Nuclear Chemistry

Chemical Reactions Nuclear Reactions

- Occur when bonds are broken or formed

-Occur when the nucleus emits particles or rays

-Atoms remain unchanged, just rearranged

-Atoms are often converted into atoms of another element

- Involve only valence electrons

- May involve protons, neutrons, or electrons

- Small energy changes may occur

- Very large energy changes may occur

Chemical Reactions vs. Nuclear Reactions

The Nucleus

• Remember that the nucleus is comprised of the two nucleons, 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.

Isotopes

• Not all atoms of the same element have the same mass because of different numbers of neutrons in those atoms.

• For example, there are three naturally occurring isotopes of uranium:– Uranium-234– Uranium-235– Uranium-238

Radioactivity

• It is not uncommon for some isotopes of an element to be unstable, or radioactive.

• We refer to these as radioisotopes.

• There are several ways a radioactive nucleus can decay into a more stable nucleus.

Types of Radioactive Decay Alpha Decay

Loss of an -particle (a helium nucleus)

He42

U23892

Th23490 He4

2+

Types of Radioactive Decay Beta Decay

Loss of a -particle (a high energy electron)

0−1 e0

−1or

I13153 Xe131

54 + e0

−1

Types of Radioactive Decay Gamma Emission

Loss of a -ray (high-energy radiation that almost always accompanies the loss of a nuclear particle)

00

Penetrating Ability

Types of Radioactive Decay Positron Emission

Loss of a positron (a particle that has the same mass as but opposite charge than an electron)

e01

C116

B115 + e0

1

Types of Radioactive Decay Electron Capture

Addition of an electron to a proton in the nucleus– As a result, a proton is transformed into a

neutron.

C116 + e0

−1 B11

5

Balancing Nuclear Reactions•In the reactants (starting materials – on the left side of an equation) and products (final products – on the right side of an equation)

Atomic numbers must balanceand

Mass numbers must balance

•Use a particle or isotope to fill in the missing protons and neutrons

Nuclear Reactions

• Alpha decay example

Nuclear Reactions

• Beta decay example

Other Types of Nuclear Reactions

Positron (0+1): a positive electron

Electron capture: Electron capture: the capture of an electron

207 207

Learning Check

What radioactive isotope is produced in the following bombardment of boron?

10B + 4He ? + 1n

5 2 0

Radioactive DecayRadioactive elements are unstable. They decay, or change into differentelements over time. Here are some facts to remember:

The half-life of an element is the time it takes for half of the material you started with to decay. Remember, it doesn’t matter howmuch you start with. After 1 half-life, half of it will have decayed.Each element has its own half-life

Each element decays into a new element C14 decays into N14 while U238 decays into Pb206 (lead)The half-life of each element is constant. It’s like a clock keeping perfect time.

Now let’s see how we can use half-life to determine theage of a rock or other artifact.

The grid below represents a quantity of C14. Each time you click,one half-life goes by. Try it! C14 – blue N14 - red

As we begin notice that no timehas gone by and that 100% of thematerial is C14

Halflives

% C14 %N14 Ratio of C14 to N14

0 100% 0% no ratio

The grid below represents a quantity of C14. Each time you click,one half-life goes by. Try it! C14 – blue N14 - red

Half

lives

% C14 %N14 Ratio of

C14 to N14

0 100% 0% no ratio

1 50% 50% 1:1

After 1 half-life (5700 years), 50% ofthe C14 has decayed into N14. The ratioof C14 to N14 is 1:1. There are equalamounts of the 2 elements.

The grid below represents a quantity of C14. Each time you click,one half-life goes by. Try it! C14 – blue N14 - red

Halflives

% C14 %N14 Ratio of C14 to N14

0 100% 0% no ratio

1 50% 50% 1:1

2 25% 75% 1:3

Now 2 half-lives have gone by for a totalof 11,400 years. Half of the C14 that waspresent at the end of half-life #1 has nowdecayed to N14. Notice the C:N ratio. Itwill be useful later.

The grid below represents a quantity of C14. Each time you click,one half-life goes by. Try it! C14 – blue N14 - red

Halflives

% C14 %N14 Ratio of C14 to N14

0 100% 0% no ratio

1 50% 50% 1:1

2 25% 75% 1:3

3 12.5% 87.5% 1:7

After 3 half-lives (17,100 years) only12.5% of the original C14 remains. Foreach half-life period half of the materialpresent decays. And again, notice the ratio, 1:7

Practice question

• If the half-life of an element is 10 days, how much of a 80 gram sample will remain after 50 days?

Practice question 2

• If you start with a 200 gram sample, and 25 grams remains after 75 years, what was the half-life of the sample?

Test question may involvegraphs like this one. The mostcommon questions are:"What is the half-life of this element?"

Just remember that at the endof one half-life, 50% of theelement will remain. Find 50%on the vertical axis, Follow theblue line over to the red curveand drop straight down to findthe answer:

The half-life of this element is 1 million years.

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.

• 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 so-called 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.

Practice Questions:

• Predict the type of decay of carbon-14 and plutonium-239.