Question

The most common radium isotope found on earth, ${}^{\mathbf{226}}\mathit{R}\mathit{a}$, has a half-life of about $\mathbf{1600}$years. If the earth was formed well over${\mathbf{10}}^{\mathbf{9}}$ years ago, why is there any radium left now?

Short answer

All of the nuclides have descended from ${}^{238}U$.

Step-by-step solution

Define the isotopes.

The two or more atoms having the same atomic number and different mass number are known as isotopes.

For example: ${}_8{}^{16}O$and${}_8{}^{15}O$

Explanation.

All of the nuclides in the decay process are in secular equilibrium, thus they just pass through the ${}^{206}Pb$on their way and the others stops along the way of${}^{222}Rn$, due to the longer life of $3.8$days radon is worst offender.

Hence, all of the nuclides have descended from ${}^{238}U$.