Mention any two trends exhibited by elements when we go from left to right across the period 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?

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?

State whether the following statement is true or false: $Na, Mg, Li, Al, Si, F, P$ and $Cl$ belong to the third period of the Modern periodic table.

When Mendeleev started his work,the number of known elements were:

Which of the following statement$(s)$ about the Modern Periodic Table are incorrect?
$(i)$ The elements in the Modern Periodic Table are arranged on the basis of their decreasing atomic number.
$(ii)$ The elements in the Modern Periodic Table are arranged on the basis of their increasing atomic masses.
$(iii)$ Isotopes are placed in adjoining group$(s)$ in the Periodic Table.
$(iv)$ The elements in the Modern Periodic Table are arranged on the basis of their increasing atomic number.