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Nuclear Chemistry

- Just the basics.
- By
- J.M.Soltmann

What is Nuclear Chemistry

- As its name implies, nuclear chemistry is the

study of the nucleus and reactions between

nuclei. - Remember that virtually all of the mass of an

atom resides in the nucleus, as does all of the

positive charge. - Nuclear energy is a much greater form of energy

than bond energy.

Radioactivity

- While most nuclei are stable, many nuclei are

unstable and spontaneously emit particles and

electromagnetic radiation. - These nuclei are refered to as radionuclides.

Nuclear Equations

- In a nuclear equation, mass numbers and atomic

numbers are balanced instead of elements. - The example here to the right depicts a

radioactive decay specifically an alpha decay. - The helium ion is called an alpha particle.

3 Common types of Radioactive Decay

- Alpha decay
- Beta decay - a ß- particle is a subatomic nuclear

particle essentially equivalent to an electron

and a ß particle is a positively charge

electron, called a positron. - Gamma decay - high energy photons are emitted

which have virtually no mass nor charge.

Nuclear electrons?

- Modern theory has shown that a neutron is

actually comprised of a proton and an electron. - So, if a nucleus emits an electron, it has really

transformed a neutron into a proton. - Also, if a nucleus absorbs an electron, it will

convert a proton into a neutron.

Common Particles in Nuclear Reactions

- Neutrons (10n)
- Protons (11p or 11H)
- Electrons (0-1e)
- Alpha Particles (42He or 42?)
- Beta- Particles (0-1e or 0-1?)
- Gamma (00?) - Gamma radiation consists of

high-energy photons, with a mass far too little

for consideration. - Positron (01e) - A positron is a positively

charged electron. It has the mass of an electron

but a positive charge.

Differentiating the Radiations

- Alpha emissions are the heaviest and thus have

the least penetrating power. - Beta emissions have masses much smaller than

protons or neutrons, so they have more

penetrating power. In terms of penetrating

power, ?100? . - Gamma emissions have essentially no mass, so they

are the most powerful. In terms of penetrating

power. ? 100? .

Try this

- Write a nuclear equation for the process when

mercury-201 undergoes electron capture.

To answer this question

- First we have to understand what mercury-201 is.

Since mercury is always atomic number 80, this

isotope is 20180Hg. - Since we are capturing an electron, the electron

must be a reactant. - Now we add up mass numbers and atomic numbers.

(201 0 201 and 80 -1 79). - Element 79 is gold, so the answer is
- 20180Hg 0-1e --gt20179Au

Try another

- Thorium-231 decays into protactinium-231.
- What is the balanced equation?
- What other particle(s) is/are involved in the

reaction?

The answers are

- 23190Th --gt 23191Pa 0-1e
- The extra particle is an electron, but because it

is being emitted, it would be called a Beta

emission.

Nuclear Transformations

- The first manmade conversion of one nucleus into

another was performed by Sir Ernest Rutherford

(1919). - Rutherford bombarded a nitrogen-14 atom with

alpha particles to produce an oxygen-17 atom plus

a proton. - 147N 42He --gt 178O 11H
- The shorthand version of this reaction is
- 147N(?,p)178O WHY???

Now try this one

- Write the balanced nuclear equation for the

process noted by the shorthand - 2713Al(n,?)2411Na

Now try this one

- Write the balanced nuclear equation for the

process noted by the shorthand - 2713Al(n,?)2411Na
- 2713Al 10n --gt 2411Na 42He

Nuclear Stability

- Why are some nuclei more stable than others?
- To be honest, there are several factors, most of

which are beyond the scope of this course. - However, there are a few easy to see indications

of nuclear stability.

Did you ever wonder?

- We know that like charges repel each other, yet a

nucleus can have dozens of positively charged

protons held together. Why? - Neutrons are a major reason. All nuclei with 2

or more protons have neutrons. The neutrons and

the protons meld by a force of nature, different

than gravity or electromagnetism, called the

strong (nuclear) force. - Because of the way this force binds the protons

and neutrons together, the ratio of protons to

neutrons is an issue.

