Question

What is the name of the element with atomic number 105?
(A)Dubnium
(B)Holmium
(C)Kurchatovium
(D)Nobelium

Answer 1

Hint: For a question like this to answer, you need to have command over the periodic table. Element of atomic number 105 is present in d-block, and is in the VII period.

Complete answer:
Writing the electronic configuration of element 105 we get, $[Rn]7{{s}^{2}},5f{{,}^{14}}6{{d}^{3}}$.

Element with atomic number 105 is Dubnium(Db), with the above given configuration. It is a highly radioactive element, with stable oxidation state +5. It has a half life of $2\times {{10}^{4}}$.the half life of dubnium greatly limits the extent of research on the element. It is a synthetic chemical element.The most stable known isotope is dubnium-268.

Considering other options,option (B), Holmium(Ho) has an atomic number of 67. It is an f-block element, and is a lanthanide. Its stable oxidation state is +3.

Considering other options, option (C), Kurchatovium, is the other name of artificial radioactive element, rutherfordium having atomic number 104.This name was suggested by the Russian scientist. It is a d-block element, with a stable oxidation state of +4.

Considering other options, option (D), Nobelium is an f-block element of atomic number 102. It is a radioactive element and is present in the actinide series. It has two stable oxidation states +2 and +3.

So, the correct answer is “Option A”.

Note:
Elements above atomic number 82 are radioactive elements. Electronic configuration helps to find out the block of the element whose atomic number is given. Decay mode of Dubnium is Alpha emission(that is alpha decay).