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

Arrange the following elements in the order of their decreasing metallic character: $Na, Si, Cl, Mg, Al$.

Up to which element was the Law of Octaves found to be applicable?

An element $X$ has a mass number of $35$ and the number of neutrons is $18$. Determine the atomic number and electronic configuration of $X$. Also, identify the group number, period number, and valency of $X$.

$(i)$ The elements of the second period along with their atomic numbers in parentheses are given below: $B(5)$,$Be(4)$,$O(8)$,$N(7)$,$Li(3)$,$C(6)$,$F(9)$.
$(a)$ Arrange them in the same order as they appear in the periodic table.
$(b)$ Which element has the largest and smallest atom?
$(ii)$ Why does the atomic radius change as we move from left to right in a period?

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.