Question

Which of the following is not a merit of Mendeleev's periodic table? (a) It helped in correcting the atomic masses of some of the elements. (b) He predicted the properties of some undiscovered elements and left gaps for them. (c) He framed the periodic table with vertical and horizontal columns and gave shape to it. (d) He gave separate places to isotopes in his periodic table.

Short answer

The correct answer is (d) He gave separate places to isotopes in his periodic table, as Mendeleev's periodic table did not account for isotopes.

Step-by-step solution

- Understand Mendeleev's Periodic Table

Identify the key aspects of Mendeleev's contribution to the periodic table which includes the prediction of undiscovered elements, correction of atomic masses based on element properties and organizing elements into a table with increasing atomic weights.

- Examine Each Option

Review each provided statement to assess its accuracy with respect to Mendeleev's periodic table. Understand that Mendeleev's periodic table was based on atomic weights and did not account for isotopes, which have the same atomic number but different atomic masses.

- Identify the Incorrect Statement

Compare each statement against historical scientific knowledge of Mendeleev's periodic table to determine which does not represent a merit of his work. Mendeleev didn't know about isotopes at the time he developed his table; isotopes were not given separate places.

Key concepts

Correction of Atomic Masses

One notable aspect of Mendeleev's Periodic Table was its role in correcting atomic masses of certain elements. Before Mendeleev, atomic weights were often determined with less precision. Mendeleev observed the properties of the elements and their compounds, and this helped him to see when an atomic mass seemed out of place with respect to an element’s position in the periodic table.

For instance, he suggested that the atomic mass of tellurium must be lower than that of iodine, despite tellurium having a higher atomic weight, so that it could fit correctly into the periodic table based on its properties. This was contrary to the existing measurements of that time. Later on, the more accurate mass values vindicated Mendeleev's adjustments, showcasing his foresight and deep understanding of elemental properties. This correction is attributed as one of Mendeleev’s meaningful contributions to chemistry.

Prediction of Undiscovered Elements

Mendeleev's periodic table also stood out for its predictive power, which at the time, seemed like an act of scientific clairvoyance. When organizing elements according to increasing atomic weights, Mendeleev noticed there were gaps in his periodic table.

These gaps did not represent mistakes but rather, they were intentional, left there by Mendeleev for elements that had not yet been discovered. He predicted the existence of these elements and went further to predict their properties based on their position relative to known elements. Noteworthy predictions include the elements gallium, scandium, and germanium, which when discovered, bore remarkable similarity to Mendeleev’s foresight. This gave his periodic table a level of credibility and acceptance that it may not have otherwise achieved.

Organization of the Periodic Table

The organization of Mendeleev's periodic table was revolutionary at the time. Mendeleev chose to arrange the elements in order of increasing atomic weight, which is akin but not identical to the modern atomic number.

Furthermore, he grouped the elements into columns, known as groups, based on recurring ('periodic') chemical properties. The layout was such that elements with similar properties fell into the same column, despite some anomalies that Mendeleev recognized but could not explain at the time (which later were clarified by the discovery of the proton and the concept of atomic number). However, the horizontal rows, known as periods, showed a progression of properties and were intended to provide a structure to understand how those properties changed gradually from one element to the next.

Isotopes

In Mendeleev’s time, the concept of isotopes was not yet understood. He based his periodic table off atomic weights, unaware that certain elements could have atoms with different masses. Isotopes are variants of a particular chemical element that have the same number of protons (and hence the same atomic number) but different numbers of neutrons, leading to different atomic weights.

Today, we know that isotopes of an element occupy the same position on the periodic table because they have identical electron configurations and similar chemical behaviors, which are largely independent of the atomic mass. Mendeleev did not give separate places to isotopes in his table simply because the knowledge to do so did not exist at that time. The later discovery of isotopes added another layer to the dense fabric of the periodic table and required the concepts underlying it to evolve further.