A) Cl, Br
B) Ca, Cl
C) Na, Ca
D) Na, Cl
Correct Answer: D
Solution :
Key Idea The elements of the same period have same number of shells. Cl, Br \[_{17}Cl\] : 2, 8 ,7 \[_{35}Br\] : 2, 8, 18, 7 Ca, Cl \[_{20}Ca\] : 2, 8, 8, 2 \[_{17}Cl\] : 2, 8, 7 Na, Ca \[_{11}Na\] : 2, 8, 1 \[_{20}Ca\] : 2, 8, 8, 2 Na, Cl \[_{11}Na\] : 2, 8, 1 \[_{17}Cl\] : 2, 8, 7 In the following pairs only Na and \[Cl\] have same number of shells i.e., 3-shells hence, they are placed in third period of periodic table.
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