Periodic Table — Periodic Properties & Variation of Properties — Question 28

Electronic configuration of lighter elements shows that the elements which have an even number of proton, for example, atomic numbers like $_2^4\text{He}$, $_6^{12}\text{C}$ etc., have their mass numbers twice the atomic numbers except for $_4^9\text{Be}$ and $_{18}^{40}\text{Ar}$.
Elements which have an odd number of protons like $_3^7\text{Li}$, $_5^{11}\text{B}$ etc., have their mass number twice the atomic numbers + 1 (A = 2Z + 1) except $_7^{14}\text{N}$ and $_1^1\text{H}$

Elements with neutron:proton ratio 1.5 and above are considered as radioactive as the nucleus of such elements becomes unstable causing their radioactive decay. For example, in Uranium $(_{92}^{235}\text{U})$:

No. of protons (p) = 92
No. of neutrons (n) = 235 - 92 = 143

n : p = $\dfrac{143}{92}$ = 1.5543

A neutron:proton ratio greater than 1.5 makes Uranium radioactive.