Question

Answer the following question regarding the modern version of Mendeleev's periodic table.

• Were the elements of the group on the extreme left hand known when Mendeleev made periodic table?
• Name all the elements of this group in the ascending order of atomic numbers.

Answer 1

When Mendeleev made periodic table, the elements of the group on the extreme left hand were not known.

• These elements were discovered after the original Mendeleev's periodic table was published.
• So to accommodate these elements called noble gases, a separate group called group zero was added to the extreme left of the periodic table in the modern version of Mendeleev's periodic table.

The elements of this group in the order of their ascending atomic numbers are as follows:

• Helium< Neon< Argon< Krypton< Xenon

Element	Symbol	Atomic number
Helium	$\mathrm{He}$	$2$
Neon	$\mathrm{Ne}$	$10$
Argon	$\mathrm{Ar}$	$18$
Krypton	$\mathrm{Kr}$	$36$
Xenon	$\mathrm{Xe}$	$54$