Besides gallium, which other elements have since been discovered that were left by Mendeleev in his Periodic Table? (any two)

Which element has

$(a)$ two shells, both of which are completely filled with electrons?

$(b)$ the electronic configuration $2,\,8,\,2 ?$

Lithium, sodium, potassium are all metals that react with water to liberate hydrogen gas. Is there any similarity in the atoms of these elements ?

Which of the following statements is not a correct statement about the trends when going from left to right across the periods of periodic Table.

Use Mendeléev’s Periodic Table to predict the formulae for the oxides of the following elements :

$K,\, C,\, Al,\, Si,\, Ba$.

The position of three elements $A$, $B$ and $C$ in the Periodic Table are shown below

$ (a)$ State whether $A$ is a metal or non-metal.

$(b)$ State whether $C$ is more reactive or less reactive than $A$.