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ATOMIC STRUCTURE
CONTINUED
Introduction to
Nuclear Chemistry
• I can describe a stable/unstable
nucleus and explain the different
forms of nuclear decay.
In the late 1800s, scientists discovered that certain elements naturally gave off
rays of energy. By the early 1900s, scientists began to believe that the nucleus of
the atom, though it was extremely small, could produce a tremendous amount of
energy.
By the late 1930s, scientists began to realize that atoms could potentially be used
to create a weapon of unbelievably destructive power.
As World War II began to expand, Albert Einstein wrote a letter to the president
stating that the US must pursue a nuclear weapon before the Germans or other
enemies could if America was to win the war.
A secret project code named the MANHATTAN PROJECT was developed to
pursue the creation of an atomic weapon.
The result was the creation of two very powerful atomic bombs named FAT MAN
and LITTLE BOY which were used against the cities of HIROSHIMA and
NAGASAKI, JAPAN, effectively brining the World War II to an end.
In this unit, we will explore the basic principles of NUCLEAR CHEMISTRY
including RADIOACTIVE DECAY, NUCLEAR FISSION and NUCLEAR FUSION.
Stability of the Nucleus
• Most atoms have a very stable nucleus
– it does not change over time.
• This stability is due to a force called the
STRONG NUCLEAR FORCE.
– This force is created by NEUTRONS.
– Not well understood.
What is an UNSTABLE
NUCLEUS?
• Too few or too many No makes the
nucleus unstable.
• Over time, it breaks apart, releasing
particles and energy.
How Many Neutrons Make A
Nucleus Stable?
• For elements 1 – 20, need equal numbers
of P+ and No.
• For elements 21 – 83, increasingly more
and more No than P+ are needed.
• Beyond element 83, there are no stable
atoms!!!
What happens to an UNSTABLE
NUCLEUS?
• An UNSTABLE nucleus breaks apart bit by
bit, releasing particles and energy along the
way.
• This PROCESS is RADIOACTIVE DECAY.
Types of RADIOACTIVE DECAY
• Over time, an unstable nucleus will break
down until it reaches a stable form. This is
known as RADIOACTIVE DECAY.
• There are 3 types of RADIOACTIVE
DECAY:
• Alpha [ α ] (make a fish sign)
• Beta
[ β ] (make a “B” with a long side)
• Gamma [ γ ] (just make a “Y”)
Alpha Decay
• During ALPHA DECAY, a small piece of
the nucleus is given off.
• This “piece” of the nucleus consists of:
– 2 Protons and 2 Neutrons and is called an
ALPHA PARTICLE!
• An ALPHA PARTICLE is symbolized as:
4
2
•
α
α
QUESTION
What is the ATOMIC NUMBER and ATOMIC MASS NUMBER of an
ALPHA PARTICLE?
• Since the ALPHA PARTICLE is:
• 2 P+ and 2 No, the atomic number = 2 and
the mass number = 4!
• That is why the symbol is:
4
2
α
What happens to a nucleus that
has LOST an ALPHA PARTICLE?
• Once an ALPHA PARTICLE is
LOST, the nucleus of the atom is
changed, and is transformed into
an atom of another element!
PROBLEM
What would happen if a URANIUM-238
atom lost an ALPHA PARTICLE?
Some needed information:
1. The AMN of this isotope is 238, so the
NUCLEON NUMBER is 238.
2. On the PT, the AN of Uranium is 92.
• To find the newly formed atom, SUBTRACT 4
from the MASS NUMBER and SUBTRACT 2
from the ATOMIC NUMBER.
• Use the NEW ATOMIC NUMBER to locate the
new element on the Periodic Table:
238 – 4 =
92 – 2 =
234
90
Th
The new element is now THORIUM!
Practice Problems
• For each of the following ISOTOPES,
determine the resulting element after an
ALPHA DECAY has occurred:
• Pb-210
Np -240
At – 210
• HINT: use your Periodic Table to
determine the atomic number of each
isotope!