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?

$(a)$ State Modern Periodic Law.
$(b)$ Elements $A, B, C$ and $D$ have atomic numbers $1, 8, 11$ and $19$ respectively. Choose the odd element and give reason for your answer.

An element $A$ has an atomic number of $17$. To which group and period does it belong?

Mendeleev predicted the existence of certain elements not known at that time and named two of them as Eka-silicon and Eka-aluminium.
$(a)$ Classify these elements as metals,non-metals,or metalloids.
$(b)$ How many valence electrons are present in each one of them?

If an element $X$ is placed in group $14$,what will be the formula and the nature of bonding of its chloride?

$(a)$ How does electropositivity of elements get affected as we move :
$(i)$ down a group
$(ii)$ across a period?
$(b)$ Which atomic property increases both as we move across a period or down a group?