Explain in terms of chemical reactivity why the elements in group 18 on the modern periodic table were not identified by Mendeleev's at that time

elements tend not to react with other elements, there were no oxide compounds for Mendeleev to study. –Group 18 elements are generally unreactive. sodium ion is smaller because the sodium atom lost one electron. –An Na+ ion is smaller because it has one fewer electron shell.

Explanation:

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