Three elements A, B and C have atomic numbers 7, 8 and 9 respectively.
(a) What would be their positions in the modern periodic table ? (Mention group and period both)
(b) Arrange A, B and C in decreasing order of their size.
(c) Which one of the three elements is most reactive and why ?

Explanation

The electronic configuration of these elements are :
(a) A ( Z = 7) 2, 5, B (Z = 8) 2, 6, C (Z = 9) 2, 7
Position of element A = 15th group and 2nd period
Position of element B = 16th group and 2nd period
Position of element C = 17th group and 2nd period.
(b) In general, atomic size decreases along a period. Therefore, decreasing order of size is `A gt B gt C`
(c) The element C (Z = 9) is fluorine. It is the most reactive element since it needs only one electron to acquire a noble gas configuration.