An element $\mathrm{X}$ belongs to group $2$ and another element $\mathrm{Y}$ belongs to group $15$ of the periodic table. What is the number of valence electron in $\mathrm{Y}$? Explain how you have arrived at your answers.

An element $\mathrm{X}$ belongs to group $2$ and another element $\mathrm{Y}$ belongs to group $15$ of the periodic table. What is the valency of $\mathrm{Y}$? Explain how you have arrived at your answers.

How do the following change on going from left to right in a period of the periodic table?.

i. Chemical reactivity of elements.

ii. Nature of oxides of elements.

Explain why all the elements of a group have similar chemical properties

Explain why all the elements of a period have different chemical properties.

The atomic radii of three elements $\mathrm{X}, \mathrm{Y}$ and $\mathrm{Z}$ of a period of the periodic table are $186 \mathrm{pm}, 104 \mathrm{pm}$ and $143 \mathrm{pm}$ respectively. Giving a reason, arrange these elements in the increasing order of atomic numbers in the period.