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: Element 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. What are the merits of the modern periodic table of elements?

• The first period of the modern periodic table has only two elements whereas second period has eight elements because the first electron can take maximum of 2 electrons only and in 2^nd period the maximum number of electron which can be put in the 2^nd shell is 8.
• Elements in the same group show similar properties but the elements in different groups show different properties because of the difference in electronic configuration.

(c)(c) (i) F; Br

(ii) Li; Li

• Fluorine and chlorine have been placed in the same group because both of them have 7 valence
• Merits:
  1. It is based on atomic number of
  2. Explains the reason behind showing similar and dissimilar properties in same and different groups
  3. Explains periodicity in the properties of
  4. There are no anomalies in the arrangement of