Merits of Mendeleev’s Periodic Table
The study of a huge number of elements is condensed into a few groups of elements, with elements belonging to the same group sharing similar properties. The known elements were classified using Dobereiner’s Triads, Newland’s law of octaves, and Mendeleev’s periodic table. The periodic table of Mendeleev and its merits are discussed further below.
Mendeleev’s Periodic Table
The periodic law, proposed by Mendeleev in 1869, states that when elements are grouped in increasing atomic mass order, their properties would be repeated at regular intervals or periods. Mendeleev created the periodic table by arranging all 63 elements known at the time in horizontal rows in order of increasing atomic masses, but with similar properties grouped together in the same vertical column.
There were seven horizontal rows called periods and eight vertical columns named groups in Mendeleev’s periodic table. Noble gases were unknown at the time. As a result, there was no group of noble gases in Mendeleev’s original periodic table.
Mendeleev left several gaps in his table to ensure that elements with similar properties were placed in the same vertical column. Mendeleev named the undiscovered elements at the time that the gaps in the periodic table were left as eka-boron, eka-aluminium, and eka-silicon. Eka-boron was renamed scandium, eka-aluminium was renamed gallium, and eka-silicon was renamed germanium once they were discovered later.
To ensure that elements with similar properties were grouped together, Mendeleev arranged a few elements in the wrong order of their atomic masses, placing the element with the lower atomic mass last and the element with the higher atomic mass first. When noble gases were found, Mendeleev’s periodic table was able to accommodate them. It may also predict the properties of other elements based on their periodic table positions.
Merits of Mendeleev’s classification of elements
Mendeleev’s periodic table is the first systematic attempt to classify elements based on their fundamental properties. The following are the merits of Mendeleev’s element classification.
- It was able to anticipate the properties of several elements based on their periodic table positions.
This method was used to predict the properties of then-undiscovered elements including gallium, scandium, and germanium. Mendeleev predicted the properties of the unknown element eka-aluminium based on its position in the periodic table in 1871. The actual properties of this element (called gallium) and eka-aluminium are listed below.
The properties of eka-aluminium predicted by Mendeleev are almost identical to the actual properties of the gallium element, as seen in the table. In 1875, four years after Mendeleev’s table was published, the element gallium was discovered, and its properties matched up amazingly well with eka-aluminium, fitting into the table exactly where he had predicted.
- When noble gases were identified, Mendeleev’s periodic table could accommodate them.
When a new group of elements known as noble gases was discovered, it was given its own spot in the periodic table as a separate group. It did not change Mendeleev’s periodic table’s original arrangement. Since noble gases are chemically inert, they are classified as a separate group. Noble gases were discovered much later than expected because they are chemically inert and exist in extremely low concentrations in the atmosphere.
- It predicted the existence of a number of elements that had not yet been discovered.
Mendeleev’s periodic table had gaps for elements like gallium (Ga), scandium (Sc), and germanium (Ge) that had not yet been discovered. When these elements were discovered later, they were inserted in the gaps between the existing elements without disturbing them. It was able to predict the existence and properties of three elements with similar properties to boron, aluminium, and silicon. Eka-boron, Eka-aluminum, and Eka-silicon are the names of these elements. He gave them names by prefixing the name of the preceding element in the same group with the Sanskrit numeral Eka (one). Eka-boron is the name given to the gap he left for the undiscovered element that comes after boron, and Eka-aluminium is the name given to the gap he left for the undiscovered element that comes after aluminium. Eka-silicon is the name given to the gap he has left for the undiscovered element that comes after silicon. They were isolated and given the names scandium, gallium, and germanium. Their oxides and halides have experimentally determined atomic weights, physical properties, and chemical formulas that were identical to those anticipated by Mendeleev. The missing periods indicated the presence of elements that were yet to be discovered. Since he expected the presence of elements that had not yet been identified, Mendeleev’s periodic table had certain gaps.
Question 1: How did Mendeleev predict the existence of some elements in his periodic table that had yet to be discovered?
When Mendeleev proposed his periodic table, he identified gaps in the table and predicted that undiscovered elements with properties suitable for filling those gaps existed.
Question 2: Mendeleev predicted the properties of two elements based on their positions in the periodic table. Name them.
Since some elements were not discovered at the time, their properties were expected based on their positions in Mendeleev’s periodic table. The properties of Gallium(eka-Aluminium) and Scandium(eka-Boron) were predicted based on their positions in Mendeleev’s periodic table.
Question 3: In Mendeleev’s original periodic table why was the noble group of elements missing?
The noble gases were not included in Mendeleev’s original periodic table because they were not discovered at the time.
Question 4: Write a few demerits of Mendeleev’s periodic table.
Some demerits of Mendeleev’s periodic table are:
- It was unable to fix the position of isotopes.
- It was unable to explain why the atomic masses of some elements were in the wrong order.
- In the periodic table, hydrogen could not be assigned to a correct position.
Question 5: What factors guided Mendeleev to classify the elements in the periodic table?
Mendeleev was guided by two factors:
- The increasing atomic masses of the elements.
- Elements with similar properties are grouped together.
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