as that energy is shared in collisions.

Working The peak is at about 3.5 μm = 3.5 × 10⁻⁶ m, which is longer than visible wavelengths (up to 7 × 10⁻⁷ m) and shorter than microwave wavelengths (from about 10⁻³ m), so it is in the infrared region. Infrared radiation corresponds to transitions between a molecule's vibrational levels.

CED 3.11.A.1.ii · Read this in Fix

1. AThe intensity (brightness) of the beam
   A student who thinks a more intense beam causes higher-energy kinds of transition picks this. Intensity determines how many photons arrive, and so how many molecules absorb, not which kind of transition occurs.
2. BThe wavelength of the radiation in the beam Correct
   Each kind of transition is associated with a region of the spectrum, and a region is a range of wavelengths. Knowing the wavelength identifies the region and so the kind of transition.
3. CThe speed at which the radiation travels
   A student who thinks different kinds of radiation travel at different speeds picks this. All electromagnetic radiation travels at the same speed in a vacuum, so the speed does not distinguish the regions.
4. DThe length of time the beam is switched on
   A student who thinks energy from many photons builds up in a molecule until a higher-energy transition occurs picks this. A longer exposure allows more absorptions of the same kind; it does not change the kind of transition.

Working The kind of transition is related to the spectral region of the radiation (microwave: rotational; infrared: vibrational; ultraviolet/visible: electronic), and the region is identified by the wavelength (or frequency). Intensity and exposure time affect only how many photons are absorbed, and all electromagnetic radiation travels at the same speed.

CED 3.11.A.1 · Read this in Fix

1. ADiagram 1
   A student who thinks microwave radiation makes bonds vibrate picks the diagram of the atoms moving toward and away from each other. Vibrational transitions are associated with infrared radiation.
2. BDiagram 2
   A student who thinks every absorption moves an electron to a higher level picks the diagram of an electron moving up. Electronic transitions are associated with ultraviolet/visible radiation.
3. CDiagram 3
   A student who thinks absorbed radiation directly makes molecules travel faster picks the diagram of the whole molecule moving. The microwave photon changes the rotational level of the molecule.
4. DDiagram 4 Correct
   Microwave radiation is associated with transitions in molecular rotational levels: the molecule turns end over end with more energy.

CED 3.11.A.1.i · Read this in Fix

1. AIt holds for CO only: a vibration needs a bond between atoms, which Ar atoms lack. Correct
   A molecular vibration is the motion of bonded atoms toward and away from each other. CO molecules have a bond and vibrational energy levels, and CO absorbs infrared radiation. Ar is monatomic, so its atoms have no vibrational levels.
2. BIt holds for both: Ar atoms vibrate back and forth, just as the atoms in a CO molecule do.
   A student who thinks single atoms vibrate in the same way as the atoms in a molecule picks this. A vibrational transition involves atoms moving relative to the atoms they are bonded to; an Ar atom has no bond.
3. CIt fails for both: infrared is absorbed when electrons, not atoms, move to higher levels.
   A student who thinks every absorption moves an electron to a higher level picks this. Infrared radiation is associated with vibrational transitions, which CO undergoes; electronic transitions are associated with ultraviolet/visible radiation.
4. DIt holds for Ar only: the bond in CO keeps its atoms a fixed distance apart, so they do not vibrate.
   A student who thinks a covalent bond is rigid picks this. The atoms in CO move toward and away from each other about an average distance, and it is Ar, with no bond, that has no vibrational levels.

CED 3.11.A.1.ii · Read this in Fix

1. AIt is adequate: ultraviolet radiation makes the spring vibrate more violently than infrared does, so all three are shown.
   A student who thinks higher-energy radiation just makes the same motion more vigorous picks this. Ultraviolet radiation is associated with a different kind of change, a transition in electronic energy levels, which the model cannot show.
2. BIt fails: absorption in all three regions involves electrons moving between levels, and the model has no electrons.
   A student who thinks every absorption moves an electron to a higher level picks this. Microwave and infrared absorption are associated with rotation and vibration, which the model does show.
3. CIt is limited: it shows the motions behind microwave and infrared absorption but no electron levels for ultraviolet. Correct
   Turning end over end represents rotation (microwave) and the balls moving on the spring represents vibration (infrared). Ultraviolet radiation is associated with transitions in electronic energy levels, and the model has nothing that represents electrons or their energy levels.
4. DIt is limited: the spring wrongly suggests that a bond stretches, so only the rotation linked to microwaves is shown correctly.
   A student who thinks a covalent bond is rigid picks this. Bonded atoms do move toward and away from each other, and transitions between the vibrational levels are associated with infrared radiation; the spring is the appropriate part of the model.

