Which of the following gives the correct increasing order of the atomic radii of $O$,$F$ and $N$?

The element with atomic number $14$ is hard and forms acidic oxide and a covalent halide. To which of the following categories does the element belong?

The order of decreasing atomic size is:

The electrons in the atoms of four elements $A, B, C$ and $D$ are distributed in three shells having $1, 3, 5$ and $7$ electrons in the outermost shell respectively. State the period in which these elements can be placed in the modern periodic table. Write the electronic configuration of the atoms of $A$ and $D$ and the molecular formula of the compound formed when $A$ and $D$ combine.

When Mendeleev started his work,the number of known elements were:

What type of oxide would Eka-aluminium form?