Chemistry: Principles and Practice (3rd Edition)

The number of neutrons (N) = A-Z where A is the mass number and Z is the atomic number. The number of protons (P)= Z where Z is the atomic number. Therefore, the neutron-to-proton ratio $(\frac{N}{P})$ is given by the expression $\frac{N}{P}=\frac{A-Z}{Z}$. (a) $\frac{N}{P}=\frac{A-Z}{Z}=\frac{12-6}{6}=1.00$ (b) $\frac{N}{P}=\frac{40-20}{20}=1.00$ (c) $\frac{N}{P}=\frac{90-40}{40}=1.25$ (d) $\frac{N}{P}=\frac{138-56}{56}=1.46$ (e) $\frac{N}{P}=\frac{208-82}{82}=1.54$