Arrange the following metals in decreasing order of their atomic size: $Ca, Mg, Ba, Be$.

$(a)$ Electropositive nature of the element$(s)$ increases down the group and decreases across the period.
$(b)$ Electronegativity of the element decreases down the group and increases across the period.
$(c)$ Atomic size increases down the group and decreases across a period (left to right).
$(d)$ Metallic character increases down the group and decreases across a period.
On the basis of the above trends of the Periodic Table,answer the following about the elements with atomic numbers $3$ to $9$:
$(a)$ Name the most electropositive element among them.
$(b)$ Name the most electronegative element.
$(c)$ Name the element with the smallest atomic size.
$(d)$ Name the element which is a metalloid.
$(e)$ Name the element which shows maximum valency.

Which of the given elements $A, B, C, D$ and $E$ with atomic numbers $2, 3, 7, 10$ and $30$ respectively belong to the same period?

How many vertical columns are there in the modern periodic table and what are they called?

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 the major air pollutants.
$(a)$ Identify the element $X$
$(b)$ Write the electronic configuration of $X$
$(c)$ Write the balanced chemical equation for the thermal decomposition of ferrous sulphate crystals?

The three elements $A$,$B$,and $C$ with similar properties have atomic masses $X$,$Y$,and $Z$ respectively. The mass of $Y$ is approximately equal to the average mass of $X$ and $Z$. What is such an arrangement of elements called? Give one example of such a set of elements.