Smaller Atoms vs Bigger Atoms

- In smaller atoms, most stable atoms have neutron

to proton ratios of about 1.00. - As isotopes increase in atomic number, most

stable isotopes have increasingly larger ratios

of neutrons to protons. - To our knowledge, any isotope with an atomic

number greater than or equal to 84 would be

radioactive.

Some stability trends

- Of the 265 known stable isotopes
- 157 of them have even numbers of protons and

neutrons. - 53 of them have an even number of protons but an

odd number of neutrons. - 50 of them have an odd number of protons but an

even number of neutrons. - Only 5 of them have odd numbers of both protons

and neutrons.

Magic Numbers

- For some reason, nuclei with 2,8,20,28,50 or 82

protons and/or 2,8,20,28,50,82, or 126 neutrons

are generally more stable than isotopes without

these numbers. - When we think of substances that shield

radiation, we tend to think of lead. The most

common isotope of lead is 20882Pb that means it

has 82 protons and 126 neutrons.

Decays and Half-lifes

- When a radioactive substance decays, the amount

of that particular isotope will decrease. - We call the rate of decay the half-life, because

it is the time needed for exactly 1/2 of the

isotope to decay.

More on Half-life

- If we examine the graph to the right, we see that

we started with 50 g of the isotope. Each

subsequent point represents half of the previous

mass (50 to 25 to 12.5 to 6.25 to 3.125 to 1.5625

to .78125). - Each point is approximately 24 days apart The

half-life for this substance is 24 days.

Calculations with half-life

- Although it is possible to determine the amount

remaining of a radioisotope using natural logs

ln(Nt/N0)-kt we do not need to do this. - We only work with whole number increments of the

half-life.

For example

- The half-life of an isotope is 8 days. If we

start with 100 grams of the isotope, how much is

present in 32 days? - 32 days/(8 days/half-life) 4 half-lives.
- Each half-life divides the previous mass in half.

- 100g/2 50g/2 25g/2 12.5g/2 6.25g
- There would be 6.25 g of that isotope left.

You try one

- The half-life of Bismuth-211 is 185 years. How

much time would it take for a 360 g sample to

decay to 11.25 g?

You try one

- The half-life of Bismuth-211 is 185 years. How

much time would it take for a 360 g sample to

decay to 11.25 g? - 360g/2180g/290g/245g/222.5g/211.25g.
- That is 5 half-lives.
- 5 half-lives185 years/half-life 925 years.

Fusion vs. Fission

- Fission and Fusion are two types of highly

exothermic nuclear reactions, different than the

decays covered earlier. - Fusion means to bring two smaller nuclei together

to make a larger nucleus. - 136C 136C --gt 2511Na 11p
- Fission means to break a larger nucleus into 2 or

more smaller nuclei. - 23592U 10n --gt 13752Te 9740Zr 2 10n

How much Energy are we talking about?

- In fusion and fission, a tiny, almost meaningless

mass of each affected nucleus is converted to

energy. - Einstein theorized that the amount of energy was

dependent on the mass lost and the square of the

speed of light. E mc2.

So, how much energy is that?

- Well if each uranium atom in a given fission

process loses the mass of one electron

(9.11x10-31 kg) - E mc2
- E (9.11x10-31 kg) (3.0x108 m/s)2
- E 8.2x10-14 J

But that seems like a small number!

- 8.2x10-14 J is a small amount, but that was for

just one atom or uranium. If we had 1 mg of

uranium (about the mass of a cystal of salt),

that would contain roughly 2.5 x 1018 atoms of

uranium. - 8.2x10-14 J/atom 2.5 x 1018 atoms 2.1x105 J
- That is almost enough energy to handle the

electrical needs of this school for a day - from

a tiny starting mass.

Think about this

- If we could convert the .25 kg mass of a banana

peel (using our Mr. Fusion power supply) into

pure energy, - E mc2
- E (.25 kg)(3.0x108 m/s)2
- E 2.25x1016 J
- Thats enough energy to run New York City for a

year (with enough energy left over to go Back to

the Future)!