Nuclear structure and NMR
Inside the Nucleus: Protons and
Neutrons (Nucleons)
atomic mass
number (#
nucleons)
atomic
number (#
protons)
𝐴
𝑍S
Element’s
Symbol
• Elements with same 𝑍 but
different 𝐴 are called
isotopes. For example:
• 126C and 146C are isotopes of
carbon
Atom
Atomic nucleus
• Proton: Charge = +𝑒, mass
𝑚𝑝 = 1.672 × 10−27 kg
• Neutron: Charge = 0, mass
𝑚𝑛 = 1.675 × 10−27 kg
• Nucleon: collective term for
protons and neutrons
2
What Holds the Nucleus Together?
• Could it be electric
force?
+
+
+
• No!
• Gravity?
• Too weak; can’t
overcome electric
repulsion
• Answer: The Strong
Nuclear force:
+
+
+
Balance between
electric and strong
nuclear forces
proton
neutron
3
Turning one of the protons in an atomic nucleus into a neutron
will __________ the total electric repulsion within the nucleus
and will __________ the total attraction within the nucleus due
to the strong nuclear force.
A. increase; decrease
B. increase; not change
C. decrease; decrease
D. decrease; not change
4
Nuclear Stability
• For first 20 elements in
periodic table, have
(about) an equal number
of neutrons and protons
in stable nuclei
• Farther in periodic table,
with more protons, need
an increasing number of
neutrons to act as “glue”
to hold the nucleus
together
5
Announcements
• Today’s Lab is NOT optional, and is graded, but
we won’t do an experiment.
• Equation sheet will slight modification, we’ll
go over that in lab today
• WS due tomorrow no later than 1:00.
• If you’ve been contacted about missing PSs
please make sure you respond accordingly.
• Photo today!
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Radioactive Decay
Conservation of electric charge and nucleon number
𝛽 − decay: 10n → 11p + 00e−
𝛽+
decay:
1
1p
→
1
0 +
0n + 0e
Electron capture: 11p + 00e− → 10n
𝛼 decay: 𝐴𝑍P →
𝐴−4
𝑍−2𝐷
+ 42He
7
The nucleus of an isotope of Polonium 214
84Po emits an alpha
particle 42He . What is the other daughter nucleus in the decay
reaction?
A.
212
84Po
B.
210
82Po
C.
212
80Pb
D.
210
82Pb
8
Question: Why is there instability with
too many neutrons in the nucleus?
•
•
•
•
Neutrons each occupy one orbital
in the nucleus, in order of
increasing energy (like electrons
in an atom)
The number of available neutron
orbitals increases as the atomic
number increases
When neutron number gets too
high, the final neutrons are at
very high energy, so are very
weakly bound within nucleus
Usual result: beta decay
9
Nuclear Magnetic Resonance (NMR)
B
Magnetic
moment m
Atomic nucleus
•
B
𝐸 = +𝜇𝐵
(anti-aligned)
Energy levels
(𝐵 ≠ 0)
E  2 B
𝐸 = −𝜇𝐵
(aligned)
ZEEMAN EFFECT
Energy level
(𝐵 = 0)
Idea of NMR:
– send photons with energy 𝐸𝑝ℎ𝑜𝑡𝑜𝑛 = 2𝜇𝐵 to drive transition between split
energy levels
– Detect emitted photons from atoms that resonate (have energy gaps matching
your initial photons)
•
•
Each type of atom has its characteristic magnetic moment, for example:
For 11H,𝜇 = 1.41 × 10−26 J/T, and for 136C,𝜇 = 3.55 × 10−27 J/T
10
For Example: Ethanol
In actual NMR use, further shifts due to specific electronic
environments of atoms (“chemical shift”)
Ethanol
B = constant
Frequency of infrared light
11
The reason that specific elements can be identified using NMR
spectroscopy is that _____________ .
A. Different nuclei have different magnetic moments
B. Each type of atom gives off a characteristic discrete
visible spectrum
C. Different atoms have different number of electrons
12
Magnetic Resonance Imaging (MRI):
Using NMR to map tissues
• send photons with energy
𝐸𝑝ℎ𝑜𝑡𝑜𝑛 = 2𝜇𝐵 to drive
transition between split
energy levels
• Vary the magnetic field
spatially in desired MRI
region using magnetic field
gradients
• Scan radio frequency (RF)
and measure NMR signal
strength (indicating
concentration of H nuclei)
within the desired region
Energy levels
(𝐵 ≠ 0)
E  2 B
Energy level
(𝐵 = 0)
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