Past Year - 3 Mark Questions

Class 10
Chapter 5 Class 10 - Periodic Classification of Elements

## 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)

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

(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

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