Question

(a) Explain why, the first period of the modern periodic table has only two elements whereas second period has eight elements
(b) Why do elements in the same group show similar properties but the elements in different groups show different properties?
(c) For each of the following triads, name the element with the characteristics specified below:

Elements	Least atomic radius	Chemically least reactive
(i) F, Cl, Br (ii) Li, Na, K	.................... ....................	.................... ....................

(d) State one reason for keeping fluorine and chlorine in the same group of the periodic table.
(e) What are the merits of the modern periodic table of elements?

Answer 1

(a) The first period has only two elements because the first shell (K shell) can take a maximum of only two electrons. The second period has eight elements because the second shell (L shell) can accommodate a maximum of 8 electrons. The number of elements in a period depends upon the number of shells.

(b) The properties of an element depends on the number of valence electrons present in an atom of the element. Elements of a particular group have similar electronic configurations and the same number of valence electrons in their atoms. Hence, all the elements belonging to a certain group show similar properties. However, the electronic configurations of elements change on moving from left to right in a periodic table. Different groups have different valencies. Therefore, elements in different groups show different properties.

(c)

Elements	Least atomic radius	Chemically least reactive
(i) F, Cl, Br	F	Br
(ii)Li, Na, K	Li	Li

(d) Fluorine and chlorine have a valency of 1 and show similar chemical properties. So, they are placed in the same group.

(e) The merits of the Modern Periodic Table of Elements are as follows:
1. It is based on the atomic number of elements.

2. The modern periodic table clearly explains the reason behind elements showing similar properties in a group and dissimilar properties in different groups.

3. This also explains the reason for the periodicity in the properties of elements.

4. The modern periodic table explains the reason for a repetition in the properties of elements after 2,8,18 and 32 elements.

5. In the modern periodic table, there are no anomalies in the arrangement of elements.