why were no members of group 8A (18) of the modern periodic table found not found on Mendeleev's table?

Answer 1

Mendeleev's periodic arrangement of elements was first presented in March 1869. It did not have the form of a table (as we picture it today) but of a list arranged in columns.

The elements were arranged by order of atomic mass (proton + neutrons, averaged over the various isotopes of the same element). The atom was not yet known to be made up of protons and neutrons; therefore, the table had lots of gaps (for example, between Carbon at 12 and Nitrogen at 14, there is no element of mass 13.
The larger gap between an element of Period 7A (for example Fluorine at 19) and the next element of period 1A (Sodium at 23) was no worst than the gap from, let us say, silver (108) to cadmium (112).

Period 8A are known as "noble gases", elements whose outer electron shell is full. They are chemically inert and, therefore, difficult to detect using chemical means.

Helium was "discovered" by studying the spectrum of the Sun (in Greek: Helios) in 1868. However, it was not found in nature until 1895 (William Ramsey); therefore, it could not have been chemically analysed by Mendeleev.

Neon 1898 (Ramsey and Travers) (neo = new)
Argon 1894 (Rayleigh and Ramsey) (I do not know where the name comes from)
Krypton 1898 (Ramsey and Travers) (kryptos = hidden)
Xenon 1898 (Ramsey and Travers) (Xenon = stranger or strange)
Radon (Rutherford and Owens) -- comes from the disintegration of radium (hence the name)

Answer 2

18 On Periodic Table

Answer 3

The noble gases are quite rare. They also are hard to isolate.

Answer 4

BECAUSE BY HIS TIME THE NOBLE GASES HAD NOT BEEN DISCOVERED