Shalini Pandey answered this
Please refer the answers given by your friend , the answers are correct.
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Soniya Dash answered this
mendellevs periodic table is based on atomic mass and chemical reactivity of elements, mainly with hydrogen and oxygen..
When Mendeléev started his work, 63 elements were known. He examined the relationship between the atomic masses of the elements and their physical and chemical properties. Among chemical properties, Mendeléev concentrated on the compounds formed by elements with oxygen and hydrogen. He selected hydrogen and oxygen as they are very reactive and formed compounds with most elements. The formulae of the hydrides and oxides formed by an element were treated as one of the basic properties of an element for its classification. He then took 63 cards and on each card he wrote down the properties of one element. He sorted out the elements with similar properties and pinned the cards together on a wall. He observed that most of the elements got a place in a Periodic Table and were arranged in the order of their increasing atomic masses. It was also observed that there occurs a periodic recurrence of elements with similar physical and chemical properties. On this basis, Mendeléev formulated a Periodic Law, which states that ‘the properties of elements are the periodic function of their atomic masses’.
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Soniya Dash answered this
hope it helps...
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Soniya Dash answered this
ok sry if it is too bookish.....
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Uma answered this
After the failure of Newlands Law of Octaves, scientists continued to correlate the properties of elements with their atomic masses.Dmitri Ivanovich Mendeleev (1834-1907), a Russian scientist, also tried to relate the properties of elements with their atomic masses.
Mendeleev was the first person to introduce the concept of a periodic table. His periodic table was based on the atomic mass as well as the properties of elements. He arranged elements in the increasing order of their atomic masses and grouped the elements having similar properties together. Mendeleev formulated the periodic law in 1869.
Mendeleev’s Periodic Law:
This law states that the properties of elements are a periodic function of their atomic masses. This means that if elements are arranged in the increasing order of their atomic masses, then their properties will be repeated after regular intervals.
Only 63 elements were known when Mendeleev first started his work. He arranged these elements in the form of a table in the increasing order of their atomic masses. This table is known as Mendeleev’s periodic table as shown in the table. His table contains vertical columns called ‘groups’ and horizontal rows called ‘periods’.
Mendeleev’s periodic table
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Uma answered this
Achievements of Mendeleev’s periodic table:
Mendeleev left some gaps in his periodic table. He had predicted that some elements were yet to be discovered. He left these gaps deliberately so that these undiscovered elements could get a place in his periodic table.
Mendeleev named the undiscovered elements using the Sanskrit word Eka (meaning one) as a prefix, with the name of the preceding element in the same group. For example, gallium was not discovered in Mendeleev’s time. Therefore, he left a gap for it in his periodic table and named it Eka-aluminium. He also predicted the properties of these undiscovered elements based on their positions in the periodic table. A comparison of the properties of gallium as predicted by Mendeleev and its actual properties is given in the following table.
Property | Eka-aluminium | Gallium |
Atomic mass | 68 | 69.7 |
Density (g/cm³) | 5.9 | 5.93 |
Melting point (°C) | Low | 30.15 |
Oxides formula | Ea2O3 | Ga2O3 |
Chlorides formula | EaCl3 | GaCl3 |
Comparison of the properties of Eka-aluminium and gallium
In the periodic table, Ea is used as the symbol for Eka-aluminium. It can be clearly observed from the table that Mendeleev’s predictions were almost exact. This extraordinary achievement made Mendeleev’s periodic table very popular.
Noble gases were not discovered at the time when Mendeleev gave the periodic table. These were discovered in recent times as they are very inert and are present in very low concentrations in the atmosphere. When these gases were finally discovered, they got a place in his periodic table as a separate column. The accommodation of these gases in the periodic table did not disturb the positions of other elements. This underlined the strength of Mendeleev’s periodic table.
Although Mendeleev arranged the elements in the increasing order of their atomic masses, there were instances where he placed an element with a slightly higher atomic mass before an element with a slightly lower atomic mass.
For example, cobalt, whose atomic mass is 58.9, was placed before nickel whose atomic mass is 58.7. This was done to maintain consistency in the properties of the elements present in a group i.e., to group the elements with similar properties together.
Though, Mendeleev’s periodic table was a huge success at that time, it still had many limitations. Now, let us discuss the limitations of Mendeleev’s periodic table.
Limitations of Mendeleev’s periodic table:
1. Hydrogen’s position was not justified in Mendeleev’s periodic table. He positioned hydrogen in the first column above alkali metals. He did so because hydrogen and alkali metals have similar properties. For example, hydrogen reacts with halogens, oxygen, and sulphur to form compounds whose formulae are similar to those of alkali metals.
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Hydrogen and alkali metals reacting with halogens
However, hydrogen also resembles halogens in many ways. Like halogens, hydrogen is a gas, and exists as a diatomic molecule (H2). It forms covalent compounds like halogens unlike alkali metals. Hence, it can also be placed above the halogen group.
Therefore, Mendeleev was not able to explain the position of hydrogen. In other words, the position of hydrogen in Mendeleev’s periodic table was not justified. This was the first limitation of Mendeleev’s periodic table.
2. The discovery of isotopes revealed another limitation of Mendeleev’s periodic table. Since Mendeleev’s periodic table was based on atomic masses of elements, isotopes should be placed in different columns despite the fact that they represent the same element.
Atoms of the same elements having different number of neutrons are called isotopes. Isotopes have the same number of protons and electrons, but different number of neutrons. For example, the isotopes of chlorine are Cl-35 and Cl-37. They have the same atomic number, but different atomic masses.
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