(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?
(a) The 1st period has two elements because the 1st electron shell of an atom can take a maximum of two electrons only. The 2nd period of the periodic table has 8 electrons because the maximum no. of electrons which can be put in the 2nd shell of an atom is 8.
(b) The elements in the same group show similar properties because they have similar electronic configuration (having the same number of valence electrons) whereas the elements of different groups have different electronic configurations (different number of valence electrons) due to which they show different properties.
(c) (i) F; Br
(ii) Li; Li
(d) Fluorine and chlorine have been placed in the same group because both of them have 7 valence electrons.
(e) Merits of modern periodic table:
(i) The modern periodic table is based on the atomic numbers of elements which is the most fundamental property of elements.
(ii) It helps us to understand why elements in a group show similar properties but elements in different groups show different properties.
(iii) It explains the reasons for the periodicity in properties of elements.
(iv) It tells us why the properties of elements are repeated after 2, 8, 18 and 32 elements.