Humboldt State University ® Department of Chemistry

Richard A. Paselk

Chem 110	General Chemistry	Fall 2003
Lecture Notes::Lec 39_10 December	© R. Paselk 2003
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Nuclear Chemistry

In Chem 109 we defined chemistry as the study of matter and its transformations. Most of chemistry is restricted to transformation involving electrons. However in its broadest sense our definition also includes some specific types of nuclear transformation, where atoms of one element are transformed into atoms of another element. This week we are going to survey some aspects of this nuclear chemistry.

Review

Atoms consist of three different types of particles: electrons, protons and neutrons (the common form of one very important atom, hydrogen, has only two kinds: a proton and an electron). The protons and neutrons reside in a small inner portion called the nucleus while the electrons reside in a relatively large cloud centered on the nucleus. (Note the incredible density of the nucleus - Neutron star, which can be thought of as nuclear matter, of 2 solar masses has radius of 10km.) Important properties of these particles are listed in the table below:

Particle Charge Relative Mass Mass Electron (e-) -1 1/1840 9.11 x 10-28g Proton (p or H+) +1 1.0 1.67 x 10-24g Neutron (n) 0  1.0  1.67 x 10-24g

Some important terms which you must know are:

• Atomic number (Z) - the number of protons in the nucleus. This number is characteristic of a given element.
• Atomic mass number (A) - the sum of the protons and neutrons in a given atom (p + n).
• Atomic mass - the actual mass of an average atom in a sample. The characteristic atomic masses for Earth are shown on periodic tables.
• Atomic Mass Unit: the atomic mass unit = amu is a unit of mass for atoms. It is defined as 1/12 the mass of one atom of ¹²C, where the mass of ¹²C is defined as 12 exactly.

Isotopes are forms of elements which differ only in the number of neutrons. This means different isotopes of the same element have essentially the same chemical properties but slightly different physical properties. They can also differ substantially in terms of their nuclear stability.

Nuclear Stability

In parallel with chemical systems we've seen in the past, the stability of a nucleus is characterized by two factors:

1. Its thermodynamic stability, that is the amount of energy given off when the nucleus decays. (Note that under normal conditions one of the ways we characterized thermodynamic stability for chemical reactions, equilibrium conditions, does not apply in any practical way. This is because the equilibrium so favors products that the reverse reactions are not seen except under extraordinary conditions of high energy (high temperature) such as in a particle accelerator or a supernova explosion.).
2. Its kinetic stability, that is how rapidly a collection of nuclei decays. (Note that for an individual nucleus this is expressed as the probability it will decay during a given time interval. For a long lived species, the probability of decay is very low and vice-versa.)

Some observations of nuclear stability which provide hints to its nature are (overhead - zone of stability):

• There are NO stable nuclides with 84 or more protons - Bi is the "last" stable element, that is the last element with at least one stable isotope.
• For light nuclei a 1:1 ratio of neutrons to protons is generally stable (Calcium is the heaviest nuclei for this to be true). Many of these elements also have heavier isotopes.
• For nuclei heavier than Ca the ratio of neutrons to protons grows with atomic number to give a stable nuclide. (Note the trend of Atomic mass vs. Z on the Periodic Table).
• Nuclides with even numbers of protons and neutrons tend to be more stable (see table in text).

Why should these trends occur? The ratio in part can be rationalized by proton repulsion vs. neutron 'glue.'

Radioactive Decay

Although we have so far restricted our discussions to stable isotopes and nuclides, many nuclides are unstable and undergo nuclear decay. Since nuclear decay constitutes the most common form of nuclear transformation, we'll begin our discussion with it.

Let's begin with three common types of mass-energy losses, or radioactive emissions, that take place during nuclear decay, originally all referred to as "rays", but now two are more properly referred to as particles:

1. Alpa (a) particles: these are the largest radioactive emissions. They consist of particles identical to helium nuclei = ⁴₂He.
2. Beta (b) particles: these are identical to electrons = ⁰_-1e.
3. Gamma (b) rays: these are high energy photons, generally of greater energy than x-rays.

When we look at nuclear decay we write equations that are very similar to chemical equations, except we keep track of nucleon by displaying both atomic mass number and atomic number. We also balance them similarly, balancing charge as in regular chemical equations, however, we balance nucleon number instead of mass, e.g.:

Note that in this equation we have a change in charge of -2 and of nucleon number of -4, thus the process has emitted 2 protons and 2 neutrons = an alpha particle. So the balanced equations is:

²³⁸₉₂U Æ ²³⁴₉₀Th + ⁴₂He

Let's try some additional examples:

Thorium decay:

²³⁴₉₀Th Æ ? + ⁰_-1e

Balancing charge (=+1) and mass (no change):

²³⁴₉₀Th Æ ²³⁴₉₁Pa + ⁰_-1e

Carbon-14 decay:

¹⁴₆C Æ ¹⁴N + ?

balancing charge

¹⁴₆C Æ ¹⁴₇N + ⁰_-1e

Radon decay:

²¹⁹Rn Æ ? + ⁴₂He

balancing

²¹⁹₈₆Rn Æ ²¹⁵₈₄Po + ⁴₂He

Lead decay:

²¹⁰₈₂Pb Æ ²¹⁰₈₃Bi + ?

balancing

© R A Paselk

Last modified 12 December 2003