Nuclear chemistry

1. Radioactivity
Chapter 25

2. • Nuclear chemistry
• study of the structure of atomic nuclei
• changes they undergo.
Nuclear Radiation

3. • Wilhelm Roentgen (1845–1923)
• 1895-invisible rays were emitted when
electrons bombarded the surface of
certain materials.
The Discovery of Radioactivity
• caused photographic plates to darken.
• named the invisible high-energy
emissions X rays.

4. • Henri Becquerel (1852–1908) was
studying phosphorescence
• minerals that emit light after being
exposed to sunlight
The Discovery of Radioactivity
•phosphorescent
uranium salts
produced
spontaneous
emissions that
darkened
photographic plates.

5. • Marie Curie (1867–1934)
and her husband Pierre
(1859–1906) took
Becquerel’s mineral
sample (called
pitchblende) and isolated
the components emitting
the rays.
The Discovery of Radioactivity
• darkening of the photographic plates was
due to rays emitted specifically from the
uranium atoms present in the mineral
sample.

6. The Discovery of Radioactivity
• Marie Curie named the process by
which materials give off such rays
radioactivity
• the rays and particles emitted by a
radioactive source are called
radiation.

7. • isotopes are atoms of the same
element that have different numbers of
neutrons.
Types of Radiation
• Isotopes of atoms with unstable nuclei
are called radioisotopes
• emit radiation to attain more stable
atomic configurations in a process
called radioactive decay
• lose energy by emitting one of several
types of radiation.

8. Why do some atoms decay?
• The nucleus contains tightly packed
protons and neutrons (nucleons)
• The strong nuclear force keeps the
nucleons packed together even though
protons want to push each other away
• Stable atoms have a neutron to proton
ratio of about 1:1

9. • As atomic number increases, more
neutrons are required to have enough of a
strong force to keep the protons pushed
together
• The neutron to proton ratio for stable
atoms increases to 1.5:1

10. Band of Stability
• When the number of
protons and neutrons are
plotted, the stable nuclei
are found within the “band
of stability”
• Radioactive isotopes are
outside the band of
stability
– They will undergo nuclear
reactions to become more
stable
– All elements higher than
atomic# 83 are radioactive

12. Basic Assessment Questions
Question 3
Topic
26
Calculate the neutron-to-proton ratio for .

13. Types of Nuclear Radiation
• Alpha
• Beta
• Gamma

14. Alpha Radiation
• Release of 2 protons and 2 neutrons
– Equivalent to a He nucleus
– Charge of 2+
– Mass = 4 amu
• Largest and slowest
– Least penetrating  can be stopped by paper
• Changes to a different element with a lower
atomic mass and lower atomic number
• Example: Polonium-212 (atomic# 84) is
converted to Lead-208 (atomic# 82)

16. Beta Radiation
• Decay of a neutron into a proton and
electron
– Electron is emitted, proton stays
– Forms a new element b/c of addition of proton
• Decay of the proton into a neutron and
positron (like a positive electron)
– The positron is emitted as a beta particle
• Faster than alpha particles  can be
stopped by aluminum foil

18. Gamma Radiation
• Not a particle
• Electromagnetic wave with short
wavelength and high frequency & energy
• No mass, no charge
• Very fast  speed of light
• Stronger than X-ray
• Stopped by several centimeters of lead

21. • Transmutation: changing one element into
another through radioactive decay
– Adding or removing a proton changes the
atomic number, resulting in a different element
• Half-Life: amount of time for half of a
sample of a radioactive element to decay
into something else
– Can range from a fraction of a second to
billions of years
– Amount remaining=initial amount(1/2)t/T
• t=total time
• T=half-life

23. Half-life
n
i
f m
m )
(2
1

mf: final mass
mi: initial mass
n: # of half-lives

24. 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?

25. Nuclear Decay
• Alpha Emission
He
Th
U 4
2
234
90
238
92 

parent
nuclide
daughter
nuclide
alpha
particle
Numbers must balance!!

