Question

The element with atomic number $18$ belongs to which group:
a) Halogens
b) Alkali metals
c) Alkaline earth metals
d) Noble gases

Answer 1

In the periodic table groups are the vertical columns and the periods are the horizontal lines. There are eighteen groups and seven periods in the modern periodic table. Noble gases family is the ${18^{th}}$ group which implies that they tend to be unreactive (inert).

Complete step by step solution:
The element with atomic number $18$ will have the following electronic configuration-
$X = 1{s^2}2{s^2}2{p^6}3{s^2}3{p^6}$
The Group number can be calculated by knowing the electronic configuration of elements. But in the of p-block elements it can be calculated by the following formula –
$G = 12 + x$ And $x$ is the number of electrons present in the outermost shell.
From the given question the electrons in the outer orbital are $6$ , so the group number will be
$G = 12 + x \\\ \Rightarrow G = 12 + 6 \\\ \Rightarrow G = 18 \\\$
Period numbers can be calculated by knowing the number of highest occupied orbits. From the electronic configuration we can see that the highest occupied orbital is $3$ .
So the conclusion that can be drawn from the above discussion is that an element with atomic number $18$ belongs to the P- block element as the outer orbital filled is ‘p’. And the Group number is $18$ which is popularly known as the noble gas family. The element identified as Argon ( $Ar$ ).
They are also called the inert gases or the rare gases. The elements of these groups have completely filled orbitals, so they have very stable configurations. These elements due to completely filled orbital don't have affinity to gain electron or to lose electron, thus known as inert gas which do not react in normal conditions.
They occur in the atmosphere, and Argon is obtained in the largest amounts being the cheapest.
Hence the correct option is (d).

Note:
The elements are colourless, odourless monoatomic gases. Noble gases can diffuse through glass, rubber, plastic materials and some metals. Thus they are difficult to be handled in laboratories. The major use of Argon is to provide an inert atmosphere in metallurgical processes.