Classification of Elements and Periodicity in Properties: Limitations of Dobereiner՚s Trids, Newlands՚ Law of Octaves (For CBSE, ICSE, IAS, NET, NRA 2022)

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  • The arrangement of elements in the modern periodic table is organized into eight vertical columns (groups) and seven horizontal rows (periods) based on their atomic number.
  • In 1829, Dobereiner arranged certain elements with similar properties in groups of three in such a way that the atomic mass of the middle element was nearly the same as the average atomic masses of the first and the third elements.
  • Based on the electronic configuration, elements can be classified into four types in the periodic table, viz s-block, p-block, f-block, and d-block elements. More than 70 - 80 % of the known elements are metals.
  • Elements located at the top of the periodic table are nonmetals and are less than 20.
  • In a group, as the atomic number increases, the metallic character increases. In a period, there is a decrease in the metallic character from left to right.
  • Also, the chemical, as well as the physical properties, vary with their atomic number.
  • Elements of the same group exhibit similar chemical properties because of the similar valence shell electronic configuration.

A Few Triads Proposed by Dobereiner

The Newlands Law of Octaves

Limitations of Dobereiner՚s Trids

  • The triads given by Dobereiner were helpful in grouping some elements with similar characteristics together, but he could not arrange all the elements known at that time into triads.

Periodic Trends in Properties of Elements

  • Atomic sizes – With the increase in atomic number the atomic radii in the group will increase. While, the atomic radii decrease from left to right in a period.
  • Ionization enthalpies – it Decreases down a group but increases across a period.
  • Electron gain enthalpies – Tends to become less negative down a group and more negative across a period.
  • Electro negativity – Electro negativity decreases down a group but increases across a period.
  • Chemical reactivity – Chemical reactivity is lowest in the centre and is highest at the two extremes of a period.

Newlands՚ Law of Octaves

  • John Newlands proposed the law of octaves by stating that when elements are arranged in order of increasing atomic masses, every eighth element has properties similar to the first.
  • Newlands called it law of octaves because similar relationship exists in the musical notes also

Newlands՚ Law of Octaves

Newlands Law of Octaves

Limitations of Newland՚s Octaves Are

  • It was only up till calcium that the classification of elements is done via Newland՚s Octaves.
  • The discovery of noble gases added to the limitations of this method since they couldn՚t be included in this arrangement without disturbing it completely.

Mendeleev՚s Periodic Table

  • Mendeleev՚s Periodic Law: The physical and chemical properties of the elements are a periodic function of their atomic masses.
  • The inclusion of these newly discovered elements did not disturb the periodic table. Examples include germanium, gallium, and scandium.
  • Mendeleev arranged the elements known at that time in order of increasing atomic masses and this arrangement was called periodic table. Elements with similar characteristics were present in vertical rows called groups. The horizontal rows were known as periods.
  • In the periodic table, the elements are arranged in vertical rows called groups and horizontal rows known as periods.
  • There are nine groups indicated by Roman Numerals as I, II, III, IV, V, VI, VII, VIII and zero. Group VIII consists of nine elements which are arranged in three triads. The zero group contains elements belonging to inert gases or noble gases and elements present have zero valency.
  • There are seven periods (numbered from 1 to 7) or, horizontal rows in the Mendeleev՚s periodic table.

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