Question

One of the fundamental particles is missing in one of the isotopes. Then particle and isotope are respectively:
A) neutron, protium
B) neutron, tritium
C) proton, protium
D) electron, tritium

Answer 1

It is the most common isotope of hydrogen, which is found largely in nature, sometimes more than $99.98\%$. Its mass number and atomic number, both are $1$. It is also used as a medicine to help to reduce the amount of acid produced in your stomach and intestine.

Complete answer:
Protium $\left( {{}^1{H_1}} \right)$ is the isotope. It has only one proton and no neutron. So, the missing fundamental particle is the neutron. Protium is the most common isotope of hydrogen, which is found largely in nature, sometimes more than $99.98\%$. The nucleus of Protium consists of only a single proton. So, its mass number and atomic number, both are $1$. To be precise its mass is $1.007825$ amu.
Usually, hydrogen has three naturally occurring isotopes: Protium $\left( {{}^1{H_1}} \right)$, Deuterium $\left( {{}^2{H_1}} \right)$, and Tritium $\left( {{}^3{H_1}} \right)$.
Among all three, protium is the most prevalent hydrogen isotope. It is not generally found in the monatomic form. But it can combine with itself or with other elements.
Hence the correct answer is option C.

Additional information: Hydrogen is usually found as diatomic hydrogen gas or ${H_2}$. It mostly combines with other atoms to form new compounds. But monoatomic hydrogen is rare. The $H - H$ bond is one of the strongest bonds in existence in nature. It has a bond dissociation enthalpy of almost $435.9kJ/mol$ at a temperature of $298K$. As a result, ${H_2}$ dissociates to a very minor extent until it reaches up to a higher temperature. At $3000K$, the degree of dissociation is only about $7.9\%$.
Protium has large usage as a selective “proton pump inhibitor”. It is a medicine to help to reduce the amount of acid produced in your stomach and intestine. It is also used in the treatment of acidic diseases of the stomach and intestine.

Note:
It should be noted that deuterium is also an isotope of hydrogen that consists of one proton, one neutron, and one electron. Deuterium has a large application in nuclear magnetic resonance studies. Tritium is also an isotope of hydrogen that consists of one proton, two neutrons, and one electron. We should remember that tritium is highly radioactive (the most stable radioisotope) having a half-life of $12.32$ years. Other highly unstable nuclei of hydrogen (${}^4{H_1}$ to ${}^7{H_1}$) have been established in the laboratory, but these do not usually occur naturally.