Question

In the final step of the uranium- 238 disintegration series, the parent nuclide decays into lead-206 and an alpha particle. Identify the parent nuclide.

Short answer

The parent nuclide is polonium-210.

Step-by-step solution

Understanding Alpha Decay

In alpha decay, the nucleus emits an alpha particle which consists of 2 protons and 2 neutrons. Therefore, the atomic number of the resulting atom decreases by 2 and the mass number decreases by 4.

Identifying the Daughter Nuclide

The given problem states that the final nuclide in the decay series is lead-206. Lead (Pb) has an atomic number of 82. So, we start from Pb-206 and know that its precursor would have 2 more protons and 4 additional mass units due to the emission of an alpha particle.

Calculate the Parent Nuclide

To find the parent nuclide, we add 2 to the atomic number and 4 to the mass number of lead-206. This results in an atomic number of 84 and a mass number of 210.

Identify the Parent Element

The element with atomic number 84 is polonium (Po). Therefore, the parent nuclide is polonium-210.

Key concepts

Alpha Decay

Alpha decay is one type of radioactive decay where an unstable nucleus emits an alpha particle. An alpha particle is made up of 2 protons and 2 neutrons, essentially the same as a helium nucleus. This emission results in two significant changes to the original nucleus:

• The atomic number decreases by 2, as it loses 2 protons.
• The mass number decreases by 4, as it loses a total of 2 protons and 2 neutrons.

Alpha decay typically occurs in very heavy elements that have an excess of protons and neutrons. This decay helps the atom move toward a more stable configuration by shedding some of its mass.

Alpha particles are relatively heavy and carry a double positive charge. They are not very penetrating, as they can be stopped by a piece of paper or the outer layers of human skin. However, they can cause significant damage if ingested or inhaled because they are highly ionizing.

Uranium-238 Disintegration Series

The uranium-238 disintegration series is a sequence of radioactive decays that uranium-238 undergoes until it becomes a stable nuclide. Each step in this series involves the transformation of one radioactive element into another through the release of alpha and sometimes beta particles.

In the uranium-238 decay series, the process begins with uranium-238 itself. This unstable isotope undergoes a series of 14 distinct decay events, ultimately resulting in the formation of lead-206, which is stable:

• Uranium-238 (U-238) decays to thorium-234 (Th-234) via alpha decay.
• Th-234 undergoes beta decay to become protactinium-234 (Pa-234).
• This sequence continues with further alpha and beta decays through several more elements until it becomes lead-206 (Pb-206).

The completion of the disintegration series to lead means that the material is no longer radioactive. This decay chain emphasizes the natural radioactive processes that return elements to stability over time.

Nuclear Reactions

Nuclear reactions involve changes in an atom's nucleus and can result in the production of different elements. These reactions release a significant amount of energy due to the strong forces inside the nucleus.

There are several types of nuclear reactions, but they generally fall into two categories:

• Fission: A heavy nucleus splits into two smaller nuclei, releasing energy in the process. This is the principle behind nuclear power plants and atomic bombs.
• Fusion: Two light nuclei combine under extreme temperatures and pressures to form a heavier nucleus. This reaction powers the sun and other stars.

Alpha decay is a type of nuclear reaction because it entails changes in the nucleus of the parent nuclide. The energy released during such decays is part of what drives the Earth's natural heat.

Understanding these reactions helps us harness nuclear energy for power and comprehend the natural radioactive processes that occur in nature.