In Mendeleev's periodic table some pairs of elements do not follow increasing order of atomic weight known as anomolous pair. The correct example of anomolous pair is:
Correct answer: B. Th, Pa
- A. Cl, Ar
- B. Th, Pa
- C. Te, In
- D. Co, Nb
Explanation
In Mendeleev's periodic table, Th (thorium) and Pa (protactinium) form an anomalous pair because they do not adhere to the order of increasing atomic weight. Mendeleev sometimes placed elements out of order based on atomic weight to maintain the periodicity of their chemical properties. This approach highlights Mendeleev's emphasis on chemical behavior over strict adherence to atomic weight. Other pairs, such as Cl and Ar, follow modern periodic trends based on atomic numbers. Similarly, Te and In, as well as Co and Nb, do not form anomalous pairs in Mendeleev's table since they were not repositioned for maintaining periodic trends.
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About Periodic Properties and Trends
Periodic properties arise from electron configuration and effective nuclear charge, producing trends in atomic and ionic radius, ionization energy, electron affinity, electronegativity, metallic character and reactivity. Comparisons run across periods and down groups, with attention to common exceptions. The topic also relates these trends to the behavior of s-block and p-block elements.
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