Positron Emission and Nuclear Decay in GOB Chemistry

Positron Emission

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Definition and Mechanism

Positron emission is a type of nuclear decay that occurs when an unstable nucleus emits a positron. A positron is the antiparticle of the electron, possessing the same mass but a positive charge. This process is important in nuclear chemistry and medical imaging (e.g., PET scans).

• Positron Symbol: $^{0}_{+1}e$ or $\beta^+$

• General Reaction: A proton in the nucleus is converted into a neutron, emitting a positron.

• Equation Example: $\text{Einsteinium:}\; ^{253}_{99}\text{Es} \rightarrow ^{253}_{98}\text{Cf} + ^{0}_{+1}e$

Additional info: Positron emission decreases the atomic number by one but leaves the mass number unchanged.

Balanced Nuclear Equations for Positron Emission

When writing nuclear equations for positron emission, ensure that both the mass number and atomic number are balanced on both sides of the equation.

• Uranium (U-235): $^{235}_{92}\text{U} \rightarrow ^{235}_{91}\text{Pa} + ^{0}_{+1}e$ Explanation: Uranium-235 emits a positron, converting into Protactinium-235.

• Radon (Rn-222): $^{222}_{86}\text{Rn} \rightarrow ^{222}_{85}\text{At} + ^{0}_{+1}e$ Explanation: Radon-222 emits a positron, converting into Astatine-222.

Additional info: The atomic number decreases by one, while the mass number remains the same.

Nuclear Decay Series

Types of Nuclear Decay

Nuclei can undergo several types of decay, including alpha decay, beta decay, and gamma emission. Each type changes the composition of the nucleus in a specific way:

• Alpha Decay ($\alpha$): Emission of a helium nucleus ($^{4}_{2}\text{He}$), decreases mass number by 4 and atomic number by 2.

• Beta Decay ($\beta$): Emission of an electron ($^{0}_{-1}e$), increases atomic number by 1.

• Positron Emission ($\beta^+$): Emission of a positron ($^{0}_{+1}e$), decreases atomic number by 1.

• Gamma Emission ($\gamma$): Emission of energy, no change in mass or atomic number.

Example: Decay Series of Thorium-225

Given: Thorium-225 ($^{225}_{90}\text{Th}$) undergoes 3 alpha decays, 4 beta decays, and a gamma emission. The product is:

• Stepwise Decay: $^{225}_{90}\text{Th} \xrightarrow{3\alpha} ^{213}_{84}\text{Po}$ $^{213}_{84}\text{Po} \xrightarrow{4\beta} ^{213}_{88}\text{Ra}$ $^{213}_{88}\text{Ra} \xrightarrow{\gamma} ^{213}_{88}\text{Ra}$ (gamma emission does not change the nucleus)

• Final Product: Radium-213 ($^{213}_{88}\text{Ra}$)

Summary Table: Nuclear Decay Effects

Decay Type	Symbol	Change in Mass Number	Change in Atomic Number
Alpha ($\alpha$)	$^{4}_{2}\text{He}$	-4	-2
Beta ($\beta$)	$^{0}_{-1}e$	0	+1
Positron ($\beta^+$)	$^{0}_{+1}e$	0	-1
Gamma ($\gamma$)	$\gamma$	0	0

Additional info: Understanding nuclear decay is essential for predicting the products of radioactive processes and for applications in medicine, energy, and environmental science.