Chapter 21 Nuclear Chemistry
Section 21.1 Types of Radioactivity
Objectives: Analyze Common Sources of Background
Radiation, Compare and Contrast Alpha, Beta and
Gamma Radiation, Apply the concept of Half-Life of a
Radioactive Element
In a chemical reaction, what is the main
subatomic particle involved?
The ELECTRON
In a nuclear reaction, what is the main area of the atom
involved?
The NUCLEUS
https://www.youtube.com/watch?v=hORaebYWDwk
The Nucleus
 Remember that the nucleus is comprised of the
protons and neutrons.
 The number of protons is the atomic number.
 The number of protons and neutrons together is
effectively the mass of the atom.
Nuclear Notation
A nucleus can LOSE or GAIN protons and
neutrons (Adding or losing Protons changes
the identity of an element)
When writing nuclear equations, it is important
to indicate the isotopes of the given elements.
Isotopes
 Not all atoms of the same element have the same mass
due to different numbers of neutrons in those atoms.
 There are three naturally occurring isotopes of uranium:
 Uranium-234
 Uranium-235
 Uranium-238
Discovery of Radioactivity
Henri Becquerel discovered that uranium
compounds spontaneously give off radiation.
Discovery of Radioactivity (cont.)
Marie and Pierre Curie concluded that a
nuclear reaction was taking place within
the uranium atoms.
Radioactivity is the spontaneous emission of
radiation by an unstable atomic nucleus.
**Objects do not become radioactive when
subjected to radiation unless they actually
absorb radioactive elements.
Radiation vs. Radioactivity: What’s the
difference?
Nuclear radiation is made up of matter or energy that
has been released by a substance
During a nuclear reaction, what can an unstable atomic
nucleus do?
It can gain or lose protons and/or neutrons
Radioactivity
 It is not uncommon for some nuclides of an element to
be unstable, or radioactive.
 We refer to these as radionuclides.
 There are several ways radionuclides can decay into a
different nuclide.
Stop!
 Get Highlighter out and highlight any vocabulary
words or words you do not know…
 Create 4 questions about the content you just
covered.
Types of
Radioactive Decay
Radioactive Decay
Radioactive Decay – the release of radiation by
radioactive isotopes (radioisotopes)
Alpha Decay
 Streams of alpha particles
 Helium nuclei which consist of 2 protons and 2 neutrons
 Alpha radiation does NOT deeply penetrate into matter
and is easily stopped; the particles have a + charge and are
relatively large in comparison to other forms of radiation
 A new nucleus with an atomic number that is 2 less, and a
mass number that is 4 less than the original nucleus
Alpha Decay:
Alpha Particle- consist of a He nuclei with 2 protons and 2
neutrons (α)
Loss of an -particle (a helium nucleus)
4
2
238
92
U

He
234
90
4
2
Th+ He
Must be balanced! Sum of mass numbers and atomic
numbers must be the same on the left and right.
Try Thorium 230 (Th-230) and Radon 222 (Rn-222)
230Th
90
→
226Ra
88
+
4He
2
222Rn
→
218Po
+
4He
86
84
2
Radioactive Decay (cont.)
Alpha radiation is not very penetrating—a
single sheet of paper will stop an alpha
particle.
Beta Decay
 Beta particles : high energy electron with a 1- charge; represented 2
ways:
 Smaller & faster than alpha, so greater penetrating power; stopped by
thick materials
 A beta decay results in a new nucleus with an atomic number that is 1
greater than that of the original and a mass number that is the same
 This electron is NOT from outside the nucleus, it is produced by the
change of a neutron into a proton and an electron
 Carbon-14 undergoes beta decay, so does Sulfur -35
Beta Decay:
Beta Particle- a high energy electron with a 1- charge
Loss of a -particle (a high energy electron)
0
−1
131
53
I


0
or −1
131
54
e
Xe
+
0
−1
e
Try Magnesium 27 (Mg-27) and Sulfur 35 (S-35)
27Mg
12
35S
16
→
→
27Al
13
35Cl
17
0 e
-1
+
+
0
e
-1
Beta Blockers
Can be stopped by heavy clothing, sheets
of metal, blocks of wood
Gamma Decay
 Gamma Ray : high energy form of electromagnetic
radiation w/o mass or charge
 Most penetrating and causes the greatest damage
 Energy is the only “product” of gamma decay;
represented by:
 Occurs with other types of decay, and does not affect
mass number or atomic number
Gamma Decay:
Gamma Ray – high-energy form of electromagnetic radiation
without charge or mass (γ)
(Gamma typically does not occur alone – occurs with β or α)
Loss of a -ray (high-energy radiation that almost always
accompanies the loss of a nuclear particle)
0
0

Gamma Blockers
Can be stopped by thick blocks of lead or
concrete
Positron Emission
 Is a type of radioactive decay called beta plus decay.
 Positron emission results in a new nucleus with an
atomic number that is 1 less than that of the original
and a mass number that is the same
 A proton becomes a neutron. From this process a
positron and a neutrino are ejected from the nucleus.
Positron Emission:
Loss of a positron (a particle that has the same mass as
but opposite charge than an electron)
0
1
11
6
C
e
11

