The two kinds of element are argon and helium. These two element were discovered in 1892 by a Scottish chemist Williams Ramsay.
These two elements are known as noble gas or inert gas because of how hard they find it to react with other elements.
Elements back then were discovered through their reactivity with other elements but It took so long to discover these elements(argon and helium) because of their unreactive nature . Combine with the fact that they were odorless, tasteless and colorless it was more hard to discover them.
Your Question: There are two kinds of elements that didn't appear on the periodic table until after 1892. What kinds are they and why do you think it took so long to discover them?
The Answer: Moseley’s analysis enabled chemists to refine the table further and reveal more gaps, which suggested that there were more new elements waiting to be discovered, with atomic numbers 43, 61, 72 and 75. Scientists subsequently found these elements—now known as technetium, promethium, hafnium and rhenium, respectively.
Explanation: Physicist Henry Moseley discovered the atomic number of each element using x-rays, which led to more accurate organization of the periodic table. We will cover his life and discovery of the relationship between atomic number and x-ray frequency, known as Moseley's Law.
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In the process of solvation the polar solute ions are surrounded by the polar dipoles of the solvent molecule. For example when NaCl dissolves in water the Na⁺ cation is surrounded by the partially negatively charged O atoms of water molecules. Similarly, Cl⁻ anion is surrounded by the partially positively charged H atoms of water molecules.