An element $A$ has an atomic number $16$. To which group and period does it belong?

$(a)$ What is the basis of arranging the elements in the Modern Periodic Table?
$(b)$ How does this remove the problem of positioning of isotopes?

Which of the following elements would form anions? Why?
$(i)$ $Na$ $(ii)$ $O$ $(iii)$ $Mg$ $(iv)$ $Al$ $(v)$ $Cl$

Write the formula of the product formed when the element $A$ (atomic number $19$) combines with the element $B$ (atomic number $17$). Draw its electronic dot structure. What is the nature of the bond formed?

Where would you locate the element with electronic configuration $2, 8$ in the Modern Periodic Table?

Can the following groups of elements be classified as Dobereiner's triad?
$(a)$ $Na, Si, Cl$
$(b)$ $Be, Mg, Ca$
Atomic mass of $Be = 9, Na = 23, Mg = 24, Si = 28, Cl = 35, Ca = 40$. Explain by giving reasons.