Question

The starting element in the incomplete period of the long form of the periodic table:
(A) Ca
(B) Rn
(C) Fr
(D) Th

Answer 1

Hint: All elements till atomic number 94 are naturally occurring and so the table cannot be incomplete till here. Elements from atomic number 95 onward are synthetically made and so complete information about them is not available. Also elements from atomic number 87 to 118 lie in the last period.

Complete step by step solution:
-In the long form of the periodic table the elements are arranged on the basis of the modern periodic law. The modern periodic law states that:
“The physical and chemical properties of an element are periodic functions of their atomic masses.”
-An arrangement of elements in order of their increasing atomic numbers is called the periodic table. There are 18 vertical columns called ‘group’ and 7 horizontal rows called’ period’ in a modern periodic table. In total there are 118 elements in the periodic table including all the metals, metalloids and non-metals.
-All the elements within a group have similar electronic configuration of the outermost shell and hence have similar chemical properties as well.
While all the elements within a period have an increasing number of valance electrons and the outermost shell is same for all of them. With an increase in the energy level of the atom, the number of energy sub-levels also increases.
-The periodic table has 4 blocks: s-block, p-block, d-block and f-block.
-In the periodic table the first 94 elements are naturally occurring while the elements from 95 to 118 have been synthetically synthesised in labs and reactors only. These synthetically made elements have not yet been studied properly and it is assumed that after their properties might change after getting sufficient information regarding them. Elements from atomic number 87 to 118 lie in the last (seventh) period.
-The last period of the periodic table is incomplete which is the seventh period. The first element of the seventh period is Francium, denoted as Fr and having an atomic number of 87. It has an electronic configuration of: [Rn] $7{s^1}$.

So, the correct option will be: (C) Fr.

Note: Francium is an extremely radioactive element with a half-life of only 22 minutes and its most stable isotope is Francium-223. It is the second rarest naturally occurring element and also the second most electropositive element (after Caesium).