Smita studied about the Modern Periodic Table. She placed hydrogen, lithium, sodium, and calcium in the same group in the periodic table. However, her teacher told her that she has placed one element incorrectly in the periodic table.(Atomic numbers of H, Li, Na, and Ca are 1, 3, 11, and 20 respectively) Which element has been placed incorrectly by Smita?

Elements containing same number of valence electrons are present in the same group of the modern periodic table. The number of valence electrons present in an element can be easily found out by writing its electronic configuration.
Electronic configuration of H: 1
Electronic configuration of Li: 2, 1
Electronic configuration of Na: 2, 8, 1
Electronic configuration of Ca: 2, 8, 8, 2
Hence, it can be easily noted from the electronic configurations that all the elements except calcium contain one valence electron. Therefore, H, Li, and Na are present in the same group.

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