List any two properties of the elements belonging to the first group of the modern periodic table.

It was found that the law of octaves was applicable only up to:

The elements $A$,$B$,$C$,$D$,and $E$ have atomic numbers $9$,$11$,$17$,$12$,and $13$ respectively. Which pair of elements belongs to the same group?

List the anomalies of Mendeleev's periodic table which were resolved by the modern periodic law.

$D$öbereiner's system of classification into triads was not found to be useful as he could identify only:

In Mendeleev's Periodic Table,the elements were arranged in the increasing order of their atomic masses. However,cobalt with an atomic mass of $58.93 \ amu$ was placed before nickel having an atomic mass of $58.71 \ amu$. Give the reason for the same.