How many vertical columns are there in the modern periodic table and what are they called?

Can the following groups of elements be classified as Dobereiner's triad?
$(a)$ $Na, Si, Cl$
$(b)$ $Be, Mg, Ca$
Atomic mass of $Be = 9, Na = 23, Mg = 24, Si = 28, Cl = 35, Ca = 40$. Explain by giving reasons.

Give an account of the process adopted by Mendeleev for the classification of elements. How did he arrive at Periodic Law?

Which of the following statements about the Modern Periodic Table is correct?

State whether the following statements are true or false: In the Modern Periodic Table,a zig-zag line separates metals from non-metals. The borderline elements—boron,silicon,germanium,arsenic,antimony,tellurium,and polonium—are intermediate in properties and are called metalloids or semimetals.

$D$öbereiner's system of classification into triads was not found to be useful as he could identify only: