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 element with electronic configuration (2,8,7) means 17 is chlorine.

The no. of atomic number(Z) = no. of electrons(e)

Therefore, the atomic number is 17.

(b) An atom with electronic configuration 2, 8, 7 would be chemically similar to F (At. No. 9) because it has electronic configuration 2,7, Both have 7 electrons in the outermost orbit.