Introduction to Nuclear Chemistry

Electrons play the dominant role in Chemical Reactions. Chemical reactions involve the making and breaking of chemical bonds and these bonds involve the transfer or sharing of electrons. Nuclear reactions involve changes in the nucleus. These changes involve very large changes in energy. Nuclei that spontaneously emit radiation (high energy particles or electromagnetic radiation) are said to be radioactive.

Uses – The high energy makes it easy to follow very small amounts of matter. A major use is tracer analysis; we can follow what happens to different compounds and elements by following the radiation. We can follow substances through organs in our bodies or through the environment.

Nuclear reactions are also useful because of the large amount of energy emitted. In medicine the radiation is more harmful to fast growing tissue such as cancer tumors than to normal tissue. NR can be used to produce electrical energy and to produce weapons of mass destruction.

Radioactivity

¹⁴₆C Here the 6 is the atomic number = # protons in nucleus and 14 is the mass number = #protons and neutrons.

Protons and neutrons are called nucleons.

¹⁴₆ C and ¹²₆ C are called isotopes of carbon. They differ by the number of neutrons.

Nuclei that are radioactive are called radionuclides and atoms containing these are called radioisotopes.

Nuclear Reactions

One common type of nuclear reaction is the spontaneous emission of an alpha particle

²³⁸₉₂U ® ²³⁴₉₀ Th + ⁴₂He

Alpha is the first letter in the Greek alphabet and alpha rays were the first type of radioactivity discovered. The helium-4 nuclei (without the two electrons associated with helium atoms) is emitted from the nucleus of uranium-238. Note how atomic number and mass number is conserved. 90 + 2 = 92 and 234 + 4 = 238. In all the reactions that follow, the number of protons and neutrons is the same on both sides of the equations.

⁴₂ He = ⁴₂ a

Complete the following reactions, assuming the radioisotopes emit alpha particles

²²⁶₈₈ Ra =

²²²₈₆Rn =

A second common nuclear reaction is the spontaneous emission of a beta particle, symbolized by 0 –1e or by 0 -1 b. Beta emission is equivalent to the conversion of a neutron to a proton and an electron. The electron is emitted by the nucleus as a high energy particle, and the nucleus has changed into an element with one higher atomic number and the same mass number.

¹₀n® ¹₁p + ⁰_-1e

¹⁴₆ C ® ¹⁴₇N+ ⁰_-1 e

Note how 6 = 7-1 and 14 = 14 + 0 above. Complete the following reactions assuming beta (e^-) emission.

¹³¹₅₃ I ® ?

¹⁸₆ O ® ?

Gamma radiation (or gamma rays, ⁰₀ g) consist of high energy electromagnetic radiation (higher energy than x-rays) and is emitted by a nucleus relaxing from an excited state to a lower energy state. The high energy state usually comes from an earlier emission process.

²³⁸₉₂U ® ²³⁴₉₀Th^* + ⁴₂ He

²³⁴₉₀ Th^* ® ²³⁴₉₀ Th + ⁰₀g ® ⁰_-1e + ²³⁴₉₁Pa

There is no change in atomic number or mass number with gamma ray emission. The above emission processes (alpha, beta, and gamma) are the most common types of radioactivity.

Another type of radioactivity (usually from synthetic or man-made isotopes) comes from the emission of a positively charged electron (called a positron). The emission of a positron is equivalent to the conversion of a proton in the nucleus into a neutron and a positron. This positron is emitted with high energy, reducing the atomic number by one without changing the mass number.

¹₁ p ® ¹₀ n + ⁰₁ e

¹¹₆ C® ¹¹₅ B + ⁰₁e

This positively charged electron will react with a negative electron with the total conversion of mass into the energy of two gamma rays.

⁰_-1 e + ⁰_-1e ® 2 ⁰₀ g

This is the basic for Positron Emission Tomography (PET scan), where molecules labeled with a positron emitter can be followed in the body. By following gamma ray emission, this can be used as a diagnostic tool or to study how the body interacts with certain molecules.

Electron capture (usually from synthetic isotopes) has the same effect on atomic number and mass number as positron emission. Positron emission is more common in small nuclei while electron capture tends to happen with larger nuclei. As the name suggest, a nucleus can capture a electron, converting a proton into a neutron.

¹₁p + ⁰_-1 e ® ¹₀ n

²⁰¹₈₀ Hg + ⁰_-1 e ® ²⁰¹₇₉ Au

Summary of Nuclear Reactions

	Atomic # = # Protons Z	Mass # = # Protons + # Neutrons A
Alpha a = ^42 He	-2	-4
Beta b = ^0-1e	+1	0
Gamma g = ^00g	0	0
Positron +b = ^01e	-1	0
e^- Capture	-1	0

Band of Stability

How are all of those positive protons held together in the nucleus? The repulsion of the positive charges is overcome by what is called the strong nuclear force which works at the extremely small distances of a nucleus (10^-15 m). At these small distances, neutrons and protons experience a strong attraction for each other, while the positive charged protons also have a repulsive force due to their electrical charge. From Z (atomic number) = 1 to 20, there are about the same number of neutrons as protons in the nucleus (e.g. ⁴⁰₂₀Ca has 20 protons and 20 neutrons). Above 20 protons, nuclei need more and more neutrons to hold the protons together. For example, Pb-206 has 82 protons and 124 neutrons. A graph of stable isotopes with the y-axis as the number of neutrons and the x-axis as the number of protons reveals a band of stability showing what combinations of p and n are stable.

Isotopes above the band have too many n and so

¹₀ n ® ⁰_-1e + ¹₁ p so these element are b emitters

Isotopes below the band have too many p and so

¹₁ p ® ¹₀ n + ⁰₁e so these elements are positron emitters (or tend to undergo electron capture)

Isotopes to the northeast of the band (Z>83) have too many protons and neutrons and tend to emit alpha particles.

Radioactive Series

Elements that cannot achieve stability with one reaction, can have a series of reactions.

3 natural series

U-238 undergo 14 steps to reach Pb-206 (8 a and 6 b)

U-235 undergoes ? steps to reach Pb-207

Th-232 undergoes ? steps to reach Pb-206

Magic Numbers – Atoms having 2, 8, 18, and 32 electrons (noble gases) are very stable. There are also numbers of protons and neutrons that tend to produce stable nuclei. These numbers are called "magic numbers," and are listed below.

neutrons – 2, 8, 20, 28, 50, 82, 126

protons – 2, 8, 20, 28, 50, 82

also even numbers of protons and neutrons are more stable and more abundant than odd numbers.

Nuclear Transmutations

A nuclear transmutation represents the change of an element into a different element. These can be both natural and human made (synthetic).

The first reaction listed below is a reaction Rutherford did using an alpha particle emitting material, producing the products shown. Particle accelerators can also be used to produce transmutations. Particle accelerators accelerate protons, electrons, and nuclei, producing new isotopes.

Rutherford ⁴₂He (a) + ¹⁴₇N → ¹⁸₉ F^* → ¹⁷₈ O + ¹₁ p

²⁰⁹₈₃ Bi + ⁶⁴₂₈Ni → ²⁷²₁₁₁X + ¹₀ n