NUCLEAR CHEMISTRY
RADIOACTIVITY – WHAT IS IT?
• ALL SUBSTANCE ARE MADE OF ATOMS. THESE HAVE ELECTRONS (E) AROUND
THE OUTSIDE, AND A NUCLEUS IN THE MIDDLE. THE NUCLEUS CONSISTS OF
PROTONS (P) AND NEUTRONS (N), AND IS EXTREMELY SMALL. (ATOMS ARE
ALMOST ENTIRELY MADE OF EMPTY SPACE!)
• IN SOME TYPES OF ATOM, THE NUCLEUS IS UNSTABLE, AND WILL DECAY INTO
A MORE STABLE ATOM. THIS RADIOACTIVE DECAY IS COMPLETELY
SPONTANEOUS. THE ENERGY THAT IS RELEASED FROM THE NUCLEUS OF THE
ATOM IS RADIATION.
RADIOACTIVITY – WHAT IS IT?
When an unstable nucleus decays, there are three ways that it can do so. It may give out:
an alpha particle (we use the symbol

a beta particle (symbol

a gamma ray (symbol
)
)
)
RADIOACTIVITY – ALPHA PARTICLES





Alpha particles are made of 2 protons and 2 neutrons.
This means that they have a charge of +2, and a mass of 4
(the mass is measured in "atomic mass units", where each proton & neutron=1)
We can write them as
, or, because they're the same as a helium nucleus,
.
Alpha particles are relatively slow and heavy.
They have a low penetrating power - you can stop them with just a sheet of paper.
Because they have a large charge, alpha particles ionize (pull electrons from) other atoms
strongly.
RADIOACTIVITY – BETA PARTICLES

Beta particles have a charge of minus 1, and a mass of about 1/2000th of a proton. This means that beta
particles are the same as an electron.
We can write them as



or, because they're the same as an electron,
, written in isotope
notation as
They are fast, and light.
Beta particles have a medium penetrating power - they are stopped by a sheet of aluminum or
plastics.
Beta particles ionize atoms that they pass, but not as strongly as alpha particles do.
RADIOACTIVITY – GAMMA WAVES




