The position of eight elements in the Modern Periodic Table is given below where atomic numbers of elements are given in the parenthesis.

Period No.
2	Li (3)	Be (4)
3	Na (11)	Mg (12)
4	K (19)	Ca (20)
5	Rb (37)	Sr (38)

(i) Write the electronic configuration of Ca.

(ii) Predict the number of valence electrons in Rb.

(iii) What is the number of shells in Sr?

(iv) Predict whether K is a metal or a non-metal.

(v) Which one of these elements has the largest atom in size?

(vi) Arrange Be, Ca, Mg and Rb in the increasing order of the size of their respective atoms.

The detailed answers for each sub-question are as follows:

(i) Electronic configuration of $Ca$: The atomic number of Calcium ($Ca$) is $20$. Its electronic configuration is $2, 8, 8, 2$.

(ii) Number of valence electrons in $Rb$: Rubidium ($Rb$) belongs to Group 1 (alkali metals), so it has $1$ valence electron.

(iii) Number of shells in $Sr$: Strontium ($Sr$) belongs to Period 5, which means it has $5$ electron shells.

(iv) Metallic character of $K$: Potassium ($K$) is a metal. As mentioned in the text, alkali metals (lithium, sodium, potassium) are highly reactive metals that easily lose their valence electron.

(v) Largest atom in size: Rubidium ($Rb$) has the largest atomic size among the given elements. Atomic size increases down a group and decreases from left to right across a period.

(vi) Increasing order of atomic size: $Be < Mg < Ca < Rb$.