Dobereiner's Triads and Newlands' Law of Octaves Questions in English

(N/A) Only one triad of Dobereiner's triads exists in the columns of Newlands' octaves. The triad formed by the elements $Li$, $Na$, and $K$ of Dobereiner's triads also occurred in the columns of Newlands' octaves.
Dobereiner's triads:
$\begin{matrix} Li & Ca & Cl \\ Na & Sr & Br \\ K & Ba & I \end{matrix}$
Newlands' octaves:

$H$	$Li$	$Be$	$B$	$C$	$N$	$O$
$F$	$Na$	$Mg$	$Al$	$Si$	$P$	$S$
$Cl$	$K$	$Ca$	$Cr$	$Ti$	$Mn$	$Fe$
$Co$ and $Ni$	$Cu$	$Zn$	$Y$	$In$	$As$	$Se$
$Br$	$Rb$	$Sr$	$Ce$ and $La$	$Zr$	-	-

(A) The main limitation of Dobereiner's classification was that he could not arrange all the elements known at that time into groups of triads based on their chemical properties.
Only a limited number of elements could be grouped into triads,making the system insufficient for the classification of all elements.

(D) Limitations of Newlands' Law of Octaves:
$(i)$ It was not applicable throughout the entire arrangement. It was applicable only up to calcium. The properties of the elements listed after calcium did not show resemblance to the properties of the elements above them.
$(ii)$ Elements discovered after Newlands' time did not follow the Law of Octaves.
$(iii)$ The position of cobalt and nickel in the same group as fluorine $(F)$ and chlorine $(Cl)$,which have different properties,could not be explained.
$(iv)$ The placement of iron far away from cobalt and nickel,despite iron having similar properties to them,could not be explained.

(A) John Newlands,an English scientist,arranged the then-known elements in the order of increasing atomic masses and found that every eighth element had properties similar to the first.
This was known as the Law of Octaves.
However,this law was found to be applicable only up to Calcium $(Ca)$,which is the $20^{th}$ element in the periodic table.
After Calcium,every eighth element did not possess properties similar to the first.

(N/A) The arrangement of these elements is known as a $Dobereiner$ triad.
According to $Dobereiner's$ law of triads,when elements are arranged in groups of three in the order of increasing atomic masses,the atomic mass of the middle element is approximately the arithmetic mean of the atomic masses of the other two elements.
An example of such a set is the triad of Lithium $(Li)$,Sodium $(Na)$,and Potassium $(K)$.
Atomic mass of $Li = 6.9$,Atomic mass of $K = 39.1$.
Average mass = $(6.9 + 39.1) / 2 = 46 / 2 = 23$,which is the atomic mass of Sodium $(Na)$.

(A) The two sets of elements with similar properties are:
$(i)$ $F$ and $Cl$ (both are halogens).
$(ii)$ $Na$ and $K$ (both are alkali metals).
$(b)$ The given sequence represents Newland's Law of Octaves,which states that when elements are arranged in increasing order of their atomic masses,the properties of every eighth element are a repetition of the properties of the first element.

(B) No,because these elements do not possess similar chemical properties,which is a fundamental requirement for Dobereiner's triads,even though the atomic mass of silicon $(28)$ is approximately the average of the atomic masses of sodium $(23)$ and chlorine $(35)$,i.e.,$(23 + 35) / 2 = 29$.
$(b)$ Yes,because these elements belong to the same group (alkaline earth metals) and exhibit similar chemical properties. Furthermore,the atomic mass of magnesium $(24)$ is the average of the atomic masses of beryllium $(9)$ and calcium $(40)$,i.e.,$(9 + 40) / 2 = 24.5$,which is very close to $24$.

(N/A) According to Newlands' Law of Octaves,certain elements were placed in the same slot to fit the periodic arrangement.
The two pairs of elements placed in the same slot are:
$(i)$ Cobalt $(Co)$ and Nickel $(Ni)$
$(ii)$ Cerium $(Ce)$ and Lanthanum $(La)$

(N/A) Newlands' law of Octaves states that when elements are arranged in the order of increasing atomic masses,the properties of every eighth element are a repetition of the properties of the first element. This pattern is analogous to the musical notes in an octave (sa,re,ga,ma,pa,dha,ni,sa),where the eighth note is the same as the first.

(A) John Newlands,an English chemist,proposed the Law of Octaves in $1864$.
He arranged the elements known at that time in increasing order of their atomic masses.
Newlands assumed that only $56$ elements existed in nature and that no more elements would be discovered in the future.
Therefore,the correct option is $A$.

(D) Johann Wolfgang $D$öbereiner attempted to arrange elements with similar properties into groups of three,known as triads.
In his system,the atomic mass of the middle element was approximately the arithmetic mean of the atomic masses of the other two elements.
However,this system was not widely applicable because $D$öbereiner could identify only $5$ such triads from the elements known at that time.
Therefore,the correct option is $D$.

(C) John Newlands proposed the Law of Octaves in $1866$.
He arranged the elements in increasing order of their atomic masses and observed that every eighth element had properties similar to the first element.
This law was found to be applicable only up to $Calcium$ $(Ca)$, which is the $20^{th}$ element in the periodic table.
Beyond $Calcium$, the properties of the eighth element did not correspond to the first element.

(FALSE) The statement is False. John Newlands assumed that only $56$ elements existed in nature and that no more elements would be discovered in the future. The number $63$ is associated with the number of elements known at the time of Mendeleev's Periodic Table.

(B) The statement is False. In Newlands' Law of Octaves,he adjusted two elements in the same slot (e.g.,cobalt and nickel) to fit elements into his table,but he did not put three elements in the same slot. Furthermore,he did place some unlike elements in the same slot,but the claim regarding 'three elements' is incorrect.