Question

What is an element $140$ ?

Answer 1

The number $140$ is the atomic number of the element. Using the atomic number, the position of this element can be decided in the periodic table. Remember above atomic numbers $108$ are unstable and many of them have not been discovered yet.

Complete answer:
Each element in the modern periodic table has a fixed position depending upon its atomic number and electronic configuration. The elements are arranged in an increasing order of their atomic numbers which is the total number of protons present inside their nucleus.
With an increase in the number of protons and neutrons, the atomic mass of an atom increases making its nucleus heavier. Extremely heavy nuclei are unstable and tend to disintegrate on their own to form lighter particles that enhance the stability.
Uranium is the heaviest stable atom known and all elements placed in the modern periodic table beyond uranium are very unstable and have short life times. These elements are sensitive towards radioactive decay. Thus the series of elements with atomic number higher than Uranium is known as transuranic series.
Many members of the transuranic series are yet to be discovered and have only been predicted. The elements are given a name only after their discovery. These elements are named according to the systematic naming of transuranic elements. Each digit of the atomic number is translated into a Latin or Greek word that is a numerical root.
The element with atomic number $140$ has not been discovered yet. Its name is written as
$1 = un$
$4 = quad$
$0 = nil$
Hence, the element $140$ is named $unquadnilium$ .

Note:
Even though the element has not been discovered, its position in the modern periodic table is reserved in the bases of its atomic number. Its electronic configuration based on its atomic number suggests that it is part of the extended periodic table.