Periodic Classification of Elements - Describe Dobereiner triads, Newlands law of octaves, and Mendeleev periodic table

Dobereiner's Triads: In 1817, Johann Wolfgang Dobereiner identified groups of three elements with similar properties. He showed that when these elements were written in the order of increasing atomic masses, the atomic mass of the middle element was roughly the average of the atomic masses of the other two elements.

Newlands' Law of Octaves: In 1866, John Newlands arranged elements in increasing order of atomic masses. He found that every eighth element had properties similar to those of the first, comparing this to the octaves found in music. This law was applicable only up to Calcium ($Ca$).

Mendeleev's Periodic Law: The properties of elements are a periodic function of their atomic masses. Mendeleev arranged the then-known $63$ elements based on their atomic masses and the similarity of their chemical properties (specifically the formulae of their hydrides and oxides).

Mendeleev's Table Structure: The horizontal rows are called 'periods' and the vertical columns are called 'groups'. Mendeleev left gaps for elements like Scandium ($Sc$), Gallium ($Ga$), and Germanium ($Ge$), naming them Eka-boron, Eka-aluminium, and Eka-silicon respectively.

Limitations of Mendeleev's Classification: It could not assign a fixed position to Hydrogen ($H$), isotopes of elements posed a challenge as they have different atomic masses but similar chemical properties, and in some cases, the order of atomic mass was subverted (e.g., Cobalt with mass $58.9$ was placed before Nickel with mass $58.7$).