Question

Which period is incomplete in the modern periodic table?
A) ${{1}^{st}}$
B) ${{2}^{nd}}$
C) ${{6}^{th}}$
D) ${{7}^{th}}$

Answer 1

In a periodic table there are some elements whose properties are not discovered completely, so, without knowing their complete behavior we can’t decide their positions and similarities with other elements in the periodic table.

Complete step by step answer:
We know that most of the elements from group seven in the periodic table are identified in labs and their properties have not been studied properly or not discovered completely.
These elements are unstable and very less information is available of these elements. The seventh period of the Periodic Table is incomplete. The only incompleteness (a few years ago) was the IUPAC confirming and naming of the elements with atomic number 113, 115, 117 and 118. However, they were discovered and confirmed, and appropriately named.
113 - Nh or Nihonium,
115 - Mc or Moscovium,
117 - Ts or Tennessine,
118 - Og or Oganesson (Named after a scientist who’s still alive)
Some elements like Uut (113), Uup (115), Uuh (116), Uus(117) and Uuo(118) have not been recognised by IUPAC. Moreover these elements are highly unstable and we don't have much information on them.
All other periods are complete because they contain elements whose properties are properly discovered.

So, the correct answer is option “D”.__

Note:
7th period is considered as an incomplete period even though it has 32 elements. Because it consists of elements whose properties have not been studied properly yet.