In Mendeleev's Periodic Table,gaps were left for elements to be discovered later. Which of the following elements found a place in the periodic table later?

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

The order of decreasing atomic size is:

An element is placed in $2^{nd}$ Group and $3^{rd}$ Period of the Periodic Table,burns in the presence of oxygen to form a basic oxide.
$(a)$ Identify the element.
$(b)$ Write the electronic configuration.
$(c)$ Write the balanced chemical equation when it burns in the presence of air.

$(a)$ Amongst the following elements,identify the ones that would form anions:
$K, O, Na, F, Ca, Cl, Mg$
$(b)$ Write the electronic configuration of the anions identified above.

Compare the radii of two species $X$ and $Y$. Give reasons for your answer.
$(a)$ $X$ has $12$ protons and $12$ electrons.
$(b)$ $Y$ has $12$ protons and $10$ electrons.