Mendeleev predicted the existence of certain elements not known at that time and named two of them as Eka-silicon and Eka-aluminium.
$(a)$ Name the elements which have taken the place of these elements.
$(b)$ Mention the group and the period of these elements in the Modern Periodic Table.

In Mendeleev's Periodic Table,the elements were arranged in the increasing order of their atomic masses. However,cobalt with an atomic mass of $58.93 \ amu$ was placed before nickel having an atomic mass of $58.71 \ amu$. Give the reason for the same.

The atomic radii of the elements of the second period are given below:

Second period elements	$B$	$Be$	$O$	$N$	$Li$	$F$	$C$
Atomic Radii $(pm)$	$88$	$111$	$66$	$74$	$152$	$64$	$77$

$(i)$ Arrange these elements in decreasing order of their atomic radii.
$(ii)$ Are the elements now arranged in the pattern of a period in the periodic table?
$(iii)$ Name the element which has the $(a)$ smallest and $(b)$ largest atomic number.
$(iv)$ From the above data, infer how the atomic size or atomic radius of the elements changes as we move from left to right in a period.
$(v)$ Name one metal, one non-metal, and a metalloid out of these elements.
$(vi)$ Why does atomic radius decrease as we move from left to right in a period?

Which of the following statements is not correct regarding the trends observed when moving from left to right across the periods of the Periodic Table?

The element with atomic number $14$ is hard and forms acidic oxide and a covalent halide. To which of the following categories does the element belong?

The electronic configurations of $4$ elements $A, B, C,$ and $D$ are as follows:
$A: 2, 1$
$B: 2, 2$
$C: 2, 8, 2$
$D: 2$
$(a)$ Which of these elements belong to the same group? Name them.
$(b)$ Which of these elements belong to the same period? Name them.
$(c)$ Which of these elements is inert? Name it.