Question

When polonium-218 decays, it emits a beta particle followed by an alpha particle. The resulting isotope is:
[A] Lead-214
[B] Bismuth-214
[C] Lead-222
[D] Bismuth-212
[E] Astatine-212

Answer 1

Hint: To answer this, you must know that the atomic number of polonium is 84. Remember that removal of alpha particles decreases atomic number by 2 and mass number by 4. Similarly, beta particles decrease atomic number by 1. You can use this to find the correct answer.

Complete answer:
In radiochemistry, when a reaction takes place new elements are formed unlike chemical reaction where no new elements are formed.
To answer the given question, we need to discuss the mass and charge of alpha and beta particles.
We know that the charge of an alpha-particle is +2 electrons whereas the charge of a beta-particle is -1electron. We also know that the mass of an alpha particle is four times the mass of a hydrogen atom and the mass of a beta-particle is equal to the mass of an electron.
When an alpha-particle is emitted from an atom, the atomic number decreases by 2 and the mass number decreases by 4 and when a beta-particle is emitted, the atomic number increases by 1 and the mass number remains the same.
In the question, polonium-218 is given to us. We know that the atomic number of polonium is 84. From polonium, a beta particle is removed which will increase its atomic number by 1 i.e. it will become 85 and the mass number remains the same. Then, an alpha particle is removed i.e. atomic number decreases by 2 and becomes 83 and the mass number decreases by 4 i.e. becomes 214.
Therefore, the new element formed has the atomic number of 83 i.e. bismuth. We can write the reaction as-
     \[{}_{85}^{218}Po-{}_{-1}^{0}\beta -{}_{2}^{4}He\to {}_{83}^{214}Bi+E\]

Therefore, the correct answer is option [B] Bismuth-214.

Note:
In the radioactive disintegration series, the first is the 4n series which is the thorium series where 6-alpha and 4-beta particle are released from thorium and it gives us an isotope of lead, which is \[{}_{82}^{208}Pb\]. We get these series by dividing the mass number by 4. If it is completely divisible, the atom will undergo 4n series disintegration but if the remainder is 1, 2 or 3, it will be in (4n+1), (4n+2) or (4n+3) series respectively.