Periodic Classification Of Elements (Early Attempts)
Making Order Out Of Chaos – Early Attempts At The Classification Of Elements
As the 19th century progressed, the list of known elements grew rapidly. Scientists had accumulated a vast amount of information about the physical and chemical properties of these elements and the compounds they formed. It became increasingly difficult to study each element individually. There was a pressing need to organise this knowledge in a systematic way, to find some underlying principle that would simplify the study of elements and their reactions.
Classification, in science, helps in understanding complex systems by grouping entities with similar characteristics. In the case of elements, scientists hoped to find a way to arrange them such that elements with similar properties fell together, making it easier to predict the properties of other elements in the group and to study the relationships between them.
Several scientists attempted to classify elements based on different criteria available at the time, primarily their atomic masses and properties. These early attempts, while having limitations, were crucial steps towards the development of the modern periodic table we use today.
Döbereiner’s Triads
One of the earliest notable attempts at classifying elements was made by the German chemist Johann Wolfgang Döbereiner in 1829. Döbereiner observed that certain groups of three elements exhibited similar chemical properties. He called these groups triads.
The striking observation Döbereiner made was related to the atomic masses of the elements within a triad. When the three elements in a triad were arranged in the increasing order of their atomic masses, he found that the atomic mass of the middle element was approximately the arithmetic mean of the atomic masses of the other two elements.
This suggested a mathematical relationship between the elements in these specific groups, linking their atomic mass to their chemical behaviour.
Example: Let's re-examine the triad of Lithium (Li), Sodium (Na), and Potassium (K) and demonstrate the approximate relationship of their atomic masses.
Answer:
The known atomic masses of these elements are approximately: Lithium (Li) = 6.9 u, Sodium (Na) = 23.0 u, Potassium (K) = 39.0 u.
Arranging them in increasing order of atomic mass: Li, Na, K.
According to Döbereiner's observation, the atomic mass of the middle element (Sodium) should be close to the average of the atomic masses of the other two (Lithium and Potassium).
$$ \text{Average Atomic Mass} = \frac{\text{Atomic Mass of Li} + \text{Atomic Mass of K}}{2} $$
Substituting the values:
$$ \text{Average Atomic Mass} = \frac{6.9 \text{ u} + 39.0 \text{ u}}{2} = \frac{45.9 \text{ u}}{2} = 22.95 \text{ u} $$
The calculated average atomic mass (22.95 u) is very close to the actual atomic mass of Sodium (23.0 u), supporting Döbereiner's finding for this triad.
Some other examples of Döbereiner's Triads include:
| Triad Elements | Atomic Mass (u) | Mean of First and Third |
|---|---|---|
| Li, Na, K | 6.9, 23.0, 39.0 | (6.9 + 39.0) / 2 = 22.95 |
| Ca, Sr, Ba | 40.1, 87.6, 137.3 | (40.1 + 137.3) / 2 = 88.7 |
| Cl, Br, I | 35.5, 79.9, 126.9 | (35.5 + 126.9) / 2 = 81.2 |
| S, Se, Te | 32.1, 79.0, 127.6 | (32.1 + 127.6) / 2 = 79.85 |
Limitations of Döbereiner’s Triads:
While Döbereiner's triads showed a compelling relationship for certain sets of elements, this classification system had significant limitations:
1. Limited Scope: Döbereiner could only identify a limited number of such triads from the known elements. Many elements discovered at that time could not be grouped into triads that followed this rule.
2. Not Universally Applicable: The triad system did not provide a framework for classifying all the known elements, leaving many elements unclassified or grouped seemingly arbitrarily outside of triads.
3. Approximation: The atomic mass of the middle element was often only *approximately* the average. In some cases, the deviation was noticeable.
Due to these limitations, Döbereiner's triads, although a pioneering effort highlighting a relationship between atomic mass and properties, could not be developed into a complete system for classifying all elements.
Newlands’ Law Of Octaves
Following Döbereiner, the English scientist John Newlands in 1865 proposed another method of classification. He arranged the elements known at that time in the increasing order of their atomic masses.
Newlands observed a repeating pattern in the properties of these elements. He noticed that the properties of every eighth element were similar to the properties of the first element, much like the notes in a musical scale (Sa, Re, Ga, Ma, Pa, Dha, Ni, Sa'), where the eighth note is a repetition of the first.
Based on this observation, he formulated the Law of Octaves, stating that when elements are arranged in increasing order of atomic masses, the properties of the eighth element are a repetition of the properties of the first element.
Newlands' arrangement of elements based on his Law of Octaves is represented in a table similar to a musical scale:
| Sa (Do) | Re | Ga | Ma (Fa) | Pa (So) | Dha (La) | Ni (Ti) |
|---|---|---|---|---|---|---|
| Li | Be | B | C | N | O | F |
| Na | Mg | Al | Si | P | S | Cl |
| K | Ca | Cr | Ti | Mn | Fe | |
| Co and Ni | Cu | Zn | Y | In | As | Se |
| Br | Rb | Sr | Ce and La | Zr | — | — |
In this table, elements like Li, Na, K in the first column share similar properties (highly reactive metals). Similarly, F and Cl in the seventh column are reactive non-metals (halogens).
Limitations of Newlands’ Law of Octaves:
Despite being a significant step, Newlands' Law of Octaves also suffered from several drawbacks:
1. Applicability Limited to Lighter Elements: The Law of Octaves was found to be valid only for elements up to Calcium (Ca). For heavier elements, the properties of the eighth element did not resemble those of the first element in the sequence.
2. Assumption of Limited Elements: Newlands believed that only 56 elements existed and that no more elements would be discovered. This assumption was quickly proven wrong as more elements were found, and their properties did not fit into Newlands' existing pattern.
3. Placement of Elements:
- To fit elements into his table, Newlands sometimes had to place two elements in the same slot. For example, Cobalt (Co) and Nickel (Ni) were placed together in one position.
- He also placed elements with very dissimilar properties in the same column. For instance, Co and Ni (metals) were placed in the same column as Fluorine (F), Chlorine (Cl), and Bromine (Br), which are non-metals (halogens) and have distinctly different chemical behaviours. Also, Iron (Fe), a metal, was placed in the same row as oxygen and sulphur, which are non-metals.
4. Ignorance of Noble Gases: The noble gases (like Helium, Neon, Argon, etc.) were not known at Newlands' time. When these elements were discovered later, their inclusion in the arrangement completely disrupted the 'every eighth element' pattern, as the noble gases would typically fall between the elements Newlands had placed, shifting the position of the eighth element.
Even with these limitations, Newlands' Law of Octaves was important because it was the first attempt to establish a logical basis for classifying elements based on atomic mass and periodic repetition of properties. It provided a glimpse of the periodic behaviour that would be more accurately described in later classifications.