The anomalous pairs of elements are specific elements in the early development of the periodic table whose arrangement defied the strict order of increasing atomic weights, instead prioritizing their chemical properties.
Understanding Anomalous Pairs in the Periodic Table
In the original periodic table proposed by Dmitri Mendeleev, elements were primarily arranged in order of their increasing atomic weights. This arrangement successfully grouped elements with similar chemical properties. However, a few elements, when placed strictly by their atomic weights, would not align with elements sharing similar chemical characteristics. To maintain the integrity of the periodic law—that properties recur periodically—Mendeleev and subsequent chemists made strategic reversals for these pairs. These are known as anomalous pairs or inverted pairs.
These reversals ensured that elements with similar reactivities and valencies were placed in the correct groups, even if it meant placing a slightly heavier element before a lighter one. This demonstrated the powerful predictive and organizational capability of the periodic table, highlighting that chemical properties were the ultimate determinant of an element's position.
The Key Anomalous Pairs of Elements
The most recognized anomalous pairs of elements, where a heavier element was placed before a lighter one to align with chemical properties, are:
Pair Number | Element 1 (Heavier) | Atomic Weight (approx.) | Element 2 (Lighter) | Atomic Weight (approx.) | Justification for Placement (Chemical Properties) |
---|---|---|---|---|---|
1 | Argon (Ar) | 39.95 amu | Potassium (K) | 39.10 amu | Argon, an inert gas, belongs with other noble gases. Potassium, an alkali metal, belongs with highly reactive metals. |
2 | Tellurium (Te) | 127.60 amu | Iodine (I) | 126.90 amu | Tellurium, a metalloid, aligns with the oxygen group. Iodine, a halogen, aligns with fluorine, chlorine, and bromine. |
3 | Cobalt (Co) | 58.93 amu | Nickel (Ni) | 58.69 amu | Cobalt's properties are closer to those of iron and other elements in its group, while nickel aligns with its own group members. |
The Resolution: Atomic Number
The anomalies arising from atomic weight ordering were ultimately resolved with the discovery of the atomic number by Henry Moseley in 1913. Moseley's work demonstrated that the fundamental organizing principle of the periodic table is the number of protons in an atom's nucleus (its atomic number), not its atomic weight. When elements are arranged strictly by increasing atomic number, these anomalous pairs naturally fall into their correct positions according to their chemical properties, without any reversals.
For example:
- Argon has an atomic number of 18, and Potassium has 19. (Ar then K)
- Tellurium has an atomic number of 52, and Iodine has 53. (Te then I)
- Cobalt has an atomic number of 27, and Nickel has 28. (Co then Ni)
This fundamental shift from atomic weight to atomic number solidified the structure and predictive power of the modern periodic table.