Question

Which of the following is a radioactive substance?
A. Uranium
B. Radium
C. Thorium
D. All of above

Answer 1

In order to solve this question, we would understand the definition of radioactive substance which states that those materials whose constituent atoms are unstable so these substances decay naturally. These atoms radiate alpha, beta and gamma rays which are radioactive and have high penetrating power. The only difference with X-rays is that they cannot be turned off like X-ray so radioactive decay is a continuous process. For example Iridium- $92$ are emitters of gamma radiation.

Complete answer:
Let us check each option and examine their radioactivity.
So first Uranium. Uranium has isotope configuration $_{92}{U^{238}}$ It has $92$ protons, $92$ electrons and $146$ neutrons and it is the most abundant radioactive substance found on earth. The half-lives of the natural isotope of uranium is $4.5$ billion years. It’s continuously decaying and found in less concentration.

Radium has isotope configuration $_{88}R{a^{226}}$ It is most stable isotope of radium having half-lives of $1600$ years while other radium isotope has half-lives of $11.43$ days too. It is a radioactive substance and it is continuously decaying. This isotope has $88$ protons, $88$ electrons and $50$ neutrons.

Thorium has isotope configuration $_{90}T{h^{227}}$ It has $90$ protons, $90$ electrons and $47$ neutrons and it is most stable radioactive isotope of half-lives $18.68$ days. It is quite reactive and it catches fire when finely powdered.

So the answer is option D.

Note: It should be remembered that all radioactive substances have unstable nuclei and they emit neutrons which can initiate chain reaction. These radioactive substances are a high source of nuclear energy and are very useful as $1gm$ of Uranium can provide electricity to a city over $50years$ but the only problem is radioactive reactions are uncontrollable.