When a radioactive substance emits an alpha particle, its position in the periodic table is lowered by
A. One place
B. Two places
C. Three places
D. Four places

Hint: In the given question, we need to determine the position of a radioactive substance that emits an alpha particle in the periodic table. For this, we have to use the concept of whenever an alpha particle is emitted from the nucleus of a radioactive atom, the atomic number is decreased by two and mass number is also decreased by four units.

Complete step by step solution:
We know that the atomic number as well as mass number of a radioactive atom is both decreased by two and four respectively whenever an alpha particle is released from the atom's nucleus. Mathematically, it is represented as,
\[{}_Z{X^A} \to {}_{Z - 2}{Y^{A - 4}} + {}_2H{e^4} \\ \]
As per their atomic number, elements are arranged in the periodic table. An element's atomic number is decreased by two by the emission of alpha-particles from its atom, which causes the element to change into a new variant that is located two steps earlier in the periodic table. Hence, when a radioactive substance emits an alpha particle, its position in the periodic table is lowered by two places.

Therefore, the correct option is B.

Additional Information: A pair of protons and two neutrons combine to form an alpha particle. Both protons and neutrons are tightly bonded to one another and combine to form a single particle. The resultant particle and the helium-\[4\] nucleus are identical. Typically, alpha decay results in the production of alpha particles.

Note: Many students may write the explanation without the mathematical representation of an alpha particle. But mathematical representation gives a clear idea about the solution. So, we need to explain the key concept using mathematical expressions.