Nuclear Chemistry
Brown, LeMay Ch 21AP Chemistry
2
21.1: Radioactivity• Result of unstable nuclei
Nucleons: particles in the nucleus, n0 and p+
Radioisotopes: atoms that containing radioactive nuclei (or radionuclides)
Nuclear reactions or equations: express products of radioactive decay, fusion, or fission
Radioactive decay: process in which a radionuclide spontaneously decomposes
3
Most common types of radioactive decayType Symbol Description Travels in air…
Ex:
alpha
or
energized He nucleusHe
4 2+2
A few cm; cannot penetrate
human skin
U23892
He42
Th234
90+
4
Alpha decay
5
Ex:
Same as:
beta or
High energy electron
e0
-1
~300 cm; can penetrate skin, but
rarely
I13153
Xe131
54e
0-1
+
n 1 0
p 1
1e
0-1
+
A neutron converts to a proton and
electron
6
Ex:
gamma
orphoton
0
0
Very far; can be stopped by ~5 cm of
Pb
Pu244
94+
0 0Pu
24494
*
• Represents energy emitted (i.e., radiation) when nucleons in an unstable radionuclide reorganize to become more stable
• Usually not written in a nuclear reaction.
7
Ex:
Same as:
positron • Antimatter (positively charged) e; collides with e- and both are annihilated as gamma rays are created
e0
1
C116
B 11
5e
0 1
+
p 1 1
n 1
0e
0 1
+
A proton converts to a positron and
neutron
8
9
Ex:
Same as:
Electron capture
• Capture of inner shell e- by nucleus
Rb8137
Kr 81 36
+
p 1 1
n 1
0
A proton and electron convert to a
neutron
e0
-1
e0
-1
e0
-1+
10
21.2: Nuclear stabilityStrong nuclear force: pulls nucleons together to
form nuclei (actually acts on quarks)* Weak nuclear force: responsible for changes
in flavor of quarks
• Nuclei become unstable (radioactive) if the neutron-to-proton ratio “strays” too far from “normal range”
• * Nuclear shell model: when p and n fill nuclear shells, atoms are unusually stable:“Magic numbers” 2, 8, 20, 28, 50, 82, 126
11
A radionuclide will decay until a stable ratio exists:
• If too many n, n will be converted to p by emission.
• If too few n, p will be converted to n by positron emission or electron capture.
• Nuclei with p ≥ 84 undergo emission 1
12
21.4: Rates of DecayHalf-life (t½):• Time for ½ a radioactive (i.e., having an
unstable p/n ratio) material to decay (form 2 or more stable atoms)
1/21/2 t0.693
t2ln k
13
21.6: Mass-energy relationshipsE = m c2
(mass in kg)
Mass → energy Mass lost during radioactive decay is released as
energy
Energy → mass Mass defect (m): mass difference between a nucleus
and its constituent nucleons; the nuclear bonding energy must be added to a nucleus to break it into its nucleons
When energy is added, the nucleons separate and gain mass
14
21.7 & 21.8: Fission & FusionFission: splitting of a nucleus; some mass is
lost, which results in release of energy (ex: nuclear power plants, “atomic” bombs)
Ba +13956 Kr +94
36 3 n + energy10 U235
92n + 10
15
Fusion: combination of 2 nuclei; some mass is lost, which results in release of energy (ex: stars, “H” bombs)
H + 31
H
21
He +42
n10
+ energy
16
* Fundamental Particles and the Standard Model of the Atom
6 flavors of quarks:
Inc Mass
↓
Q = -1/3 Q = 2/3
Down (d) Up (u)
Strange (s)Charm (c):
discovered 1974 at 1.5 GeV
Bottom (b): discovered 1978 Top (t)
17
* Ordinary matter is made of:
p+: u-u-d quark tripletn0: u-d-d quark triplete-: one of 6 leptons
decay: d quark in a n changes into u quark, making a p