In a particular case of alpha decay, and isotope of uranium, u with a mass number of 238 and an atomic number of 92 emits and alpha particle and transforms into thorium, which has an atomic number of 90. write the equation for this decay process

₉₂U²³⁸   →   ₉₀Th²³⁴  + ₂He⁴  

Explanation:

Alpha beta and gamma radiations are the examples of ionizing radiations. When an atom is an excited state and having high energy, the atom is in unstable state. The excess of energy is released by the atom to get the stability. The released energy is in the form of radiations which may include alpha, beta, gamma, X-ray etc.

Alpha decay:

Alpha radiations are emitted as a result of radioactive decay. The atom emit the alpha particles consist of two proton and two neutrons. Which is also called helium nuclei. When atom undergoes the alpha emission the original atom convert into the atom having mass number less than 4  and atomic number less than 2 as compared to parent atom the starting atom.

Properties of alpha radiation:

Alpha radiations can travel in a short distance.

These radiations can not penetrate into the skin or clothes.

These radiations can be harmful for the human if these are inhaled.

These radiations can be stopped by a piece of paper.

₉₂U²³⁸   →   ₉₀Th²³⁴  + ₂He⁴  

"record the time to complete a chemical reaction" would most likely be part of a lab procedure.