Gamma rays are waves, not particles.
This means that they have no mass and no charge. So we sometimes write
.
Gamma rays have a high penetrating power - it takes a thick sheet of metal such as lead, or concrete
to reduce them significantly.
Gamma rays do not directly ionize other atoms, although they may cause atoms to emit other particles
which will then cause ionization.
We don't find pure gamma sources - gamma rays are emitted alongside alpha or beta particles. Strictly
speaking, gamma emission isn't 'radioactive decay' because it doesn't change the state of the nucleus. It
just carries away some energy.
RADIOACTIVITY – SUMMARY
Particles that ionize other atoms strongly have a low penetrating power, because they lose energy each
time they ionize an atom. Therefore, alpha particles are easy to stop and gamma rays are hard to stop.
ISOTOPE NOTATION
• ISOTOPE NOTATION INCLUDES ADDITIONAL INFORMATION ABOUT AN ISOTOPE. IN ADDITION
TO THE CHEMICAL SYMBOL, THE MASS NUMBER AND THE ATOMIC NUMBER ARE INCLUDED.
THIS ALLOWS INFORMATION ABOUT THE NUCLEUS TO BE DETERMINED.
The isotope notation for an atom of uranium-238 is:
 The mass number is 238.
This is the sum of protons and neutrons in the nucleus. (Protons + Neutrons = Mass)
 The atomic number is 92.
This is the quantity of protons (and electrons) in the atom. It also leads to the
chemical symbol for the isotope.
 The chemical symbol “U” is obtained from the name or by looking up the atomic
number on the periodic table.
NUCLEAR DECAY REACTIONS
• SOME ELEMENTS ARE NATURALLY UNSTABLE
• THEY TEND TO UNDERGO ALPHA-DECAY OR BETA-DECAY TO BECOME
MORE STABLE.
• THEY MAY TAKE SEVERAL STEPS IN ORDER TO ACHIEVE THIS, THUS WE
OBSERVE DECAY CHAINS (ALSO KNOWN AS SERIES DECAY) FOR MOST
RADIOACTIVE ELEMENTS.
• FOR EXAMPLE, IT TAKES U-238 14 STEPS OF ALPHA AND BETA DECAY TO
BECOME COMPLETELY STABLE
IDENTIFYING ALPHA AND BETA DECAY REACTIONS
Alpha Decay:
Alpha decay is one process that unstable atoms can use to become more stable. During alpha decay, an atom's nucleus
sheds two protons and two neutrons in a packet that scientists call an alpha particle.
Since an atom loses two protons during alpha decay, it changes from one element to another. For example, after
undergoing alpha decay, an atom of uranium (with 92 protons) becomes an atom of thorium (with 90 protons).
The reaction above would be written as:
(with the He representing the alpha particle)
IDENTIFYING ALPHA AND BETA DECAY REACTIONS
Beta decay:
During beta decay, a neutron turns into a proton and an electron. The electron is emitted and is called a beta
particle. This reaction would be written as:
BALANCING NUCLEAR DECAY REACTIONS
Examples:
1.
222
86
Rn 
218
84
Po + ____
Decay type __________________
2.
228
90
Th 
228
91
Pa + ____
Decay type __________________
DECAY CHAIN OF U-238
• STEP 1: ALPHA DECAY OF URANIUM-238
____________________________________________________
• STEP 2: BETA DECAY REACTION
____________________________________________________
• STEP 3: BETA DECAY REACTION
____________________________________________________
• NOW YOU COMPLETE THE REST ………………………
FISSION AND FUSION
• FISSION: FISSION IS THE PROCESS OF SPLITTING AN ATOM.
• FUSION: NUCLEAR FUSION IS THE PROCESS BY WHICH MULTIPLE SMALL
ATOMIC NUCLEI JOIN TOGETHER TO FORM A HEAVIER NUCLEUS.
FISSION AND FUSION
Examples:
1.
236
92
U +
1
0
n 
1
30
n +
92
36
Kr + _____
Decay type _________________
2. _____ +
2
1
H 
4
2 He
Decay type _______________
HALF LIFE
• WHEN RADIOACTIVE ISOTOPES DECAY, THEY DO SO
EXPONENTIALLY. THEIR RATE OF DECAY IS DETERMINED
THROUGH AN UNDERSTANDING OF HALF-LIFE.
• HALF-LIFE IS THE AMOUNT OF TIME IT TAKES FOR HALF OF
THE ATOMS OF AN UNSTABLE ISOTOPE TO DECAY.
• ATOMS WITH SHORT HALF-LIVES ARE MORE UNSTABLE
THAN THOSE WITH LONG HALF-LIVES.
HALF LIFE
• WHEN RADIOACTIVE ISOTOPES DECAY, THEY DO SO
EXPONENTIALLY. THEIR RATE OF DECAY IS DETERMINED
THROUGH AN UNDERSTANDING OF HALF-LIFE.
• HALF-LIFE IS THE AMOUNT OF TIME IT TAKES FOR HALF OF
THE ATOMS OF AN UNSTABLE ISOTOPE TO DECAY.
• ATOMS WITH SHORT HALF-LIVES ARE MORE UNSTABLE
THAN THOSE WITH LONG HALF-LIVES.
HALF LIFE EXAMPLES
• RADON-222 DECAYS WITH A HALF-LIFE OF 3.82 DAYS. IF AN
INITIAL SAMPLE OF THE ELEMENT CONTAINS 1000 ATOMS, HOW
LONG WILL IT TAKE FOR THE SAMPLE TO DECAY DOWN TO 125
ATOMS? YOUR TIME MUST ALWAYS START AT 0 DAYS!
•
SOLUTION 
AMOUNT
TIME
1000 ATOMS
0 DAYS
HALF LIFE EXAMPLES
• AN INITIAL SAMPLE OF ZINC-61 CONTAINS 215 GRAMS. AFTER
976 DAYS, THE SAMPLE SIZE HAS REDUCED TO 13.4375 GRAMS.
WHAT IS THE HALF-LIFE OF ZINC-61 ATOMS?
•
SOLUTION 
AMOUNT
TIME
215 GRAMS
0 DAYS
NOTE: TOTAL TIME/# HALF LIVES = LENGTH OF HALF LIFE
HALF LIFE EXAMPLES
• THORIUM-230 DECAYS WITH A HALF LIFE OF 80,000 YEARS. IF AN
INITIAL SAMPLE HAS A MASS OF 10.00 GRAMS, HOW MUCH WILL
REMAIN AFTER 320,000 YEARS?
•
SOLUTION 
AMOUNT
TIME
10 GRAMS
___ DAYS (YEARS)