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 ⁴He₂, ¹²C₆ etc., have their mass numbers twice the atomic numbers except for ⁹Be₄​ and ⁴⁰Ar_18.

Elements which have an odd number of protons like ⁷Li_3, ¹¹B₅​ etc., have their mass number twice the atomic numbers + 1 (A = 2Z + 1) except ¹⁴N₇​ and ¹​H₁

Elements with neutron: proton ratios 1.5 and above are considered as radioactive as the nucleus of such elements becomes unstable causing their radioactive decay. For example, in Uranium (²³⁵U₉₂):

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