Arrange the following elements in the order of their decreasing metallic character:
$Na, Si, Cl, Mg, Al$

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?

An element $X$,which is a yellow solid at room temperature,shows catenation and allotropy. $X$ forms two oxides which are also formed during the thermal decomposition of ferrous sulphate crystals and are major air pollutants.
$(a)$ What would be the nature (acidic/basic) of the oxides formed?
$(b)$ Locate the position of the element in the Modern Periodic Table.

$Li, Be, B$ and $C$ are the elements of the same period of the Modern Periodic Table.
$(a)$ Arrange them in increasing order of their atomic size.
$(b)$ In which shell (number) would the last electron enter for all of them?
$(c)$ Calculate the number of valence electrons in each.
$(d)$ Which element amongst them is most electropositive?

Mention any two trends exhibited by elements when we go from left to right across the period of the periodic table.

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.