(i) F
(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 that can accumulate in the 2nd shell of an atom is 8.
(b) The elements in the same group show similar properties because they have similar electronic configurations whereas the elements of different groups have different electronic configurations due to which they 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 been placed in the same group because both of them have 7 valence electrons.
(e) Merits of the modern periodic table are given below:
(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.
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