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# Dobereiner's Triads, Newlands Octaves and Mendeleev's Periodic Table Class 11 Notes | EduRev

## JEE : Dobereiner's Triads, Newlands Octaves and Mendeleev's Periodic Table Class 11 Notes | EduRev

The document Dobereiner's Triads, Newlands Octaves and Mendeleev's Periodic Table Class 11 Notes | EduRev is a part of the JEE Course Chemistry for JEE.
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What is a Periodic Table?

Certain groups of elements have similar chemical properties; we classify elements as members of the same chemical family.

Such similarity in the properties of elements has led scientists to classify them in such a form where one can study them in an organized manner.

Such an arrangement is called a Periodic Table.

• The modern science world is knowing information about more than 114 elements and have a periodic table prepared by Henry Moseley which makes the study easier.
• But before the appearance of the modern periodic table, there were many attempts that deserve respect.
• Some of the significant earlier attempts before coming on the modern periodic table are discussed below.

• He arranged similar elements in the groups of three elements called triads, in which the atomic mass of the central element was merely the arithmetic mean of atomic masses of the other two elements or all the three elements possessed nearly the same atomic masses

• It was restricted to few elements, therefore, discarded.

Newland's Law of Octaves:

• He was the first to correlate the chemical properties of the elements with their atomic masses.
• According to him if the elements are arranged in the order of their increasing atomic masses the eighth element starting from the given one is similar in properties to the first one.
• Newlands compared the similarity between the elements to the octaves of music, where every eighth note is comparable to the first.
• This was the first attempt at assigning an atomic number to each element.
• However, this method of classifying elements was met with a lot of resistance in the scientific community.

• This arrangement of elements is called as Newland's law of Octave.
• This classification worked quite well for the lighter elements but it failed in the case of heavier elements and, therefore, discarded.

Lother Meyer's Classification

He determined the atomic volumes by dividing atomic mass with its density in solid states.

He plotted a graph between atomic masses against their respective atomic volumes for a number of elements. He found the following observations.

1. Elements with similar properties occupied similar positions on the curve.
2. Alkali metals having larger atomic volumes occupied the crests.
3. Transitions elements occupied the troughs.
4. The halogens occupied the ascending portions of the curve before the inert gases.
5. Alkaline earth metals occupied the positions at about the midpoints of the descending portions of the curve.

Lothar Meyer's Arrangement of Elements

On the basis of these observations, he concluded that the atomic volumes (a physical property) of the elements are a periodic function of their atomic masses.

It was discarded as it lacks practical utility.

Mendeleev's Periodic Table:

Mendeleev's Periodic's Law

• According to him the physical and chemical properties of the elements are a periodic function of their atomic masses.
• He arranged then known elements in order of their increasing atomic masses considering the fact that elements with similar properties should fall in the same vertical columns and leaving out blank spaces where necessary.

Merits of Mendeleev's Periodic table:

1. It has simplified and systematized the study of elements and their compounds.

2. It has helped in predicting the discovery of new elements on the basis of the blank spaces given in its periodic table.

• Mendeleev predicted the properties of those missing elements from the known properties of the other elements in the same group.
• Eka - Aluminium, and Eka-silicon names were given for gallium and germanium (not discovered at the time of Mendeleev).
• Properties predicted by Mendeleev for these elements and those found experimentally were almost similar.

 Property eka-aluminium(predicted) gallium (found) eka-silicon (predicted) germanium(found) Atomic Mass 68 70 72 72.6 Density / (g/cm3) 5.9 5.94 5.5 5.36 Melting Point (K) Low 30.2 High 1231 Formula of Oxide E2O3 Ga2O3 EO2 GeO2 Formula of Chloride ECl3 GaCl3 ECl4 GeCl4

3. Atomic weights of elements were corrected. The atomic weight of Be was calculated to be 3 × 4.5 = 13.5 by considering its valency 3, was correctly calculated considering its valency 2 (2 × 4.5 = 9)

Demerits In Mendeleev's Periodic Table:

1. Position of hydrogen is uncertain. It has been placed in group 1 and 17th group because of its resemblance with both the groups.
2. No separate positions were given to isotopes.
3. Anomalous position of lanthanides and actinides in periodic table.
4. Order of increasing atomic weights is not strictly followed in the arrangement of elements in the periodic table. For e.g.-Ar(At.wt.39.94) is placed before K(39.08) and Te (127.6) is placed before I (126.9)
5. Similar elements were placed in different groups(CuIB and Hg IIB) and the elements with different properties were placed in the same groups(alkali metals IA and coinage metals IB)
6. It didn't explain the cause of periodicity.

The Modern Periodic Table

• The modern table is based on Mendeleev’s table, except the modern table arranges the elements by increasing atomic number instead of atomic mass.
• The atomic number is the number of protons in an atom, and this number is unique for each element.
• The modern table has more elements than Mendeleev’s table because many elements have been discovered since Mendeleev’s time.
• Rows of the modern periodic table are called periods, as they are in Mendeleev’s table.
• Columns of the modern table are called groups, as they are in Mendeleev’s table.

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