Question

(a) State the periodic law on which Mendeleev's periodic table table was based. Why and how was this periodic law changed?
(b) Explain why, the noble gases are placed in a separate group.

Answer 1

(a) According to Mendeleev's periodic law, the properties of elements are a periodic function of their atomic masses. This means that when elements are arranged in an increasing order of their atomic masses, the elements with similar properties repeat at regular periods.
However, Henry Moseley, a scientist, showed that the atomic number of an element is a more fundamental property and a better basis to classify elements than the atomic mass. The atomic number increases consistently by 1, from one element to the next, whereas the atomic mass does not change regularly. Also, the atomic number of an element does not change and remains fixed. This led to a change in Mendeleev's periodic law.

(b) Noble gases are placed in a separate group of the periodic table because they are inert elements. They are unreactive as their valence shells are completely filled with electrons. Their properties are different when compared to all the other elements.