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? Why?
(Atomic numbers are given in parentheses.) N(7), F(9), P(15), Ar(18)

Hint: The electronic configuration of an element determines the arrangement of electrons in the shells of an atom. The atomic number is the total number of electrons in an uncharged atom and from the atomic number, we can define the properties of an element.

Complete step by step solution:
The atomic number of an element is defined as the number of protons in the nucleus of an atom. The atomic number can also be defined as the number of electrons in an uncharged atom. The electronic configuration is the electronic arrangement of an atom. The electronic configuration of the given atom is 2, 8, 7. Hence the total number of electrons in that atom will be the sum of the electrons in each shell that is two plus eight plus seven which will be equal to seventeen. Therefore, the atomic number of the element will be seventeen.
The element chlorine has atomic number seventeen. Chlorine is a member of group 17 in the modern periodic table. The chemical properties of elements down the group are similar in the modern periodic table. Therefore, among N, F, P and Ar, chlorine that is Cl will be chemically similar to fluorine that is F as they both are members of the same group which is group 17.

Note:
-Nitrogen that is N and phosphorus that is P both reside in the same group 15. Argon that is Ar resides in group 18.
-Group 17 elements are also known as halogens.