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.
(i) Electronic configuration of Ca (20): 2, 8, 8, 2
(ii) Rb belongs to Group 1, and all Group 1 elements have one valence electron.
(iii) Sr belongs to Period 5, and thus, it has five shells.
(iv) K is a metal with electronic configuration 2, 8, 8, 1. Thus, it will donate its one electron to acquire the noble gas configuration.
(v) The atomic size increases down the group and decreases across a period. Rb is the element that has the largest atomic size.
(vi) Arranged in increasing order of size: Be < Mg < Ca < Rb
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