Band of Stability: Electron Capture & Positron Emission: Videos & Practice Problems

Isotopes located to the right of the valley of stability on the neutron-to-proton plot may undergo electron capture or positron emission to achieve greater stability. These processes, although different, share the common goal of transforming an isotope with an excess of protons into a more stable form by increasing the number of neutrons relative to protons. For example, cesium-131 can either absorb an electron (electron capture) or emit a positron (positron emission) to become xenon-131, illustrating that both processes effectively convert protons into neutrons, moving the isotope towards the valley of stability. The choice between electron capture and positron emission is complex and depends on the specific isotope, but the end result is a shift towards a more stable nuclear configuration.

Band of Stability: Electron Capture

In the context of nuclear stability, isotopes located in the upper right corner of the neutron-to-proton plot often undergo either electron capture or positron emission. Both processes serve the purpose of transforming protons into neutrons, thereby moving the isotopes closer to the band of stability, represented by a green curve on the plot. This region is characterized by an excess of protons, which necessitates a shift towards a more stable configuration.

For instance, cesium-131 can undergo electron capture, where it absorbs an electron, resulting in the formation of xenon-131 as the daughter product. Alternatively, cesium-131 can also undergo positron emission, releasing a positron and still yielding xenon-131. Despite the differing mechanisms, both processes ultimately achieve the same outcome: the conversion of cesium-131 into xenon-131.

The fundamental goal of these decay processes is to increase the neutron count while decreasing the proton count, facilitating a transition from the less stable orange region to the stable green valley of stability. This highlights the importance of understanding how isotopes in this area of the neutron-to-proton plot can utilize different pathways to achieve nuclear stability.

Band of Stability: Electron Capture Example

When strontium-80 undergoes electron capture, it absorbs an electron, which results in a transformation of the element. Strontium has a mass number of 80 and an atomic number of 38. In the process of electron capture, one proton is converted into a neutron, effectively reducing the atomic number by one while keeping the mass number unchanged. This can be represented mathematically as follows:

Initial state: Strontium-80 (₈₀Sr, atomic number 38) + e^- → Final state: Rubidium-80 (₈₀Rb, atomic number 37)

Here, the mass number remains 80 (80 + 0 = 80), while the atomic number changes from 38 to 37 (38 - 1 = 37). Referring to the periodic table, the element with atomic number 37 is rubidium. Therefore, the daughter nuclide produced from the electron capture of strontium-80 is rubidium-80.

Provide the identity of the daughter nuclide created from the positron emission of strontium-80. 


a) Yttrium-78                  
b) Radon-81                           
c) Zirconium-82                       
d) Rubidium-80

• A. Yttrium-78
• B. Radon-81
• C. Zirconium-82
• D. Rubidium-80

Answer: (D)

Here’s what students ask on this topic:

What is the band of stability in nuclear chemistry?

The band of stability, also known as the valley of stability, is a region on a neutron-to-proton plot where stable isotopes are found. It represents the optimal ratio of neutrons to protons that allows an isotope to be stable. Isotopes outside this band are unstable and tend to undergo radioactive decay to achieve a more stable configuration. For isotopes with too many protons (to the right of the band), processes like electron capture or positron emission can occur to increase the neutron-to-proton ratio and move the isotope towards stability.

What is electron capture and how does it affect an isotope?

Electron capture is a process in which an unstable nucleus absorbs an inner orbital electron, typically from the K-shell. This electron combines with a proton to form a neutron and a neutrino. The equation for electron capture can be represented as:

$p^{\mathrm{+}}+e^{\mathrm{-}}\to n^0+{\nu }^{\mathrm{e}}$

As a result, the number of protons in the nucleus decreases by one, while the number of neutrons increases by one, moving the isotope closer to the band of stability. For example, cesium-131 undergoes electron capture to become xenon-131.

What is positron emission and how does it affect an isotope?

Positron emission is a type of radioactive decay in which a proton in the nucleus is converted into a neutron while releasing a positron (the antimatter counterpart of an electron) and a neutrino. The equation for positron emission can be represented as:

$p^{\mathrm{+}}\to n^0+e^{\mathrm{+}}+{\nu }^{\mathrm{e}}$

This process decreases the number of protons by one and increases the number of neutrons by one, helping the isotope move towards the band of stability. For instance, cesium-131 can emit a positron to become xenon-131.

How do electron capture and positron emission help isotopes achieve stability?

Both electron capture and positron emission help isotopes achieve stability by altering the neutron-to-proton ratio. In electron capture, an inner orbital electron is absorbed by the nucleus, converting a proton into a neutron. In positron emission, a proton is converted into a neutron while emitting a positron. Both processes decrease the number of protons and increase the number of neutrons, moving the isotope closer to the band of stability. For example, cesium-131 can undergo either process to become xenon-131, achieving a more stable nuclear configuration.

Why is it difficult to predict whether electron capture or positron emission will occur for a specific isotope?

Predicting whether electron capture or positron emission will occur for a specific isotope is complex because it depends on various factors, including the energy levels of the nucleus, the availability of inner orbital electrons, and the specific nuclear structure of the isotope. Both processes aim to achieve the same end result—converting protons into neutrons to move towards the band of stability. However, the exact mechanism that predominates can vary widely among different isotopes, making it challenging to predict without detailed knowledge of the nuclear properties involved.