26. Nuclear Decay
• Beta Emission
e
Xe
I 0
-1
131
54
131
53 

electron
• Positron Emission
e
Ar
K 0
1
38
18
38
19 


positron

27. Nuclear Decay
• Electron Capture
Pd
e
Ag 106
46
0
-1
106
47 

electron
• Gamma Emission
– Usually follows other types of decay.

28. Types of Transmutation
• Induced transmutation
– Nucleus of an unstable isotope (radionuclide)
is struck with a high velocity charged particle
• Particle accelerator
• Need lots of energy and unstable nucleus
– Elements atomic 93 and higher (transuranium
elements)
• Natural transmutation
– Occurs naturally as a radioisotope decays to
become more stable

29. Basic Assessment Questions
Question 1
Topic
26
What element is formed when polonium-214
( ) radioisotope undergoes alpha decay?
Give the atomic number and mass number of
the element.

30. Basic Assessment Questions
Question 2
Topic
26
What element is formed when undergoes
beta decay? Give the atomic number and mass
number of the element.

31. Basic Assessment Questions
Question 4
Write a balanced nuclear equation for the
beta decay of the following radioisotope.
Topic
26

32. Nuclear Fission
• Fission = divide
• Neutron hits an unstable atom
• Nucleus splits into two fragments of about
the same mass
– Some single neutrons are released (energy)
– These neutrons can smash into other atoms
• Causes a chain reaction

33. Fission Reaction

35. Nuclear reactors
• Nuclear power plants use
the process of nuclear
fission to produce heat in
nuclear reactors.
• The heat is used to
generate steam, which is
then used to drive turbines
that produce electricity.

36. Atomic Bomb- uncontrolled fission reactions

37. • Little Boy: $2billion in research; made of
Uranium-235; equal to 20,000 tons of TNT;
140,000 people died; 2/3 of the city destroyed
• Fat Man: Plutonium-239; 70,000 people died;
40% of the city destroyed

38. Hydrogen Bomb
• 1000 times more powerful than atomic
bomb
• March 1, 1954; Bikini Atoll in Pacific
– Never in war
• Fission reaction triggers fusion of
Hydrogen isotopes

39. Nuclear Fusion
• Opposite of fission
• Two nuclei fuse together to form one
nucleus with a larger mass
– Not simple sum of masses
– Some mass lost as energy
• Requires high temperature:
Thermonuclear reaction
• Occurs in the sun and stars
– 4 H combine to form one He, 2e- and energy

40. Nuclear Fusion

41. Solar Flare

42. Radiation Detectors
• Cloud Chamber
– supersaturated water or ethanol
– radioactive particle flows through
and knocks e- off
– vapor condenses showing path
– alpha: short/thick trails; beta:
long/thin
• Bubble Chamber
– superheated liquid
– e- knocked off again
– bubbles are formed

43. Measuring Radiation
• Geiger Counter
– produces electric
current when near
radiation
– Results in clicks or a
digital reading

44. Using Nuclear Reactions in
Medicine
• Tracers: monitor body
processes
– Iodine-131
• emits beta particles
• used to detect tumors in
thyroid gland
– also used: Carbon-11
and Sodium -24

45. • Cancer Treatment
• damage cancer cells
• Gold -198 or Iridium -192 -- implanted in or
near tumor
• Cobalt-60
– outside body
– emits gamma rays

46. • Positron Emission
Tomography (PET)
– Fluorine-18 attached to
molecules that go to brain
– positrons are emitted and collide
with electrons forming 2 gamma
rays
– the gamma rays are detected
and indicate brain activity

47. • http://www.hpwt.de/Kern2e.htm
• http://www.colorado.edu/physics/2000/isot
opes/radioactive_decay3.html
• http://www.msd.k12.or.us/schools/mhs/proj
ects/Fission/frames.html
• http://www.cnn.com/SPECIALS/cold.war/e
xperience/the.bomb/history.science/