1. The earliest classification categorized element into metals and non-metals
It was difficult to classify the elements, such as boron, which exhibited the properties of both metals as well as non-metals

2. Dobereiner’s triads
Dobereiner’s law of triads states that, the atomic mass of the middle element of a triad is
approximately the arithmetic mean of the atomic masses of the other two elements.

3. EXAMPLE Ca Sr Ba 40.1 87.6 137.3 Arithmetic mean : 88.7 N P As
Arithmetic mean : 88.7
N P As
Arithmetic mean : 44.4

4. Drawbacks of this law:
A large number of similar elements could not be grouped into triads e.g., iron, manganese, nickel, cobalt, zinc and copper are similar elements but could not be placed in the triads.
It was possible that quite dissimilar elements could be grouped into triads.
Dobereiner could only classify 3 triads successfully (highlighted in the table).

5. Newlands’ Law of Octaves:
The property of every eighth element is similar to that of the first element, similar to the first and the eighth notes in the musical scale.

6. Drawbacks or Defects
Not valid for elements having atomic masses higher than calcium.
Newly discovered elements could not fit into the octave structure.
Newland s adjusted two elements in the same slot, but also put some unlike elements under the same note.

7. Mendeleev’s periodic law
Mendeleev’s periodic law states that the physical and chemical properties of all elements are a periodic function of their atomic masses.
Atomic weight and chemical reactivity are the  two parameters he chose for classifying the elements:

8. Main features of Mendeleev’s periodic table
The table had 8 vertical columns called groups, and 12 horizontal rows called periods
In every group, a gradation of physical and chemical properties of elements was observed

9. The table provided gaps for
undiscovered elements

10. The table helped predict the properties of three elements
The table helped predict the properties of three elements. These elements were named eka-boron, eka-aluminium and eka-silicon
When these elements were discovered, they were named scandium, gallium and germanium.

11. The properties of these elements were very close to those predicted by Mendeleev. Property Eka-boron Scandium
Atomicweight Oxide Eb2O Sc2O3
Specific gravity
Sulphate Eb2(SO4) Sc2(SO4)3
The properties of t

12. Merits:
The table helped in the correction of atomic mass for many elements.
For example, atomic mass of beryllium was corrected from 13.5 to 9.0. Atomic masses of indium, gold and platinum were also corrected
It predicted the existence of some elements that have not been discovered at the time the table was created.

13. Mendeleev's periodic table could accommodate noble gases when they were discovered.

14. Limitations of Mendeleev's
Position of Hydrogen
The position of hydrogen was not correctly defined. It was placed in Group I although its properties resembled both the Group I elements (the alkali metals) and the group VII elements (the halogens).

15. Grouping of Some Elements
In some cases Mendeleev placed elements according to their similarities in properties and not in increasing order of their atomic masses, while some dissimilar elements were grouped together. Thus, the position of these elements was not justified. For example, cobalt (at. mass 58.9) was placed before nickel (at. mass 58.6);

16. copper and mercury are similar in their properties but were placed separately. Copper was placed in group I although it did not resemble the elements of this group.

17. Anomalous Pair
In certain pairs of elements like, Ar (40) and K (39); Co (58.9) and Ni (58.6); Te (127.6) and I (126.9) the arrangement was not justified. For example, argon was placed before potassium whereas its atomic mass is more than potassium.

18. Isotopes
Isotopes are atoms of the same element having different atomic mass but same atomic number.
For e.g., there are three isotopes of hydrogen
with atomic mass 1, 2, and 3.
According to Mendeleev's periodic table these
should be placed at three separate places.
However isotopes have not been given separate places in the periodic table.

19. Lanthanides and Actinides
Fourteen elements that follow lanthanum called lanthanides and fourteen elements following actinium called actinides were not given proper places in Mendeleev's periodic table.
Cause of Periodicity
Mendeleev's table was unable to explain the cause of periodicity among elements.