Question

What are Anomalous pairs of elements?

Answer 1

Anomalous pairs

• Elements are organized in Mendeleev's periodic table in ascending order of their atomic weights. However, only a few elements are organized in reverse order. They are known as anomalous pairs.

Example

Argon and Potassium, Tellurium and Iodine, Cobalt and Nickel are the elements, which Mendeleev arranged in order of increasing atomic mass, not increasing property similarity.

$\mathrm{Ar}(\mathrm{mass}=39.9)$ comes before $\mathrm{K}(\mathrm{mass}=39.1)$.

• According to Mendeleev's periodic law $\mathrm{Ar}$ should come after $\mathrm{K}$ in Mendeleev's periodic table as the mass of $\mathrm{Ar}$ is more than that of $\mathrm{K}$.
• But $\mathrm{K}$ is placed before $\mathrm{Ar}$ and hence showed anomalous behaviour.

$\mathrm{Co}(\mathrm{mass}=58.9)$ comes before $\mathrm{Ni}(\mathrm{mass}=58.6)$.

• According to Mendeleev's periodic law $\mathrm{Co}$ should come after $\mathrm{Ni}$ in Mendeleev's periodic table as the mass of $\mathrm{Co}$ is more than that of $\mathrm{Ni}$.
• But $\mathrm{Ni}$ is placed before $\mathrm{Co}$ and hence showed anomalous behaviour.

$\mathrm{Te}(\mathrm{mass}=127.6)$ comes before $\mathrm{I}(\mathrm{mass}=126.9)$.

• According to Mendeleev's periodic law $\mathrm{Te}$ should come after $\mathrm{I}$ in Mendeleev's periodic table as the mass of $\mathrm{Te}$ is more than that of $\mathrm{I}$.
• But $\mathrm{I}$ is placed before $\mathrm{Te}$ and hence showed anomalous behaviour.