Chemistry · Ch 3 — Classification of Elements and Periodicity in Properties

Genesis of Periodic Classification

3.2

Genesis of Periodic Classification

3.2 Genesis of Periodic Classification

The modern Periodic Table did not appear overnight. It is the product of nearly a century of observations, guesses, corrections, and bold predictions by several scientists. Each contributor added a piece to the puzzle, and the story of how elements came to be classified is as instructive as the table itself.

Dobereiner's Triads (1829)

The German chemist Johann Dobereiner was the first to notice that elements could be grouped in sets of three — triads — that shared similar chemical and physical properties. By 1829, he had identified several such triads.

\text{Atomic weight of the middle element} \approx \frac{\text{Atomic weight of first element} + \text{Atomic weight of third element}}{2}

For example, in the triad lithium–sodium–potassium:

Element	Atomic weight
Li	7
Na	23
K	39

The atomic weight of sodium (23) is almost exactly the average of lithium (7) and potassium (39): $(7 + 39)/2 = 23$ .

Dobereiner also observed that the properties of the middle element — its reactivity, melting point, density — fell between those of the other two members. This was the first hint that atomic weight and properties were connected in a regular way.

Watch out

Dobereiner's Law of Triads worked only for a few groups (Li–Na–K, Ca–Sr–Ba, Cl–Br–I). When scientists tried to extend it to other elements, it failed. Most chemists dismissed it as coincidence, and the idea did not gain wide acceptance.

Table 3.1Dobereiner's Triads

Element	At. wt.	Element	At. wt.	Element	At. wt.
Li	7	Ca	40	Cl	35.5
Na	23	Sr	88	Br	80

De Chancourtois' Cylindrical Table (1862)

The French geologist A.E.B. de Chancourtois took a different approach. He arranged the known elements in order of increasing atomic weight and wrapped this list around a cylinder. Elements that lined up vertically on the cylinder showed similar properties. This was the first attempt to display periodic recurrence of properties in a visual form.

Note

De Chancourtois' work was published in a geological journal and did not reach many chemists. Like Dobereiner's triads, it attracted little attention at the time.

Newlands' Law of Octaves (1865)

The English chemist John Alexander Newlands made a more systematic attempt. He arranged the elements in increasing order of atomic weight and noticed a striking pattern: every eighth element resembled the first, much like the eighth note in a musical octave.

Lithium (first element) and sodium (eighth element) are both highly reactive metals. Beryllium (second) and magnesium (ninth) are both alkaline earth metals. The pattern held — but only up to calcium.

Watch out

Newlands' Law of Octaves broke down for elements beyond calcium. When new elements were discovered later, they did not fit into his neat octaves. The scientific community ridiculed his idea, comparing it to arranging elements alphabetically. However, in 1887, the Royal Society of London awarded him the Davy Medal in recognition of his pioneering work.

Table 3.2Newlands' Octaves

Element	Li	Be	B	C	N	O	F
At. wt.	7	9	11	12	14	16	19
Element	Na	Mg	Al	Si	P	S	Cl
At. wt.	23	24	27	29	31	32	35.5

Mendeleev and Lothar Meyer (1869)

The Periodic Law as we know it today owes its development to two chemists working independently: the Russian Dmitri Mendeleev (1834–1907) and the German Lothar Meyer (1830–1895).

Lothar Meyer plotted physical properties — atomic volume, melting point, boiling point — against atomic weight. He obtained a periodically repeated pattern, but unlike Newlands, he noticed that the length of the repeating pattern changed. By 1868, he had developed a table of elements that closely resembles the Modern Periodic Table. However, his work was not published until after Mendeleev's.

Important

Mendeleev is generally credited with the development of the Modern Periodic Table because he published his work first and, more importantly, because he made bold quantitative predictions that were later verified.

Mendeleev's Periodic Law

Mendeleev stated the Periodic Law for the first time:

\text{The properties of the elements are a periodic function of their atomic weights.}

He arranged elements in horizontal rows (which he called series) and vertical columns (groups) in order of increasing atomic weight, such that elements with similar properties occupied the same vertical column.

