Tro chemistry a molecular approach 37 magic numbers

Tro, Chemistry: A Molecular Approach 37
Magic Numbers
•
besides the N/Z ratio, the actual numbers of protons and
neutrons effects stability
•
most stable nuclei have even numbers of protons and neutrons
•
only a few have odd numbers of protons and neutrons
•
if the total number of nucleons adds to a magic number, the
nucleus is more stable

same idea as the electrons in the noble gas resulting in a more stable
electron configuration

most stable when N or Z = 2, 8, 20, 28, 50, 82; or N = 126

Tro, Chemistry: A Molecular Approach 38
Decay Series
•
in nature, often one radioactive nuclide changes
in another radioactive nuclide

daughter nuclide is also radioactive
•
all of the radioactive nuclides that are produced
one after the other until a stable nuclide is made
is called a decay series
•
to determine the stable nuclide at the end of the
series without writing it all out
1. count the number of  and  decays
2. from the mass no. subtract 4 for each  decay
3. from the atomic no. subtract 2 for each  decay and
add 1 for each 

Tro, Chemistry: A Molecular Approach 39
U-238
Decay Series














or







or other
combinations

Tro's Introductory Chemistry, Chapter 17 40
Detecting Radioactivity
To detect something, you need to identify what it does
•
Radioactive rays can expose light-protected
photographic film
Use photographic film to detect its presence – film
badges

Tro's Introductory Chemistry, Chapter 17 41
Detecting Radioactivity
•
Radioactive rays cause air to become ionized
An electroscope detects radiation by its ability to
penetrate the flask and ionize the air inside
A Geiger-Müller Counter works by counting
electrons generated when Ar gas atoms are ionized
by radioactive rays

42
Detecting Radioactivity
•
Radioactive rays cause certain chemicals to give off
a flash of light when they strike the chemical
A scintillation counter is able to count the number
of flashes per minute

Tro, Chemistry: A Molecular Approach 43
Natural Radioactivity
•
there are small amounts of radioactive minerals
in the air, ground, and water
•
even in the food you eat!
•
the radiation you are exposed to from natural
sources is called background radiation

Tro, Chemistry: A Molecular Approach 44
Rate of Radioactivity
•
it was discovered that the rate of change in the
amount of radioactivity was constant and different
for each radioactive “isotope”

change in radioactivity measured with Geiger counter

counts per minute

each radionuclide had a particular length of time it
required to lose half its radioactivity

a constant half-life

we know that processes with a constant half-life follow
first order kinetic rate laws
•
rate of change not affected by temperature

means that radioactivity is not a chemical reaction!

Tro, Chemistry: A Molecular Approach 45
Kinetics of Radioactive Decay
•
Rate = k N

N = number of radioactive nuclei
•
t
1/2
= 0.693/ k
•
the shorter the half-life, the more nuclei decay
every second – we say the sample is hotter
0
t
0
t
rate
rate
ln t
N
N
ln    k

Tro, Chemistry: A Molecular Approach 46