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)

(a) The atomic number of this element is 2 + 8 + 7 = 17.

(b) The elctronic configuration of given element is 2, 8, 7. Thus, it has 7 valence electrons in its outermost shell. This element will be chemically similar to that element which has 7 valence electrons in its atoms. Now we will check electronic configuration of each element by using their atomic numbers.

(i) The electronic configuration of element N having atomic number 7 is 2, 5. It has 5 valence electrons.

(ii) The electronic configuration of element F having atomic number 9 is 2, 7. It has 7 valence electrons just like that of the given element. Thus, it would be chemically similar to the given element.

(iii) The electronic configuration of element P having atomic number 15 is 2,8,5. It has 5 valence electrons.

(iv) The electronic configuration of element Ar having atomic number 18 is 2,8,8. It has 8 valence electrons.