Transcript I. The Nucleus
CHAPTER 25 Nuclear Chemistry
I. The Nucleus -Terms (p. 798-820)
Ionizing Radiation
Radiation is a form of energy transferred by waves or atomic particles Ionizing Radiation is any radiation with high enough energy to create ions (by knocking electrons out of atoms) like UV, X, gamma, and cosmic rays There are both natural sources of radiation (unstable nuclei and stars) and human created sources
Zone of Stability
Stable nuclei exist within the “zone of stability” seen on the graph…not always a 1:1 ratio of p+ to no Outside this range, nuclei are unstable and will decay (disintegrate) into new nuclei
Definitions
Nucleons = particles in nucleus (p + and n Nuclide refers to the nucleus of an atom 0 ) Nuclear Reactions involve transmutation where one element become another. Radioactive Decay is the when unstable nuclei spontaneous lose energy by emitting ionizing particles; as this changes the nucleus of the atom, this also changes the type of element
Alpha Decay Process
Daughter Nuclide Np-237 Th-234 Ra-228 Rn-222 Parent Nuclide Am-241 U-238 Th-232 Ra-226
Alpha Particle (Helium Nucleus) (4.00147 amu)
A. Mass Defect
The mass defect describes the mass lost during the formation of nuclei Difference between the mass of an atom and the mass of its individual particles.
4.00260 amu Mass of atom 4.03298 amu Mass of particles
B. Nuclear Binding Energy
Energy released when a nucleus is formed from nucleons. This contributes to the loss in mass of nucleus, described by E = mc 2 .
High binding energy = stable nucleus.
E = mc
2 E: energy (J) m: mass defect (kg) c: speed of light (3.00×10 8 m/s)
B. Nuclear Binding Energy
Iron (Fe) is the most stable nucleus!!
Unstable nuclides are radioactive and undergo radioactive decay.
CHAPTER 25 Nuclear Chemistry
II. Radioactive Decay (p. 798-820)
Types of Spontaneous Radiation
Greek symbol
Alpha particle (
)
helium nucleus
2 4 He
charge
2+
stopped by… paper
Beta particle
or
-
electron
Positron
+
positron
0 1
Gamma (
)
high-energy photon
e -1 e 0 0
γ
0 1 1+ 0
wood Lead or concrete
Other Radiation particles
proton p+
Greek symbol
1 1 p
charge
+1
neutron n 0 0 1 n 0
How does an electron get emitted from the nucleus?
Basically a neutron splits into a proton which stays in the nucleus and an electron is emitted (
decay)
n & p in nucleus
+ + +
n is “really” like a p and e together
+ +
n converted to a proton and an e is emitted
Transmutation Reactions
I Alpha Emission 238 U
92 234 90 Th
2 4 He
parent nuclide daughter nuclide alpha particle
Numbers must balance on both sides of arrow!!
238amu on left = (234 + 4amu) 92 is nucl chrg on left = 90 + 2 on right
B. Nuclear Decay
II Beta Emission 131 53 I
131 Xe
54 0 -1 e
III Positron Emission 38 19 K
38 Ar 18
0 1 e
electron positron *a proton 1 p is not the same as a positron 0 e
B. Nuclear Decay
IV Electron Capture 106 47 Ag
0 -1 e
106 Pd 46
electron
B. Nuclear Decay
V Alpha Capture followed by neutron emission 31 P
15 2 4 He
34 Cl
17 0 1 n
Alpha capture
Gamma Emission causes no change in mass or charge and…
Usually follows the previous
0 0
γ types of decay.
Beta (Negatron) Decay Process
Daughter Nucleus Osmium-187 Calcium-40
Antineutrino Parent Nucleus Rhenium-187 Potassium-40
Beta Particle (electron)
Beta Particles
Same as an electron with kinetic energy Positive or negative charge of 1 May be positively or negatively charged Can normally be stopped by 1 cm of plastic, wood, paper Exception for positron emitters
B. Nuclear Decay
Why nuclides decay…pg. 803
need stable ratio of neutrons to protons
238 U
92 234 90 Th
2 4 He 131 I
53 131 Xe
54 0 -1 e 38 19 K
38 Ar 18
0 1 e 106 47 Ag
0 -1 e
106 Pd 46
DECAY SERIES TRANSPARENCY
C. Half-life
Half-life (t ½ )
Time required for half the atoms of a radioactive nuclide to decay.
Shorter half-life = less stable.
C. Half-life
m f
m i
( 1 2 )
n m f
: final mass
m i
: initial mass n: # of half-lives
C. Half-life
Fluorine-21 has a half-life of 5.0 seconds. If you start with 25 g of fluorine-21, how many grams would remain after 60.0 s?
t GIVEN: ½ m i = 5.0 s = 25 g m f = ?
total time = 60.0 s n = 60.0s ÷ 5.0s =12 WORK : m f = m i (½) n m f = (25 g)(0.5) 12
m f = 0.0061 g
Decay Series
Many heavy elements are unstable and so they will continue to decay (be radioactive) until they finally transmute into a stable nucleus.
Here is an example of the Th-232 decay series Thorium oxide is used to in camping lanterns to intensify the brightness when on fire.
Stable isotope