Question

What’s the correct answer

Answer 1

The correct answer is: 
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    " F  and Br , because they are in the same group" .
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Note:
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Choice [B]:  "F and Br ; because they are in the same period" ; is incorrect; since  "F" and "Br" are not in the same "period" (that is, "row").
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Choice [C]:  "Na and Mg; because they are in the same group {"column"} ;  is incorrect; since:  "Na" and "Mg are NOT in the same group {"column"].
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Choice [D]:  "Na and Mg" ; because they are in the same period {"row"}; is incorrect.  Note:  "Na" and "Mg" are, in fact,  in the same period {"row"}. However, as aforementioned, {Mg and Na} are not in the same group {"column".}.  

Note:  The similiarities in physical and chemistry properties among elements are determined and organized — or tend to be so—by "groups" {"columns"} — NOT by "periods" {rows}.  
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