An atom has electronic configuration 2, 8, 7. (a) What is the atomic number of this element?
(b) To which of the following elements would it be chemically similar? (Atomic numbers are given in parentheses.)
N(7) F(9) P(15) Ar(18)

Explanation

(a) The atomic number of this element is obtained by adding all the electrons present in its electronic configuration.
` therefore` Atomic number = 2 + 8 + 7 = 17
(b) The electronic configuration of the given element = 2, 8, 7
Valence electron in its atom = 7
This element will be chemically similar to that element which has the same valence electron (7).
The electronic configuration of the above elements are:
(i) N (7): 2,5 (5 valence electron) (ii) F (9): 2, 7 (7 valence electron) (iii) P (15): 2, 8,5 (5 valence electron) (iv) Ar (18): 2, 8, 8 (8 valence electron)
Clearly, F (9) has 7 valence electrons just like that of the given element. Hence, the given element of atomic number 17 will be chemically similar to the element fluorine (F) of atomic number 9.