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. | Elements | |
| 2 | Li(3) | Be(4) |
| 3 | Na(11) | Mg(12) |
| 4 | K(19) | Ca(20) |
| 5 | Rb(37) | Sr(38) |
(i) State 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.
Answer
(i) Atomic number of Ca = 20.
So, Electronic configuration of Ca: 2, 8, 8, 2
(ii) Since Rb belongs to the fifth period , so it has 5 shells.
Electronic configuration of Rb: 2, 8, 8, 18, 1
Therefore, the number of valence electrons ( ie., the number of electrons in the outermost shell) will be 1.
(iii) Sr belongs to the fifth period . So, it has 5 shells .
(iv) K has an electronic configuration of 2, 8, 8, 1.
Metals lose electrons to become stable whereas, non-metals gain electrons to become stable.
Since K has a tendency to lose 1 electron to become stable, it is a metal.
(v)
Rb will be the largest in size as it belongs to group 1 and Period 5.
(vi) Be, Ca and Mg all belong to the same group i.e group 2 and Rb belongs to group 1. So the order of their atomic sizes will be Be < Mg < Ca < Rb
