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.

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

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.

What is the position of metalloids in the periodic table?

Two elements $P$ and $Q$ belong to the same period of the modern periodic table and are in Group $1$ and Group $2$ respectively. Compare their following characteristics in tabular form:
$(a)$ The number of electrons in their atoms
$(b)$ The sizes of their atoms
$(c)$ Their metallic characters
$(d)$ Their tendencies to lose electrons
$(e)$ The formula of their oxides
$(f)$ The formula of their chlorides

The atomic numbers of a few elements are given below: $10, 20, 7, 14$.
$(a)$ Identify the elements.
$(b)$ Identify the group number of these elements in the Periodic Table.
$(c)$ Identify the periods of these elements in the Periodic Table.