In the Modern Periodic Table, calcium (atomic number $20$) is surrounded by elements with atomic numbers $12$, $19$, $21$ and $38$. Which of these have physical and chemical properties resembling calcium?

Name two elements you would expect to show chemical reactions similar to magnesium. What is the basis for your choice ?

What were the limitations of Dobereiner’s classification ?

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

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

Element $X$ forms a chloride with the formula $XCl_2$, which is a solid with a high melting point. $X$ would most likely be in the same group of the Periodic Table as

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 $A$.