Electron Capture & Positron Emission: Videos & Practice Problems

Electron capture occurs when an unstable nucleus absorbs an electron from its inner orbital, reducing protons and increasing neutrons, resulting in a neutron formation. For example, Cesium-131 captures an electron to become Xenon-131. In positron emission, an unstable nucleus emits a positron, an antiparticle of the electron, which also reduces protons and increases neutrons. Cesium-130 emits a positron to transform into Xenon-130. Both processes help stabilize nuclei with excess protons, maintaining mass and atomic number balance.

Electron Capture

Electron capture is a process that occurs in unstable nuclei with an excess of protons. During this process, an unstable nucleus absorbs an electron from its inner electron orbital. This absorption helps to reduce the number of protons while simultaneously increasing the number of neutrons. Specifically, when a proton and an electron combine, they form a neutron, which effectively decreases the proton count and brings the nucleus closer to a balanced state.

For example, consider Cesium-131 (Cs-131) undergoing electron capture. In this reaction, the nucleus captures an electron, resulting in a transformation where the mass number remains unchanged. The mass number on both sides of the equation must be equal, and the total number of protons must also balance. In the case of Cs-131, the mass number is 131, and the atomic number is 55. After capturing an electron, the atomic number decreases by one, resulting in an atomic number of 54. Referring to the periodic table, the element with atomic number 54 is xenon (Xe). Therefore, the reaction can be summarized as:

Cs-131 + e^- → Xe-131 + ν

Here, ν represents a neutrino, which is often emitted during the process. This transformation illustrates how electron capture serves as a mechanism for unstable nuclei to achieve greater stability by adjusting their proton-to-neutron ratio.

Positron Emission

Positron emission is a type of radioactive decay where an unstable nucleus emits a positron, which is the antiparticle of the electron. The positron is denoted by the symbol e⁺, indicating its positive charge and lack of mass number, similar to the electron. When a positron encounters an electron, they annihilate each other, resulting in the production of two gamma rays. This process typically occurs in nuclei with an excess of protons, helping to reduce the proton count while increasing the number of neutrons.

In positron emission, a proton within the nucleus transforms into a neutron and a positron, effectively decreasing the number of protons. For example, consider cesium-130 (¹³⁰Cs). During positron emission, the mass number remains unchanged at 130, while the atomic number decreases by one due to the emission of a positron. The initial atomic number of cesium is 55, so after the emission, the new atomic number becomes 54, leading to the formation of xenon-130 (¹³⁰Xe). This transformation can be summarized in the following nuclear reaction:

¹³⁰Cs → ¹³⁰Xe + e⁺

In this reaction, the mass numbers on both sides are equal, and the number of protons is conserved, illustrating the principles of positron emission and its role in stabilizing unstable nuclei.

Electron Capture & Positron Emission Reaction Example

In nuclear chemistry, understanding the processes of electron capture and positron emission is essential for analyzing radioactive decay. Let's explore these two reactions in detail.

First, consider the electron capture of iridium-189. Iridium (Ir) has an atomic number of 77, which means it has 77 protons. In electron capture, an electron is absorbed by the nucleus, leading to a transformation of a proton into a neutron. The reaction can be represented as follows:

\[\text{Ir}_{77}^{189} + e^- \rightarrow \text{Os}_{76}^{189} + \nu_e\]

In this equation, the mass number remains the same at 189, while the atomic number decreases from 77 to 76, resulting in the formation of osmium (Os), another notable element due to its high density.

Next, we examine the positron emission of uranium-229. Uranium (U) has an atomic number of 92. During positron emission, a proton is converted into a neutron, and a positron (the antimatter counterpart of an electron) is released. The reaction can be expressed as:

\[\text{U}_{92}^{229} \rightarrow \text{Pa}_{91}^{229} + e^+ + \nu_e\]

Here, the mass number remains unchanged at 229, while the atomic number decreases from 92 to 91, resulting in the formation of protactinium (Pa). The positron emission signifies the release of energy and the transformation of the element.

Both processes illustrate the fascinating transformations that occur in nuclear reactions, highlighting the interplay between protons, neutrons, and electrons in the stability of atomic nuclei.

Provide a daughter nuclide when Rn-215 undergoes 2 sets of alpha decay and a positron emission.

• A.
• B.
• C.
• D.

Answer: (A)

Identify the missing species from the following nuclear reaction.

• A.
• B.
• C.
• D.

Answer: (B)

Here’s what students ask on this topic:

What is electron capture, and how does it affect the nucleus of an atom?

Electron capture is a nuclear process where an unstable nucleus absorbs an electron from its inner orbital. This electron combines with a proton in the nucleus to form a neutron. As a result, the number of protons in the nucleus decreases by one, while the number of neutrons increases by one. This process helps the nucleus achieve a more stable proton-to-neutron ratio. Importantly, the mass number (total number of protons and neutrons) remains unchanged, but the atomic number (number of protons) decreases by one, leading to the formation of a new element. For example, cesium-131 undergoes electron capture to form xenon-131, as shown in the equation:

${\mathrm{Cs}}^{131}+e^0\to {\mathrm{Xe}}^{131}$

What is positron emission, and how does it stabilize an unstable nucleus?

Positron emission is a nuclear decay process where an unstable nucleus emits a positron, which is the antiparticle of an electron. A positron has no mass number and a positive charge. During this process, a proton in the nucleus is converted into a neutron and a positron. The positron is ejected from the nucleus, reducing the number of protons by one and increasing the number of neutrons by one. This helps stabilize nuclei with an excess of protons. The mass number remains constant, but the atomic number decreases by one, resulting in the formation of a new element. For example, cesium-130 undergoes positron emission to form xenon-130, as shown in the equation:

${\mathrm{Cs}}^{130}\to {\mathrm{Xe}}^{130}+e^0$

How do electron capture and positron emission differ in terms of their mechanisms?

Electron capture and positron emission are both processes that reduce the number of protons in a nucleus, but they differ in their mechanisms. In electron capture, the nucleus absorbs an inner orbital electron, which combines with a proton to form a neutron. This decreases the atomic number by one while keeping the mass number constant. In contrast, positron emission involves a proton in the nucleus transforming into a neutron and emitting a positron (a particle with no mass number and a positive charge). The positron is ejected from the nucleus, also decreasing the atomic number by one while keeping the mass number constant. Both processes help stabilize nuclei with an excess of protons, but electron capture involves absorbing an electron, while positron emission involves emitting a positron.

What happens to the mass number and atomic number during electron capture?

During electron capture, the mass number of the nucleus remains unchanged because the total number of protons and neutrons stays the same. However, the atomic number decreases by one because a proton in the nucleus combines with the captured electron to form a neutron. This process reduces the number of protons by one and increases the number of neutrons by one, resulting in the formation of a new element with an atomic number that is one less than the original element. For example, cesium-131 undergoes electron capture to form xenon-131, as shown in the equation:

${\mathrm{Cs}}^{131}+e^0\to {\mathrm{Xe}}^{131}$

What is the role of gamma rays in positron emission?

Gamma rays are often produced as a secondary effect in positron emission. When a positron is emitted from the nucleus, it eventually encounters an electron in the surrounding environment. Since the positron and electron are antiparticles, their collision results in annihilation, converting their mass into energy in the form of two gamma rays. These gamma rays are emitted in opposite directions to conserve momentum. While gamma rays are not directly involved in the nuclear transformation during positron emission, their production is a key indicator of positron annihilation and is often used in medical imaging techniques like positron emission tomography (PET).