# Chapter 2 - Exercises - Page 81: 103

#### Work Step by Step

Isotones are those chemical species which contains same number of neutrons. The number of protons in thorium is 90. Calculate the number of neutrons as follows: \begin{align} & \text{Number of neutrons}=\text{Mass number}-\text{Number of protons} \\ & =236-80 \\ & =146 \end{align} The elements having same number of neutrons are $^{237}\text{Pa}$, $^{238}\text{U}$, $^{239}\text{Np}$, and $^{240}\text{Pu}$.

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