Arrange the following elements in increasing order of their atomic radii.
$(a)$ $Li, Be, F, N$
$(b)$ $Cl, At, Br, I$

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

An element $X$ belongs to the $3rd$ period and group $16$ of the Modern Periodic Table.
$(a)$ Determine the number of valence electrons and the valency of $X$.
$(b)$ Write the molecular formula of the compound formed when $X$ reacts with hydrogen and draw its electron dot structure.
$(c)$ Name the element $X$ and state whether it is metallic or non-metallic.

An element is placed in $2^{nd}$ Group and $3^{rd}$ Period of the Periodic Table,burns in the presence of oxygen to form a basic oxide.
$(a)$ Identify the element.
$(b)$ Write the electronic configuration.
$(c)$ Write the balanced chemical equation when it burns in the presence of air.

Three elements $B$,$Si$ and $Ge$ are

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.