State whether the following statement is true or false: $Na, Mg, Li, Al, Si, F, P$ and $Cl$ belong to the third period of the Modern periodic table.

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.

Arrange the following elements in the order of their increasing non-metallic character: $Li, O, C, Be, F$.

Find the atomic number of the element whose electronic configuration is $2, 8, 5$.

Identify the elements with the following properties and arrange them in increasing order of their reactivity:
$(a)$ An element which is a soft and reactive metal.
$(b)$ The metal which is an important constituent of limestone.
$(c)$ The metal which exists in a liquid state at room temperature.

$(a)$ State the main characteristics of elements on which the modern periodic table is based.
$(b)$ No fixed position can be assigned to hydrogen in the periodic table. Why?