Key Features of Mendeleev's System

1. He used a broader range of properties. Unlike his predecessors, Mendeleev relied not just on atomic weights but also on the empirical formulas and properties of compounds formed by the elements. For example, he looked at the formulas of oxides and hydrides to decide where an element belonged.

2. He ignored atomic weight when necessary. Mendeleev realized that if he strictly followed atomic weight order, some elements would end up in groups where they did not fit chemically. He made a crucial decision: he placed elements with similar properties together, even if it meant reversing the atomic weight order.

Note

The most famous example is tellurium (atomic weight 127.6) and iodine (atomic weight 126.9). Tellurium has a higher atomic weight than iodine, but tellurium resembles the Group VI elements (oxygen, sulfur, selenium) while iodine resembles the Group VII elements (fluorine, chlorine, bromine). Mendeleev placed tellurium in Group VI and iodine in Group VII, assuming the atomic weight measurements were incorrect. He was right about the grouping — and later, Moseley showed that atomic number, not atomic weight, is the fundamental property.

3. He left gaps for undiscovered elements. Mendeleev was so confident in his system that he left blank spaces in his table where no known element fit. He predicted that these gaps would be filled by elements yet to be discovered.

Important

Mendeleev predicted the existence and properties of two such elements:

Eka-aluminium (the element under aluminium) — later discovered as gallium (1875)
Eka-silicon (the element under silicon) — later discovered as germanium (1886)

Mendeleev's Predictions vs. Experimental Findings

The table below shows how accurately Mendeleev predicted the properties of these elements:

Table 3.3Mendeleev's Predictions for the Elements Eka-aluminium (Gallium) and Eka-silicon (Germanium)

Property	Eka-aluminium (predicted)	Gallium (found)	Eka-silicon (predicted)	Germanium (found)
Atomic weight	68	70	72	72.6
Density /(g/cm $^3$ )	5.9	5.94	5.5	5.36
Melting point /K	Low	302.93	High	1231
Formula of oxide	$E_2O_3$	$Ga_2O_3$	$EO_2$	$GeO_2$

…

Figure 3.1Mendeleev's Periodic Table published earlier.

Fig. 3.1 — Mendeleev's Periodic Table published earlier.

Drawn by us to help you understand the concept clearly, and verified to make sure it's accurate. For exams, practice from your NCERT textbook's own diagram.

Fig. 3.1 reproduces the structure of Mendeleev's Periodic Table published in 1905 — the mature form of his table, not the modern 18-column layout. Elements are arranged in numbered horizontal series (1 to 12) and vertical groups (0 and I through VIII), in order of increasing atomic weight; each cell shows the element's symbol and its atomic weight as the book prints them.

Reading the figure:

Series 1 contains hydrogen alone (H, 1.008). The noble gases — discovered only in the 1890s — occupy their own Group 0 on the left (He 4.0, Ne 19.9, Ar 38, Kr 81.8, Xe 128).
Each group column is headed by a general oxide formula (bottom row: $R_2O$ for Group I through $R_2O_7$ for Group VII and $RO_4$ for Group VIII), and groups IV–VII also carry hydride formulas ( $RH_4$ , $RH_3$ , $RH_2$ , $RH$ ) — Mendeleev's way of encoding each group's combining capacity (valence).
Group VIII holds triads: Fe–Co–Ni (series 4), Ru–Rh–Pd (series 6) and Os–Ir–Pt (series 10), with (Cu), (Ag) and (Au) noted alongside.
Dashes mark elements still undiscovered in 1905 — for example most of series 9 and several slots in series 10–12. (Gallium and germanium, the famous eka-aluminium and eka-silicon predictions of his earlier table, had been discovered by then and sit in series 5.)

Important

The figure is a visual statement of Mendeleev's Periodic Law — $\text{The properties of the elements are a periodic function of their atomic weights.}$ Arranged by increasing atomic weight, similar properties recur at regular intervals — provided one leaves gaps for missing elements and occasionally trusts chemistry over the weighing balance. …

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