Question

Why do you think the noble gases are placed in separate groups?

Answer 1

Hint: Try to recall that noble gases have completely filled valence electron shells and are chemically inert except xenon. Now, by using this you can easily answer the question.

Complete step by step solution:
There are several reasons because of which noble gases are placed separately in periodic tables. The reasons are as follow:
The general electronic configuration of noble gas is $n{s^2}n{p^6}$. You can easily say from their electronic configuration that they have completely valence electron shells.
They are extremely uncreative or inert and have no tendency to give up or receive electrons in their outermost shell as they are already completely filled.
All the noble gases have zero valency and are placed in group 18, also known as group 0 on periodic table.
Noble gases have 0 electronegativity as they are not found in a combined state.
In a period, noble gases have the highest first ionization energy compared to the rest of the elements of that period because of the presence of a completely filled valence electron shell.
Also, the electron gain enthalpy of noble gases is positive whereas all other elements of the periodic table have negative electron gain enthalpy.

Note: It should be remembered that Xenon unlike other noble gases form several compounds like $Xe{F_2},Xe{F_4},Xe{O_3},Xe{O_2}{F_2}$ etc.
Also, you should know that since inert gases normally do not form any chemical compounds, therefore their atomic radii are usually expressed in terms of Vander Waals radii and because of this, they possess the largest atomic radii in a period among all the elements.