The earliest classification categorised elements 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.
Dobereiner’s law of triads states that, the atomic mass of the middle element of a triad is the arithmetic mean of the atomic masses of the other two elements.
Drawbacks of this law:
- All the known elements could not be arranged in the form of triads and it did not hold good for elements with very low or very high atomic mass
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.
- Not valid for elements having atomic masses higher than calcium.
Newly discovered elements could not find a place in Newlands table.
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:
The main features of Mendeleev’s periodic table are:
- 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.
- The table provided gaps for undiscovered 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.
The properties of these elements were very close to those predicted by Mendeleev.
The table helped in the correction of atomic mass for many elements.
It predicted the existence of some elements that have not been discovered at the time the table was created.
The atomic weights of two pairs of elements were reversed.
Alkali metals and coinage metals were placed in the same group.
Lanthanides and actinides were not given proper place in the periodic table.
Isotopes were not placed in the periodic table.
The position of hydrogen was not clearly discussed