The atomic numbers of three elements $A, B$,and $C$ are $12, 18$,and $20$ respectively. State,giving reason,which two elements will show similar properties.

Two elements $M$ and $N$ belong to group $I$ and $II$ respectively and are in the same period of the periodic table. How do the following properties of $M$ and $N$ vary:
$(a)$ sizes of their atoms?
$(b)$ their metallic characters?
$(c)$ their valencies in forming oxides?
$(d)$ formulae of their chlorides?

On moving from left to right in a period in the periodic table,the size of the atom.

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.

An element belongs to the third period and second group of the periodic table:
$(a)$ State the number of valence electrons in it.
$(b)$ Is it a metal or a non-metal?
$(c)$ Name the element.
$(d)$ Write the formula of its oxide.

Justify the following statements:
$(a)$ Cations are smaller in size than the corresponding neutral atom.
$(b)$ The size of the atom increases as we move down a group.
$(c)$ Atomic size decreases as we move across a period.