What is the significance of the terms-'isolated gaseous atom' and 'ground state' while defining the ionisation enthalpy and electron gain enthalpy?
Hint: Requirements for comparison purposes.

Explanation

Mendeleev used atomic weight as the important property for the classification of elements. Mendeleev arranged all the known elements in the form of a table known as periodic table. He observed that some of the elements did not fit in very will with his scheme of classification if the order of atomic weight was strictly followed. He showed courage to ignore the order of atomic weights thinking that the atomic weight measurements might be incorrect. He placed the elements with similar properties together. For example, iodine has lower atomic weight than tellurium (of Group VI) but he placed iodine in Group VII alongwith fluorine, chlorine and bromine because of the similarities in their properties. He even left some spaces or gaps for some undiscovered elements. By considering the properties of the adjacent elements, he predicted the properties of the undiscovered elements. Later on, when these elements were discovered, their properties were found to be exactly similar to those predicted by Mendeleev. For example, gallium and germanium were not discovered at that time, when Mendeleev formulated his periodic table and therefore, he left gaps for these elements. He not only predicted the existence of the elements but he estimated their properties. He tentatively named these elements as eka-aluminium andeka-silicon. When these elements were discovered, the prediction of Mendeleev proved to be remarkably correct. However, after the discovery of atomic number as more fundamental property than atomic weight by Moseley in 1913, the basis of classification was changed to atomic number.