Transcript Chapter 25.2 Nuclear Transformations

Chapter 25 Nuclear Chemistry
25.2 Nuclear Transformations
Chemistry
Today we are learning to:1. See what determines the type of decay a radioisotope
undergoes
2. Understand what we mean by the term half-life
3. Calculate how much of a sample remains after each half-life
4. Understand the 2 ways that transmutation reactions can occur
Nuclear Transformations
Nuclear Stability and Decay
A force called the nuclear force is responsible for holding the nuclei of
atoms together. It is stronger than the electromagnetic force over extremely
short distances, so nuclei don’t fly apart (1.7fm) (1 femtometer = 1.0 x 10−15
meters or 0.000,000,000,000,001,7m).
More than 1,500 different nuclei are known. Of those, only 264 are stable
and do not decay or change with time. These nuclei are in a region called the
band of stability.
Nuclear Transformations
Nuclear Stability and Decay
Nuclear Stability and Decay
Nuclear Transformations
A nucleus lying outside the band of stability will be unstable and decay into
a more stable isotope
The neutron-to-proton ratio determines the type of decay that occurs
If a nucleus has too many neutrons, a neutron can change into a proton
and a high energy electron (beta particle) is shot out of the nucleus.
1
1
n →
0
0
p +
+1
e
-1
Nuclear Stability and Decay
Nuclear Transformations
Other types of decay processes other than alpha emission and beta emission
are:
i.
Electron capture: where an electron captured by a nucleus
transforms a proton into a neutron.
1
0
e →
p +
+1
i.
1
-1
n
0
Positron emission: Where a proton in the nucleus changes into a
neutron with the emission of a positively charged electron (positron).
1
1
p →
+1
0
n +
0
e
+1
25.2
Nuclear Stability and Decay
Homework questions:
Answer Questions 33-37
on page 821 of your text
book for next day (use a
periodic table to help
identify products.)
Nuclear Transformations
Half-Life
The rate of decay of an unstable radioisotope is measured by its half-life
A half-life (t1/2) is the time required for one-half of the nuclei of a
radioisotope sample to decay to products
After each half-life, half of the existing radioactive atoms have decayed into
atoms of a new element.
Half-lives can range from fractions of a second to billions of years
Nuclear Transformations
Half-Life
Nuclear Transformations
Half-Life
25.2
Half-Life
Uranium 238 decays through a series of unstable isotopes until it reaches the
stable isotope of lead-206.
Because of the long half live of uranium -238 its ratio to other isotopes in rocks
can be used to date them.
Stable Isotope
Nuclear Transformations
Half-Life
The ratio of Carbon-14 to stable carbon in
the remains of an organism changes in a
predictable way that enables the
archaeologist to obtain an estimate of its
age.
Carbon-14 dating (simple)
Carbon-14 dating (detailed)
25.1
for Sample Problem 25.1
Practice Problem (page 806).
Homework: Questions 46, 50, 54, 55, 57, 58, 60, 61 and 65
On pages 882-883 of your Chemistry text book.
Transmutation Reactions
Transmutation Reactions
The conversion of one atom of one element to a different element is called
transmutation
Transmutation can occur by:
i.
radioactive decay
ii.
when particles bombard the nucleus of an atom.
 The first artificial transmutation reaction involved bombarding nitrogen
gas with alpha particles.
25.1
Nuclear Transformations
Transmutation Reactions
• The first artificial transmutation reaction involved
bombarding nitrogen gas with alpha particles.
Transmutation Reactions
Nuclear Transformations
Bombardment by alpha particle:
Ex. 1.
Ex. 2.
14
7
4
2
He →
9
4
Be +
4
2
He →
N+
17
O+
8
12
C+
6
1
1
H
1
n
0
• The elements in the periodic table with atomic numbers above 92, the atomic
number of uranium, are called the transuranium elements.
» All transuranium elements undergo transmutation.
» None of the transuranium elements occur in nature, and all
of them are radioactive.
» Transuranium elements are synthesized in nuclear
reactors and nuclear accelerators.
25.2 Section Quiz.
1. During nuclear decay, if the atomic number
decreases by one but the mass number is
unchanged, the radiation emitted is
a. a positron.
b. an alpha particle.
c. a neutron.
d. a proton.
25.2 Section Quiz.
2. When potassium-40 (atomic number 19)
decays into calcium-40 (atomic number 20),
the process can be described as
a. positron emission.
b. alpha emission.
c. beta emission.
d. electron capture.
25.2 Section Quiz.
3. If there were 128 grams of radioactive
material initially, what mass remains after four
half-lives?
a. 4 grams
b. 16 grams
c. 8 grams
d. 32 grams
25.2 Section Quiz.
4. When transmutation occurs, the ________
always changes.
a. number of electrons
b. mass number
c. atomic number
d. number of neutrons
25.2 Section Quiz
5. Transmutation occurs by radioactive decay
and also by
a. extreme heating.
b. chemical reaction.
c. high intensity electrical discharge.
d. particle bombardment of the nucleus.
2.3 Vocabulary 25.1 Vocabulary
•
•
•
•
Radioactivity: process where materials give off high energy rays
Radiation: name give to these penetrating rays and particles
Radioisotopes: unstable isotopes that will decay into a different element
Alpha particle: high energy helium nuclei containing 2 protons and 2
neutrons
beta particle: high energy electron emitted from a nucleus
Gamma particle: high energy electromagnetic radiation emitted from a
nucleus
•
•
25.2 Vocabulary
•
•
•
•
•
•
Nuclear force: attractive force acting between nuclear particles close
together
Band of stability: region of stable isotopes
Positron: particle with the mass of an electron but a +1 charge
Half-life: time taken for half of a radioisotope to decay
Transmutation: conversion of an atom of one element to an atom of
another
Transuranium element: elements with atomic numbers above 92
END OF SHOW