Question

The elements which are radioactive and have been named after the names of planets are:
A.$Hg$
B.$Np$
C.$Pu$
D.$Ra$

Answer 1

Hint:Radioactive elements are made up of atoms whose nuclei are unstable and easily give off atomic radiation as part of the process in order to attain stability. Four elements are named after planets.

Complete step by step answer:
So ,if we see all the options given we will get following information:
A.\[Hg\]: It is the symbol for Mercury or Hergentium which is a chemical element with atomic number $80$. It is used in thermometers, barometers and other devices. Mercury is the only liquid metal. Mercury itself is not a radioactive element but its isotopes are radioactive. It is named after planet Mercury but it is not a radioactive element.
B.$Np$: It is the symbol for Neptunium having atomic number $93$. It is a radioactive element. Its position in the periodic table is just after uranium, named after planet Uranus, led to name it after Neptune, the next planet after Uranus. So this element is radioactive as well as named after a planet.
C.$Pu$: It is the symbol for Plutonium which is a radioactive element with atomic number of $94$. Since uranium had been named after planet Uranus and neptunium after the planet Neptune, element $94$was named after Pluto, which at that time was considered a planet as well. So this element also is radioactive as well as named after a planet.
D.$Ra$: It is the symbol for Radium with atomic number $88$. Radium itself and its isotopes also are radioactive elements. It is the sixth element in Group $2$ of periodic table known as alkaline metals. It is only a radioactive element however it is not created or named after a planet name.
Although $Np$, $Pu$ and $Ra$ are radioactive elements, only $Np$ and $Pu$ have been named after planets Neptune and Pluto.
 So if we see all reasons provided above for each element our answer will be option B and C.
So, the correct option is B that is $75$.

Note:
Four elements in the periodic table are named after planets. Those elements are mercury, uranium, neptunium and plutonium.