## Chemistry: Molecular Approach (4th Edition)

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 $^{240}\text{Pu}$.