The Periodic Classification: Evolution of Periodic Table, Exam Questions and Limitations

As per William Prout's hypothesis, the atomic weights of all elements was whole number multiples of the Hydrogen's atomic weight. This hinted at the fact that there is a fundamental relationship amongst elements. This hypothesis was based on Dalton's atomic theory which assumed that elements were composed of hydrogen units but it disproved that elements like chlorine (atomic weight 35.5 u) could show fractional values. The hypothesis was then rejected.

Johann Döbereiner was a German chemist who made the earliest attempts at periodic classification. He grouped elements in sets of three and called them triads. Here, the atomic weight of the middle element was approximately the arithmetic mean of the remaining two elements. All three elements in a triad has similar properties. Let us consider one example of a triad:

1. Triad: Li (7), Na (23), K (39): Mean atomic weight $=\frac{7+39}{2}=23$ , matching sodiums atomic weight.

2. Triad: $\mathbf{C}\mathbf{l}\left(35.5\right),\mathbf{B}\mathbf{r}\left(80\right),\mathbf{I}\left(127\right):$ Mean $=\frac{35.5+127}{2}\approx 81.25$ , close to bromines atomic weight 

Solutions of this NCERT chapter include questions related to genisis of periodic classification. This is useful for those students who need to practice it for their school examinations. Because of this idea, chemists started grouping elements based on their properties. However, this approach could only be applied to a few elements. For example. Fe, Co and Ni, even with identical atomic weights did not fit into the characteristics expected from a triad. This lead to the dismissal of this idea.

John Newlands in 1866 proposed the Law of Octaves where he arranged all the known elements in the order of increasing atomic mass. He noticed that every 8th element showcases properties similar to the first element. John Newland witnessed that Lithium (Li) and Sodium (Na) displayed similar behaviour. However, this law was applicable only for the first 20 elements. After the 20th element, the law did not work because of the presence of transition metals and the newly discovered elements. His table included:

• Li (7), Be (9), B (11), C (12), N (14), O (16), F (19)
• Na (23), Mg (24), Al (27), Si (28), P (31), S (32), Cl (35.5)

Here, Na , the eighth element from Li , shares similar properties.

Lothar Meyer plotted a graph of atomic volume against atomic mass for the known elements. He observed that this curve showed repeating troughs and peaks. In this curve, those elements that were at the peak has similar chemical properties. This pattern suggested a periodic relationship amongst element properties and atomic mass. His atomic volume curve strongly supported the idea of periodicity and showed that elements having similar properties will appear at regular intervals when arranged by the atomic mass. The method was impractical for memorizing element positions and lacked predictive power, limiting its utility.

• Alkali metals (e.g., Na, K) occupied maxima.
• Alkaline earth metals (e.g., Mg, Ca) were at mid-points on descending slopes.
• Halogens (e.g., $\mathrm{C}\mathrm{l},\mathrm{B}\mathrm{r}$ ) appeared on ascending slopes.
• Transition metals occupied minima

Dmitri Mendeleev created the first widely accepted Periodic Table of Elements in the year 1869. He arranged the known elements in the increasing order of atomic mass and placed elements with similar chemical properties in the same group/columns. He was able to accurately predict the properties of the elements that were not even discovered (such as Gallium and Germanium). Later on, it was found that atomic mass is the correct basis for classification instead of mass. It is the foundation for the modern periodic law.

The following points highlight the limitations of Mendeleev's periodic table:

1. There were ambiguity in Hydrogen since it showed similarities with both halogens and alkali metals.
2. Isotopes could not be placed properly since there was no way to arrange them in the periodic table.
3. Mendeleev was unable to explain why periodicity existed since protons and atomic numbers were not discovered till that time.

Different chemists proposed hypothesis starting from William Prouts to Mosley. Each of them contributed in some ways leading to the other chemist identifying the flaws and improving the periodic table. These revisions led to the formation of modern periodic table which is now used. For students of IISER and NEET enterance examinations, learning about periodic table is important since it forms the base of chemistry and most questions are asked from this topic.