A) two places
B) three places
C) five places
D) one place
Correct Answer: A
Solution :
\[\alpha -particle\] is doubly ionized helium atom. When an a-particle is emitted from the nucleus of a radioactive atom, the atomic number is reduced by 2 and mass number is reduced by 4. \[_{Z}{{X}^{A}}{{\xrightarrow{{}}}_{Z-2}}{{Y}^{A-4}}\,+\,{{\,}_{2}}H{{e}^{4}}\,(\alpha -particle)\] Elements are placed in the periodic table according to their atomic number. The emission of an \[\alpha -particle\] from the atom of an element reduces its atomic number by 2, so the element is converted into a new element which occurs 2 steps earlier in the periodic table.
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