CED 3.11.A.1 · Read this in Fix

1. AElectrons in dye molecules fall to lower energy levels and emit light of the color that is seen.
   A student who thinks a colored substance gives out light of its color picks this. In white light the color comes from the visible wavelengths that remain after the dye has absorbed the others.
2. BElectrons in dye molecules move to higher energy levels as some visible light is absorbed. Correct
   Visible radiation is associated with transitions in electronic energy levels. The dye absorbs some visible wavelengths in electronic transitions, and the wavelengths that are not absorbed pass through and give the solution its color.
3. CBonds in dye molecules vibrate with more energy as some visible light is absorbed.
   A student who thinks molecules respond to light only by vibrating picks this. Visible radiation is associated with electronic transitions; vibrational transitions are associated with infrared radiation.
4. DBonds in dye molecules break and give out their stored energy as light of the color that is seen.
   A student who thinks energy is stored in bonds and released when they break picks this. The dye molecules stay intact, and breaking bonds would require energy; the color comes from the visible wavelengths that are not absorbed.

CED 3.11.A.1.iii · Read this in Fix

1. AAn electron moves from a higher electronic energy level to a lower one. Correct
   Visible radiation is associated with transitions in electronic energy levels. In emission the molecule loses energy, so the electron moves from a higher level to a lower level and the photon carries away the difference.
2. BAn electron is promoted from a lower electronic energy level to a higher one.
   A student who confuses the directions of absorption and emission picks this. Moving an electron to a higher level requires energy, as in absorption; emission accompanies a move to a lower level.
3. CA bond goes from a higher vibrational energy level to a lower one.
   A student who thinks molecules respond to light only through the vibration of their bonds picks this. Transitions between vibrational levels are associated with infrared radiation, not visible light.
4. DA bond breaks, and the energy that was stored in it is given out.
   A student who thinks energy is stored in bonds and released when they break picks this. Breaking a bond requires energy; the visible photon is emitted when an electron moves to a lower electronic energy level.

CED 3.11.A.1.iii · Read this in Fix

1. AMeasuring the wavelengths of microwave radiation that each gas absorbs
   A student who thinks microwave radiation makes bonds vibrate picks this. Microwave radiation is associated with rotational transitions, so it gives information about how the molecules rotate.
2. BMeasuring the wavelengths of ultraviolet radiation that each gas absorbs
   A student who thinks higher-energy radiation just makes bonds vibrate more violently picks the most energetic region. Ultraviolet radiation is associated with electronic transitions.
3. CMeasuring the wavelengths of infrared radiation that each gas absorbs Correct
   Infrared radiation is associated with transitions in molecular vibrational levels, so the infrared wavelengths that each gas absorbs show the energies of its bond vibrations.
4. DMeasuring the wavelengths of visible radiation that each gas absorbs
   A student who thinks visible light is the only radiation that molecules absorb in spectroscopy picks this. Visible radiation is associated with electronic transitions; vibrational transitions are studied with infrared radiation.

CED 3.11.A.1.ii · Read this in Fix

1. AThe photons raise CO₂ molecules to higher rotational levels, and collisions then spread this energy to other molecules.
   A student who thinks infrared radiation changes the rotational level of a molecule picks this. Infrared radiation is associated with vibrational transitions; rotational transitions are associated with microwave radiation.
2. BThe photons raise electrons in CO₂ to higher energy levels, and the electrons release heat as they fall back down.
   A student who thinks every absorption moves an electron to a higher level picks this. Infrared radiation is associated with vibrational transitions, not electronic transitions.
3. CThe photons break the bonds in CO₂ molecules, and the energy that was stored in the bonds is released as heat.
   A student who thinks energy is stored in bonds and released when they break picks this. The CO₂ molecules stay intact, with more vibrational energy; breaking bonds would require energy, not release it.
4. DThe photons raise CO₂ molecules to higher vibrational levels, and collisions then spread this energy to other molecules. Correct
   Infrared radiation is associated with transitions in vibrational levels. The vibrationally excited CO₂ molecules collide with neighboring molecules and pass the energy on, which raises the average kinetic energy of the molecules of the air and so its temperature.

CED 3.11.A.1.ii · Read this in Fix