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

The atomic number of an element is $17$. Predict $(a)$ its valency,$(b)$ whether it is a metal or non-metal,$(c)$ its relative size with respect to other members of its group. Justify your answer in each case.

What property do all elements in the same column of the Periodic Table as boron have in common?

Arrange the following elements in the increasing order of their metallic character:
$Mg, Ca, K, Ge, Ga$

Write true or false for the statement: Metallic character decreases across a period and increases down a group because the effective nuclear charge acting on the valence shell electrons increases across a period and decreases down the group.

Three elements $X$,$Y$,and $Z$ have atomic numbers $7$,$8$,and $9$ respectively.
$(a)$ State their positions (Group number and period number both) in the Modern Periodic Table.
$(b)$ Arrange these elements in the decreasing order of their atomic radii.
$(c)$ Write the formula of the compound formed when $X$ combines with $Z$.