Question

Iodine-137 decays to give xenon-137, which decays to give cesium-137. What are the modes of decay in these two reactions?

Answer 1

Decay of I-137 to Xe-137

Atomic number of I is 53 and of Xe is 54

₅₃I¹³⁷   -------> ₅₄Xe¹³⁷

We see in above reaction that mass number (number of protons + number of neutrons) remains same as 137 while atomic number increases by one unit from 53 for I to 54 for Xe. This can happen by beta emission. During beta emission a neutron is converted to a proton and electron. As a result mass number remains same and number of protons increases by one. The electron is emitted out and is called as beta particle.

₀n¹ -------> ₁H¹ + _-1e⁰ (neutron -----> proton + electron)

So decay of I-137 to Xe-137 is by beta emission and may be represented as follows,

₅₃I¹³⁷   -------> ₅₄Xe¹³⁷ + _-1e⁰       

Decay of Xe-137 to Cs-137

Atomic number of Xe is 54 and of Cs is 55

₅₄Xe¹³⁷   -------> ₅₅Cs¹³⁷

We see in above reaction that mass number (number of protons + number of neutrons) remains same as 137 while atomic number increases by one unit from 54 for Xe to 55 for Cs. This can happen by beta emission. During beta emission a neutron is converted to a proton and electron. As a result mass number remains same and number of protons increases by one. The electron is emitted out and is called as beta particle.

₀n¹ -------> ₁H¹ + _-1e⁰ (neutron -----> proton + electron)

So decay of Xe-137 to Cs-137 is by beta emission and may be represented as follows,

₅₄Xe¹³⁷   -------> ₅₅Cs¹³⁷ + _-1e⁰       

Answer: Mode of decay is beta emission in both the given reactions.