Chemistry · Periodic Classification Of Properties · NEET
Mendeleev's rule was simple: place elements by increasing atomic weight. An anomalous pair is a case where he BROKE his own rule. He put a heavier element before a lighter one so that each element sat in the group whose properties it actually matched. The three pairs are Argon-Potassium (Ar-K), Tellurium-Iodine (Te-I), and Cobalt-Nickel (Co-Ni).
By atomic weight, Potassium (39.1) is lighter than Argon (39.9), so K should come first. But Argon is a noble gas (unreactive, group 18) and Potassium is a very reactive metal (group 1). To keep Ar with the noble gases and K with the alkali metals, Mendeleev put heavier Argon FIRST. So properties decided the order, not weight.
Tellurium (127.6) is heavier than Iodine (126.9). By weight, Iodine should come first. But Tellurium behaves like Sulphur and Selenium (group 16 chalcogens) and Iodine behaves like Chlorine and Bromine (group 17 halogens). To keep each in its correct family, heavier Tellurium was placed BEFORE lighter Iodine.
Cobalt (58.93) is very slightly heavier than Nickel (58.69). Strictly by weight the difference is tiny and Ni is lighter, but Cobalt was placed before Nickel to match their chemical grouping. This is the third anomalous pair. It shows even small weight orders had to be adjusted to fit properties.
Atomic weight depends on the average mass of isotopes, which does not always increase in a perfectly smooth order. Sometimes a lighter atomic number element has a higher atomic weight because of its isotopes. The TRUE order should follow atomic number (number of protons). When Moseley showed atomic number is the real basis, all three anomalies vanished - K(19) after Ar(18), I(53) after Te(52), Ni(28) after Co(27) all became correct.
Moseley used X-ray experiments and found that atomic NUMBER (protons), not atomic weight, is the fundamental property. When elements are arranged by atomic number, Ar(18) before K(19), Te(52) before I(53), and Co(27) before Ni(28) all fall in the right place naturally. So the Modern Periodic Law is based on atomic number, and the anomalies disappear.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
There are three well-known anomalous pairs: Argon-Potassium (Ar-K), Tellurium-Iodine (Te-I), and Cobalt-Nickel (Co-Ni). In each one a heavier element is placed before a lighter one.
No. The modern periodic table is based on atomic number, not atomic weight. By atomic number the order (Ar 18, K 19; Te 52, I 53; Co 27, Ni 28) is naturally correct, so there are no anomalies.
They are counted as a defect of Mendeleev's table because they violated his own periodic law (arrangement by atomic weight). This defect was one of the clues that led to using atomic number instead.
Yes. NEET often asks about the defects of Mendeleev's periodic table and why atomic number replaced atomic weight. Knowing the three anomalous pairs and their reason is a quick, high-value fact.
