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 can be obtained by adding all the electrons present in its electronic configuration. So the atomic number of the given element having electronic configuration 2, 8,5 is `2+8+7=17`.
(b) The electronic configuration of the given element 2,8,7 shows that this element has 7 valence electrons in its atoms. this element will be chemically similar to that which has the same number of valence electrons (7 valence electrons) in its atoms. To know the number of valence electrons in the elemetns `N,F,P` and `Ar` we have to write their electronic configurations by using their atomic numbers.
(i) The atomic number of `N` of 7, so its electronic is 2,5. It has 5 valence electrons (and not 7).
(ii) The atomic number of `F` is 9, so its electronic cofiguration is 2,7,. If has 7 valence electrons just like that of the given element . So the given element of atomic number 17 will be chemically similar to the element fluorine `(F)` of atomic number 9. This is becaude both of them have similar electronic configurations each having the same number of (7) valence electrons.
The atomic number of `P` is 5, so its electronic configuration is 2,8,5 (it has 5 valence electrons). The atomic number of `Ar` s 18, so its electronic configuration is 2,8,8 (it has 8 valence electrons). Thus, neither element `P` (phosphorus) nor `Ar` (argon) have 7 valence electrons in their atoms.