Write a nuclear reaction when Uranium 238 emits an alpha particle to form a Thorium(Th) nucleus.

Hint: Uranium 238 is a radioactive element and radioactive element emit $\alpha $ particles or $\beta $ particles and also $\gamma $ rays, here ${{U}^{238}}$ emits $\alpha $ particle.

Complete step by step answer:
Radioactivity:- Uranium and some of its salts emit spontaneously some visible radiation which penetrates through opaque substance and affects the photographic plate. These rays are called radioactive rays.
The spontaneous emission of rays from a substance called radioactivity and the substance are called radioactive substances.
Radioactive decay:- The atoms of radioactive elements emit $\alpha $ or $\beta $ particles and $\gamma $ rays. As a result both the atomic weight and atomic number are changed. Thus the original radioactive atom is decayed and an atom of some new element is born. This phenomena is called radioactive decay of radioactive disintegration.
When $\alpha $ particle is emitted from a Uranium atom, then it is converted into a thorium atom. This can be represented by the following equation
${}_{92}{{U}^{238}}\to {}_{2}H{{e}^{4}}+{}_{90}T{{h}^{234}}$
This shows that $\alpha $ emission results in an atom whose mass number is lower by 4 and atomic number is lower by 2 then the parent atom.

Note: When we write the equation, Uranium atom converts into Thorium atom, and along with it emits a alpha particle, alpha particle is nothing but a positively charged helium atom.