Name the following:
$(a)$ $A$ metal that can be cut with a knife.
$(b)$ $A$ metal that is liquid at room temperature.
$(c)$ $A$ metal that has $3$ valence electrons.

Study the following table in which positions of six elements $A, B, C, D, E$ and $F$ are shown as they are in the modern periodic table:

Group $\to$	$1$	$2$	$13$	$14$	$15$	$16$	$17$	$18$
Period $2$	$A$	-	-	-	$B$	-	-	$C$
Period $3$	-	-	$D$	$E$	-	-	-	$F$

On the basis of the above table,answer the following questions:
$(i)$ Name the element which forms only covalent compounds.
$(ii)$ Name the element which is a metal with valency three.
$(iii)$ Name the element which is a non-metal with valency three.
$(iv)$ Out of $D$ and $E$,which is bigger in size and why?
$(v)$ Write the common name for the family to which the elements $C$ and $F$ belong.

Why are lithium (atomic number $3$) and potassium (atomic number $19$) placed in group $1$? What will be the atomic numbers of the first two elements in group $2$?

"Hydrogen occupies a unique position in the Modern Periodic Table". Justify the statement.

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

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