Earlier Attempts of Classification
Before the modern periodic table, scientists like Dobereiner, Newlands, and Lothar Meyer made significant attempts to classify elements based on Atomic Mass. While their models had limitations, they paved the way for Mendeleev and Moseley.
1. Dobereiner's Triads (1817)
Johann Wolfgang Dobereiner, a German chemist, observed that certain elements with similar properties could be grouped into sets of three, called Triads.
The Law of Triads
"When elements are arranged in order of increasing atomic masses, groups of three elements (Triads) having similar chemical properties are obtained. The atomic mass of the middle element is approximately the arithmetic mean of the other two."
Key Triads
| Triad Name | Element 1 | Element 2 (Middle) | Element 3 | Calculation |
|---|---|---|---|---|
| Alkali Metals | Li (7) | Na (23) | K (39) | $\frac{7+39}{2} = 23$ |
| Alkaline Earth | Ca (40) | Sr (88) | Ba (137) | $\frac{40+137}{2} \approx 88.5$ |
| Halogens | Cl (35.5) | Br (80) | I (127) | $\frac{35.5+127}{2} \approx 81.25$ |
Limitation: He could identify only a few triads from the elements known at that time. Other similar elements (e.g., N, P, As) did not fit the rule perfectly.
2. Newlands' Law of Octaves (1866)
John Newlands, an English chemist, arranged the known elements in increasing order of atomic mass and compared them to musical notes.
Arrangement (Noble Gases Unknown)
| Sa (do) | Re (re) | Ga (mi) | Ma (fa) | Pa (so) | Da (la) | Ni (ti) |
| H | Li | Be | B | C | N | O |
| F | Na | Mg | Al | Si | P | S |
| Cl | K | Ca | Cr | Ti | Mn | Fe |
Example: Sodium (8th from Lithium) resembles Lithium. Magnesium (8th from Beryllium) resembles Beryllium.
Limitations
- Valid only up to Calcium: After Ca, the properties did not match well (e.g., Chromium doesn't resemble Aluminum).
- Discovery of Noble Gases: When noble gases were discovered later, they became the 9th element, destroying the octave relationship.
- Mixed Elements: He placed Cobalt and Nickel in the same slot to maintain the periodicity.
3. Lothar Meyer's Curve (1869)
Lothar Meyer, a German chemist, plotted the physical properties (like Atomic Volume, Melting Point, Boiling Point) against Atomic Mass. He obtained a periodic curve.
Key Observations from the Curve
Elements with similar properties occupied similar positions on the curve:
- Alkali Metals (Li, Na, K, Rb, Cs): Occupied the Peaks (Maxima) of the curve (Highest atomic volume).
- Halogens (F, Cl, Br, I): Occupied the Ascending portions of the curve.
- Alkaline Earth Metals (Be, Mg, Ca, Sr, Ba): Occupied the Descending portions of the curve.
- Transition Metals / Metalloids: Occupied the Bottom / Troughs of the curve.
Conclusion: Physical properties are a periodic function of atomic mass. While accurate, Mendeleev's table was preferred because it was easier to use (tabular form vs. graphical).
4. Summary Comparison
| Scientist | Basis | Contribution |
|---|---|---|
| Dobereiner | Atomic Mass | Idea of groups (Triads) |
| Newlands | Atomic Mass | Idea of periodicity (Octaves) |
| Lothar Meyer | Atomic Volume vs Mass | Periodicity in physical properties |
Classification Quiz
Test your history of chemistry. 10 MCQs with explanations.
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