Physics for Scientists and Engineers, 6e

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Physics for Scientists and Engineers, 6e

• Chapter 42 - Atomic Physics

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A hydrogen atom is in its ground state. Incident
on the atom are many photons each having an
energy of 10.5 eV. The result is that

1. the atom is excited to a higher allowed state
2. the atom is ionized
3. the photons pass by the atom without interaction

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Because the energy of 10.5 eV does not correspond
to raising the atom from the ground state to an
allowed excited state, there is no interaction
between the photon and the atom.
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When electrons collide with an atom, they can
transfer some or all of their energy to the atom.
Suppose a hydrogen atom in its ground state is
struck by many electrons each having a kinetic
energy of 10.5 eV. The result is that

1. the atom is excited to a higher allowed state and
   the electrons pass by the atom without
   interaction
2. the atom is ionized
3. the atom is excited to a higher allowed state and
   the atom is ionized

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A hydrogen atom makes a transition from the n
3 level to the n 2 level. It then makes a
transition from the n 2 level to the n 1
level. Which transition results in emission of
the longest-wavelength photon?

1. the first transition
2. the second transition
3. neither, because the wavelengths are the same for
   both transitions.

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As the electrons strike the atom, they can give
up any amount of energy between 0 and 10.5 eV,
unlike the photons in question 1, which must give
up all of their energy in one interaction. Thus,
those electrons that undergo the appropriate
collision with the atom can transfer 13.606 eV
3.401 eV 10.205 eV to the atom and excite it to
the n 2 state. Those electrons that do not make
the appropriate collision will transfer only
enough kinetic energy to the atom as a whole to
satisfy conservation of momentum in the
collision, without raising the atom to an excited
state.
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The longest-wavelength photon is associated with
the lowest energy transition, which is n 3 to n
2.
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How many possible subshells are there for the n
4 level of hydrogen?

1. 5
2. 4
3. 3
4. 2
5. 1

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The number of subshells is the same as the number
of allowed values of l. The allowed values of l
for n 4 are l 0, 1, 2, and 3, so there are
four subshells.
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When the principal quantum number is n 5, how
many different values of l are possible?

1. 5
2. 6
3. 7
4. 9

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Five values (0, 1, 2, 3, 4) of l, as follows
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And when the principal quantum number is n 5,
how many different values of ml are possible?

1. 5
2. 6
3. 7
4. 9

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Nine different values (4, 3, 2, 1, 0, 1, 2,
3, 4) of ml, as follows
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In the hydrogen atom, the quantum number n can
increase without limit. Because of this, the
frequency of possible spectral lines from
hydrogen also increases without limit.

1. true
2. false

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If the energy of the hydrogen atom were
proportional to n (or any power of n), the energy
would become infinite as n grows to infinity. But
the energy of the atom is inversely proportional
to n2. Thus, as n increases to very large values,
the energy of the atom approaches zero from the
negative side. As a result, the maximum frequency
of emitted radiation approaches a value
determined by the difference in energy between
zero and the (negative) energy of the ground
state.
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Rank the energy necessary to remove the
outermost electron from the following three
elements, smallest to largest

1. helium, neon, argon
2. neon, argon, helium
3. argon, neon, helium
4. argon, helium, neon
5. helium, argon, neon

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The higher the value of Z, the closer to zero is
the energy associated with the outermost electron
and the smaller is the ionization energy.
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In an x-ray tube, as you increase the energy of
the electrons striking the metal target, the
wavelengths of the characteristic x-rays

1. increase
2. decrease
3. do not change

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The wavelengths of the characteristic x-rays are
determined by the separation between energy
levels in the atoms of the target, which is
unrelated to the energy with which electrons are
fired at the target. The only dependence is that
the incoming electrons must have enough energy to
eject an atomic electron from an inner shell.
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It is possible for an x-ray spectrum to show the
continuous spectrum of x-rays without the
presence of the characteristic x-rays.

1. true
2. false

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If the electrons arrive at the target with very
low energy, atomic electrons cannot be ejected
and characteristic x-rays do not appear. Because
the incoming electrons experience accelerations,
the continuous spectrum appears.