Arrange the following elements in increasing order of their atomic radii.
$(a)$ $Li, Be, F, N$
$(b)$ $Cl, At, Br, I$

$(a)$ Identify the element that has two completely filled shells and the number of valence electrons in each are as: $(i)$ $1$,$(ii)$ $2$,$(iii)$ $7$,$(iv)$ $8$. $(b)$ Analyse which amongst them is inert.

Which of the following are the characteristics of isotopes of an element?
$(i)$ Isotopes of an element have same atomic masses
$(ii)$ Isotopes of an element have same atomic number
$(iii)$ Isotopes of an element show same physical properties
$(iv)$ Isotopes of an element show same chemical properties

Why does atomic size decrease as we move from left to right along a period in the periodic table?

Describe the basic character of the oxides of third period elements across the period from left to right.

It was found that the law of octaves was applicable only up to: