Nuclear Chemistry In this chapter, we will discuss concepts that contradict what you have already learned. In a nuclear reaction, elements can change into different elements. The energy that atoms emit does not come from the electrons falling back to lower energy levels but comes from the atomic nucleus itself. And the amount of energy released in an nuclear reaction is millions of times greater than the electromagnetic radiation released from excited atoms.

Nuclear Chemistry

• Henri Becquerel (1889)

• He noticed that a certain type ofrock would glow in the dark if he left it in the sunlight for a few hours.

• Discovered that these special rockswould cause his photographicto appear that it had been exposedto light.

Nuclear Chemistry

• Marie Curie (1898)

• Worked with Henri Becquerelin trying to find out what was releasing the very powerfulenergy within the rocks.

• She discovered that the energythat was being emitted from the rock was from the nuclei of some Uranium atoms. She called it NuclearRadiation.

Nuclear Chemistry

• Marie Curie and Henri Becquerel

• They worked together inBecquerel’s lab in Paris to discover the exact nature of this nuclear radiation.

Nuclear Chemistry

• 3 types of Nuclear Radiation

• Alpha particles – Small particles that are emitted from the nucleus of an atom. They are the same size as a Helium atom and have a positive charge.

4 He

2

Nuclear Chemistry

• 3 types of Nuclear Radiation• Alpha particles

Nuclear Chemistry

• 3 types of Nuclear Radiation• Alpha particles • Alpha particles emitted from the atomic

nucleus traveling a little slower than the speed of light.

• Even though these particles have the ability to destroy soft body tissue, your skin can block them.

Nuclear Chemistry• 3 types of Nuclear Radiation

• Beta particles – Particles that are emitted from the atomic nucleus that are the size of an electron. They also have a negative charge.

• Beta particles are dangerous because are traveling much faster than an alpha particle. They travel near the speed of light.

• Beta particles can travel through your skin and cause tissue damage.

Nuclear Chemistry• 3 types of Nuclear Radiation

• Beta particles

Nuclear Chemistry• 3 types of Nuclear Radiation

• Beta particles

0β -1

Nuclear Chemistry• 3 types of Nuclear Radiation

• Gamma Rays – Rays of high energy light that come from the nucleus of the atom.

• Gamma Rays have no mass.

• They are very dangerous. Gamma Rays can travel through 6 feet of solid concrete.

Nuclear Chemistry• 3 types of Nuclear Radiation

• Gamma Rays

Nuclear Chemistry• 3 types of Nuclear Radiation

• Gamma Rays

ϒ

Nuclear Chemistry• 3 types of Nuclear Radiation

Nuclear Chemistry• Writing Nuclear Reactions• A nuclear reaction occurs when an alpha

particle, beta particle, or a gamma ray is emitted from the atomic nucleus.

• Sometimes, an element can change into a different element in the process. This is called a tranmutation reaction.

Nuclear Chemistry• Transmutation Reactions• When a C-14 atom releases an alpha

particle, the carbon atom changes into a beryllium atom.

6C 2He + 4Be

Nuclear Chemistry• Transmutation Reactions• What will happen if C-14 releases a

beta particle?

6C -1β + 7N

Nuclear Chemistry• Transmutation Reactions• Write the transmutation reaction when

Uranium-235 releases an alpha particle.

Nuclear Chemistry• Transmutation Reactions• Positron and Neutron Capture• A capture is when a nucleus absorbs a

particle.

Nuclear Chemistry• Transmutation Reactions• Positron and Neutron Capture• A capture is when a nucleus absorbs a

particle.

Nuclear Chemistry• Fission – Discovered by Enrique Fermi at

University of Chicago.

Nuclear Chemistry• Fission

Nuclear Chemistry• Fission• In order to initiate fission, there must be an

exact amount of U-235.• This exact amount is called the critical mass.• It is about the size of a grapefruit.

Nuclear Chemistry• Using fission to Make Electricity• Fission of U-235 is used to produce heat in order to

boil water.• The steam generated is used to turn a turbine that produces electricity.

Nuclear Chemistry• Using fission to Make Electricity• Cadmium control rods are used to absorb neutrons

and slow down the fission process.• If the control rods are fully inserted into the U-235

core, the process of fission ceases.

Nuclear Chemistry• Fission

Nuclear Chemistry• Fusion

Nuclear Chemistry• Fusion

Nuclear Chemistry• Rates of Radioactive Decay

o Half-Life (t1/2): The amount of time required for ½ of the nuclei of a radioactive sample to emit their radioactivity.

t1/2 C-14 = 5715 years (beta decay)

t1/2 K-40 = 1,300,000,000 years (beta decay)

t1/2 U-238 = 4,500,000,000 years (alpha decay)

t1/2 Sr-90 = 28.8 years (Beta decay)

t1/2 I-131 = 0.022 years (Beta decay)

Nuclear Chemistry• Radioactive Half-Lives

Nuclear Chemistry• Rates of Radioactive Decayo What percentage of the original sample of a

radioisotope would remain if 4 half-lives elapse?

Nuclear Chemistry• Rates of Radioactive Decayo The half-life of Co-60 is 5.3 years. How

much of a 1.000 mg sample of Co-60 is left after a 15.9 year period?

Nuclear Chemistry• Rates of Radioactive Decayo Since radioactive decay is first-order, we can

use the following equation;

k = 0.693 t1/2

k = reaction (decay) constantt1/2 = half-life of radioisotope

Nuclear Chemistry• Rates of Radioactive Decayo If we know the initial amount and current

amount of a radioisotope;

ln Nt = -kt No

k = reaction (decay) constantt = timeNt = number of nuclei remaining at tNo = initial number of nuclei

Nuclear Chemistry• Energy Changes in Nuclear Reactionso Sometimes the mass of the reactants don’t

equal the mass of the products in a nucleuar reaction.

o Where did the mass go?

E = mc2

E = Energym = massc = speed of light (3.00 x 109 m/s)

Nuclear Chemistry• Energy Changes in Nuclear Reactionso Calculate the energy change for the

following nuclear reaction;

U-238 Th-234 + alpha particle

U-238 = 238.0003 amuTh-234 = 233.9942 amuAlpha particle = 4.0015 amu