Electron Capture and Positron Emission in Nuclear Chemistry

Electron Capture & Positron Emission

Introduction to Nuclear Reactions

Nuclear reactions involve changes in an atom's nucleus and can result in the transformation of one element into another. Two important types of nuclear decay processes are electron capture and positron emission, both of which affect the atomic number of the nucleus and are important in the study of nuclear chemistry.

Electron Capture

Electron capture is a process in which an unstable nucleus captures an inner orbital electron (usually from the K-shell). This electron combines with a proton to form a neutron, thereby decreasing the atomic number by one while the mass number remains unchanged.

• Definition: A proton in the nucleus captures an inner electron and is converted into a neutron.

• General Equation:

$^A_ZX + ^0_{-1}e \rightarrow ^A_{Z-1}Y$

• Effect on Nucleus: Number of protons decreases by 1; number of neutrons increases by 1.

• Example:

$^{125}_{53}I + ^0_{-1}e \rightarrow ^{125}_{52}Te$

Positron Emission

Positron emission is a type of beta decay in which a proton in the nucleus is converted into a neutron, releasing a positron (the antimatter counterpart of the electron) and a neutrino. This process also decreases the atomic number by one, with the mass number unchanged.

• Definition: A proton is transformed into a neutron, emitting a positron ($^0_{+1}e$).

• General Equation:

$^A_ZX \rightarrow ^A_{Z-1}Y + ^0_{+1}e$

• Effect on Nucleus: Number of protons decreases by 1; number of neutrons increases by 1.

• Example:

$^{134}_{55}Cs \rightarrow ^{134}_{54}Xe + ^0_{+1}e$

Positron Particle

• Definition: A positron is the antimatter equivalent of an electron, with the same mass but a positive charge.

• Symbol: $^0_{+1}e$ or $\beta^+$

Summary Table: Comparison of Electron Capture and Positron Emission

Process	Equation	Change in Atomic Number	Change in Mass Number
Electron Capture	$^A_ZX + ^0_{-1}e \rightarrow ^A_{Z-1}Y$	-1	0
Positron Emission	$^A_ZX \rightarrow ^A_{Z-1}Y + ^0_{+1}e$	-1	0

Example Problems

• Electron capture in Iodine-125:

  • Equation: $^{125}_{53}I + ^0_{-1}e \rightarrow ^{125}_{52}Te$

• Positron emission in Uranium-239:

  • Equation: $^{239}_{92}U \rightarrow ^{239}_{91}Pa + ^0_{+1}e$

Practice Problems

• Alpha decay and positron emission: Given a parent nuclide (e.g., Rn-215), determine the daughter nuclide after two alpha decays and one positron emission.

• Completing nuclear equations: Fill in missing species in nuclear reactions, such as:

$^{227}_{87}Fr \rightarrow ^{223}_{88}Ra \rightarrow ^{219}_{86}Rn$

• Identify the type of decay and the missing particles in each step.

Additional info: In nuclear equations, the sum of atomic numbers and mass numbers must be conserved on both sides of the equation. These processes are important in medical imaging (positron emission tomography, PET) and in understanding radioactive decay series.