5
B
+
0
1
e
Positron Emission
 Example:
124Ba
56
124Cs
55
Try Si -26 and La -125
+ 0e
1
Practice Problem Cont…
 Write a balanced equation for the nuclear reaction
which neon-23 decays to form sodium-23 and
determine the type of decay.
Example Problems
p. 747 #1 and 2
#1) Alpha
226Ra
→
88
222Rn
+
4He
86
2
#2) Beta
23Ne
10
→
23Na
11
+
0
-1
e
Practice Problem…
 Write the balanced equation for the nuclear equation
for the nuclear reaction in which uranium-234 decays
to form thorium-230 and determine the type of decay.
https://www.youtube.com/watch?v=TJgc28csgV0&inde
x=11&list=PLKEmXepzBsY9Zx4EHN27HHSQyvb4Xysb4
Stop!
 Get Highlighter out and highlight any vocabulary
words or words you do not know…
 Create 4 questions about the content you just
covered.
Rates of Decay
Neutron-Proton Ratios
 Any element with more
than one proton (i.e.,
anything but hydrogen)
will have repulsions
between the protons in the
nucleus.
 A strong nuclear force
helps keep the nucleus
from flying apart.
Neutron-Proton Ratios
 Neutrons play a key role
stabilizing the nucleus.
 Therefore, the ratio of
neutrons to protons is an
important factor.
Neutron-Proton Ratios
For smaller nuclei (Z  20)
stable nuclei have a
neutron-to-proton ratio
close to 1:1.
Neutron-Proton Ratios
As nuclei get larger, it takes
a greater number of
neutrons to stabilize the
nucleus.
Stable Nuclei
The shaded region in the
figure shows what nuclides
would be stable, the socalled belt of stability.
Stable Nuclei
 Nuclei above this belt
have too many
neutrons.
 They tend to decay by
emitting beta
particles.
Stable Nuclei
 Nuclei below the belt
have too many
protons.
 They tend to become
more stable by
positron emission or
electron capture.
Stable Nuclei
 There are no stable nuclei with an atomic number
greater than 83.
 These nuclei tend to decay by alpha emission.
Radioactive Series
 Large radioactive nuclei
cannot stabilize by
undergoing only one
nuclear transformation.
 They undergo a series of
decays until they form a
stable nuclide (often a
nuclide of lead).
Some Trends
Nuclei with 2, 8, 20,
28, 50, or 82 protons
or 2, 8, 20, 28, 50, 82,
or 126 neutrons tend
to be more stable
than nuclides with a
different number of
nucleons.
Some Trends
Nuclei with an even
number of protons
and neutrons tend to
be more stable than
nuclides that have
odd numbers of these
nucleons.
Nuclear Transformations
Nuclear
transformations can
be induced by
accelerating a particle
and colliding it with
the nuclide.
Particle Accelerators
These particle accelerators are enormous, having circular
tracks with radii that are miles long.
Detecting Radioactivity
Radioactivity cannot be seen, heard or
touched – it has no smell or taste
Section
21.1
Radioactive Decay
(cont.)
There are several methods used to detect
radiation.
– photographic film
– scintillation counters
– A Geiger counter
detects ionizing
radiation, which is
radiation energetic
enough to ionize
matter with which
it collides.
Half-Life and Radioisotope Dating
The rate of spontaneous nuclear decay
cannot be changed.
Radioactive decay rates are measured in halflives.
The half-life is the time it takes for half of a
given amount of a radioactive isotope to
undergo decay. (T1/2)
Can be fractions of a second or billions of
years
Radioactive Dating
Organisms take in carbon during their lifetime. After
death, no new carbon is taken in.
C-14 remaining is measured, compared with how much
was in the material when it was alive.
Age of object can be estimated.
T1/2 of C = 5730 yrs
Other materials : K-40 (1.25 billion years), U-238 (4.5
billion years), Rb-87 (48 billion years)
Half-Life and Radioisotope Dating (cont.)
Half-Life and Radioisotope Dating (cont.)
Four different isotopes are commonly used
for dating objects: carbon-14, uranium-238,
rubidium-87, and potassium-40.
Carbon-14 dating is commonly used to measure
the age of fossils.
To date objects that are more than 60,000 years
old, carbon-14 dating cannot be used since
there is very little carbon left to measure.
Example Problems
p. 755 #3 and 4
#3) A rock was analyzed using K-40. The half life of K-40 is
1.25 billion years old. If the rock had only 25% of the K-40 that
would be found formed in a rock today – Calculate how long
ago the rock was formed.
25% of K-40
100% → 50% → 25%
1
2
1.25 x 2 = 2.5 BY
#4) Ash from an early fire pit was found to have 12.5% as
much carbon-14 as would be found in a similar sample of
ash today. How long ago was the ash formed?
12.5 % of C-14
100% → 50% → 25%→ 12.5
1
2
3
5730 x 3 = 17,190 yrs
So…
***The end result of all types
of decay will be a stable
nucleus***
Stop!
 Get Highlighter out and highlight any vocabulary
words or words you do not know…
 Create 4 questions about the content you just
covered.