Question

What were the main demerits of Mendeleev's periodic table? (i) Hydrogen has been placed in group I though it resembled to group VII as well. (ii) Position of some elements was not justified. (iii) Isotopes were not given separate places. (iv) Lanthanides and actinides were not included in the table. (a) (i), (ii) and (iii) (b) (i), (ii), (iii) and (iv) (c) (ii) and (iv) (d) (i), (iii) and (iv)

Short answer

The main demerits of Mendeleev's periodic table are that hydrogen's placement was ambiguous, some element positions were not justified, isotopes were not given separate places, and lanthanides and actinides were not included. The correct option is (b).

Step-by-step solution

Analyzing the Statement (i)

The first statement (i) mentions that hydrogen was placed in group I even though it has similarities to elements in group VII. This represents a demerit because it addresses the ambiguous placement of hydrogen, which presents characteristics of both alkali metals and halogens.

Analyzing the Statement (ii)

The statement (ii) asserts that some elements did not have a justified position in Mendeleev's periodic table. This demerit is valid because Mendeleev placed elements based on atomic weights, which led to some inconsistencies in the properties of elements when ordered in that manner.

Analyzing the Statement (iii)

The third statement (iii) points out that isotopes were not given separate places. This demerit is also correct since Mendeleev's periodic table did not account for isotopes; elements with the same atomic number but different atomic masses were not recognized as separate entities.

Analyzing the Statement (iv)

Lastly, statement (iv) indicates that lanthanides and actinides were not included. This demerit relates to the fact that Mendeleev did not have knowledge of these elements at the time, and thus they were not part of his periodic table.

Determining the Correct Option

Since all the listed demerits (i), (ii), (iii), and (iv) are factual regarding Mendeleev's periodic table, the only option that includes all of these demerits is option (b).

Key concepts

Hydrogen placement

One of the significant challenges within Mendeleev's Periodic Table is the placement of Hydrogen. While Mendeleev classified Hydrogen in Group I, which contains alkali metals known for their reactivity and single valence electron, Hydrogen also shares properties with Group VII elements – the halogens. This dual similarity presents a dilemma as Hydrogen has the electron configuration like that of alkali metals but forms diatomic molecules akin to halogens and, like them, is not a metal.

Students studying this aspect should recognize that while Mendeleev's decision was practical considering the knowledge at the time, it failed to address the unique nature of Hydrogen comprehensively. Today, we understand that Hydrogen's versatility and unique characteristics make it difficult to categorize, which is why modern periodic tables often show it separated from other groups or above both Group I and VII to reflect these shared properties.

Element positioning

Another caveat in Mendeleev's Periodic Table was the positioning of some elements. Mendeleev famously arranged elements in order of increasing atomic weight, a method that led to a periodic recurrence of properties. While this demonstrated a patterned-approach that correctly grouped many elements according to their chemical behavior, it was not without flaws.

For instance, there were a few notable exceptions where elements were ordered by Mendeleev out of strict atomic weight sequence to preserve chemical groupings – notably iodine and tellurium. This adjustment was intuitively correct but highlighted the limitations of using atomic weight as the sole criteria for organizing the elements. Today's table is organized by increasing atomic number, which amends these inconsistencies for a more coherent chemical periodicity.

Isotope representation

Mendeleev's periodic system did not account for isotopes – atoms of the same element with different numbers of neutrons and, consequently, different atomic masses. This omission is understandable given that the existence of isotopes was unknown to chemists in Mendeleev's time. However, it does present a demerit of his periodic table from a contemporary standpoint.

The discovery of isotopes later prompted a revision in the understanding and classification of elements. Students need to appreciate that the development of the concept of isotopes necessitated the change from atomic weight to atomic number as the basis for the arrangement of elements in the modern periodic table, ensuring that each element, regardless of isotopic variation, occupies its unique position.

Lanthanides and actinides inclusion

The last of Mendeleev's Periodic Table's drawbacks included in our exercise is the non-inclusion of lanthanides and actinides. In Mendeleev's time, these series of elements were either undiscovered or not understood well enough to be included in his table. It was a significant omission as these elements possess unique properties and their electrons fill up the 'f-orbitals', differing from the 's' and 'p' block elements Mendeleev had considered.

After their discovery, it became clear that these elements needed to be integrated into the periodic table. Contemporary tables include the lanthanides and actinides at the bottom to preserve the table's structure and emphasize periodic trends. For students, recognizing this historical gap helps in understanding the evolving nature of scientific knowledge and the periodic table as a living document that is updated with new findings.