State the relation between atomic number and atomic mass for light elements. State which elements are considered radioactive giving reasons.

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}$2^4\text{He}, $6^{12}\text{C}$ etc., have their mass numbers twice the atomic numbers except for ${}_4^9\text{Be}$4^9\text{Be} and ${18}^{40}\text{Ar}$.
Elements which have an odd number of protons like ${}_3^7\text{Li}$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}$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.