Question

# The elements with atomic numbers $9,17,35,53,85$ are all:A.Noble gasesB.Halogens C.Heavy metals D.Light metals

Hint: First we have to determine the electronic configuration of the elements provided the atomic numbers. And then the electronic configuration suggests to which group of the periodic table the element belongs to.

Following are the steps to determine electronic configuration:
Calculate the number of electrons in the element given and the number of electrons in the element is equal to its atomic number.
Fill the orbitals of the element in the order of $1s,{\text{ }}2s,{\text{ }}2p,{\text{ }}3s,{\text{ }}3p,{\text{ }}4s,{\text{ }}3d,{\text{ }}4p{\text{ }}and{\text{ }}5s.$
Remember that each $s$ orbital can have two electrons, each $p$ orbital has six electrons and each $d$ orbital can have 10 electrons.
Write the electron configuration for the element
Given, the atomic number of elements is $9,17,35,53,85$.
so, the electronic configuration of given elements will be as follows:
element with atomic number 9 = 2,7
element with atomic number 17= 2,8,7
element with atomic number 35= 2,8,18,7
element with atomic number 53=2,8,18,18,7
element with atomic number 85=2,8,18,18,32,7
The above data shows that all elements have seven electrons in their outermost valence shell.
These atomic numbers show that the electronic configuration is $n{s^2}n{p^5}$ and this is the configuration of the halogen group or group 17 of the periodic table.
Hence, Option B is correct.

Note:
We must know that the noble gases are the elements that belong to Group 18 of the periodic table. Metals are the elements belonging to d-block (transition metals) or f-block (inner transition metals ) of the periodic table.
In the halogen group, Atomic number 9 is of Fluorine, Atomic number 17 is of chlorine, atomic number 35 is of Bromine, atomic number 53 is of Iodine, and atomic number 85 is of Astatine.