Question

Uranium- $238$ decays into Thorium- $234$ . How do scientists explain why this happens?

Answer 1

Hint : As we know the laws of conservation of mass number and atomic number, calculate the difference in mass number and atomic number between the parent and daughter nucleus. From this calculation, we can easily find out what type of radioactive decay it is.

Complete step-by-step solution:
This is an example of radioactive decay which causes transmutation. The unstable nucleus ${}_{92}^{238}U$ (Uranium- $238$ ) releases an alpha particle and decays into ${}_{90}^{234}Th$ (Thorium- $234$ ).
${}_{92}^{238}U\to {}_{90}^{234}Th+{}_{2}^{4}\alpha$

Additional Information:
Soddy and Fajans formulated two laws based on the observations made on radioactive decay ( $\alpha$ -decay and $\beta$ -decay). These are known as Soddy-Fajans displacement laws.
Law of $\alpha$ -decay – Due to $\alpha$ -decay of a radioactive nucleus, the mass and charge of the daughter nucleus decreases by $4$ and $2$ respectively from the parent nucleus.
In addition to the well-established laws of conservation of momentum, angular momentum, mass-energy, two more conservation laws to be specially mentioned in case of radioactive decay –
Conservation of mass number – Radioactive decay does not bring about any charge in the total number of neutrons and protons. So, the mass number remains unchanged.
Conservation of atomic number (proton number) – There is no change in the total number of protons in reactant (parent) and in product (daughter nucleus $+$ emitted particle). So, the atomic number remains the same. These conservation laws also hold good in case of artificial transmutation of elements.

Note:
In both $\alpha$ and $\beta$ decay, the mass of the products (the daughter nucleus and emitted particle) is found to be less than the mass of the parent nucleus. As per Einstein’s mass-energy equivalence, this lost mass is transferred to energy and this energy is called disintegration energy.