[3 Mark Questions]

Question 13 Marks

Did Dobereiner’s triads also exist in the columns of Newlands’ octaves? Compare and find out.

Answer

Dobereiner’s triads also exist in the columns of Newlands’ octaves.
Lithium, sodium and potassium forms a Dobereiner’s triad.
Lithium, the first element of this triad is considered as the first element as Newlands’ octave, then the eighth element from it is sodium.
These elements possesses similar properties according to the law of triads and the law of octaves.
Similarly, if sodium is considered as the first element then the eighth element from it is potassium.
Moreover, sodium and potassium possess similar properties according to both the laws.
Apart from these, some other elements beryllium $(Be)$, magnesium $(Mg)$ and calcium $(Ca)$ obeys law of triads and the law of octaves.
Thus, Dobereiner’s triads also exist in the columns of Newlands’ octaves.

Question 23 Marks

Compare and contrast the arrangement of elements in mendeleev’s periodic table and the modern periodic table.

Answer

Mendeleev’s periodic table		Modern periodic table
$1$.	Mendeleev’s periodic table consists of seven periods and eight groups.	$1$.	Modern periodic table consists of seven periods and eighteen groups.
$2$.	Transition elements are not separated in the Mendeleev's periodic table.	$2$.	Transition elements are placed in a separate groups in the modern periodic table.
$૩$.	In Mendeleev’s periodic table, elements are arranged in increasing order of their atomic masses.	$3$.	In the modern periodic table, elements are arranged in increasing order of their atomic numbers.
$4$.	Period number and group number of an element cannot be predicted.	$4$.	Period number and group number of an element can be determined easily.
$5$.	Mendeleev’s periodic table has descripancies and limitations.	$5$.	Modern periodic table is almost errorless.
$6$.	Periodicity in the properties of elements cannot be explained.	$6$.	Periodicity in the properties of elements can be explained.

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Question 33 Marks

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

Answer

In the modern periodic table, elements having same number of electrons in the valence shell show similar chemical properties.
Magnesium has two electrons in the valence shell, hence all the elements such as beryllium $(Be)$, calcium $(Ca)$ and strontium $(Sr)$ having two electrons in the valence shell show similar chemical properties.

Elements of group $2$
Element	Atomic number	Electronic configuration
		K L M N O
Beryllium $(Be)$	$4$	$2 2$
Magnesium $(Mg)$	$12$	$2 8 2$
Calcium $(Ca)$	$20$	$2 8 8 2$
Strontium $(Sr)$	$38$	$2 8 18 8 2$

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Question 43 Marks

Write the limitations of Mendeleev’s classification.

Answer

The limitations of Mendeleev’s classification of elements are as follows:
$(1)$ Position of hydrogen in periodic table:
Electronic configuration of hydrogen resembles with alkali metals.
Like alkali metals, hydrogen combines with halogens, oxygen and sulphur to form compounds having similar formulae.
For example,

Table

Compounds of hydrogen	Compounds of sodium
$HCl$	$NaCl$
$H_2O$	$Na_2O$
$H_2S$	$Na_2S$

Hence, many properties of hydrogen are similar to those of alkali metals; so it should be placed along with alkali metals of group $IA$.
Just like halogens $(F_2, Cl_2, Br_2, I_2),$ hydrogen also exists as a diatomic molecule $(H_2).$
Hydrogen combines with metals and non-metals to form covalent compounds.

Table

Element	Ionic compounds with metals	Covalent compounds with non-metals
Hydrogen	$Na^+H^-$ (Sodium hydride) $Ca^{2+}(H^-)_2$ (Calcium hydrine)	$CH_4$ (Methane) $NH_3$ (Ammonia) $H_2O$ (Water)
Chlorine (Halogen)	$Na^+Cl^-$ (Sodium hydrine) $Ca^{2+}$ $(Cl^-)_2$ (Calcium chloride)	$CCl_4$ (Carbon tetrachloride) $NCl_3$ (Nitrogen trichloride)

Therefore, many properties of hydrogen are similar to those of halogens, hence it can be placed in halogen group.
In short, position of hydrogen in the periodic table is a matter of controversy.
$(2)$ Position of isotopes : [2 Marks]
Isotopes were discovered long after Mendeleev gave his periodic classification of elements.
Isotopes: Atoms of the same element having same atomic number but different atomic masses are known as isotopes.
Isotopes of any element have similar chemical properties, but different atomic masses.
Thus, isotopes of all elements posed a challenge to Mendeleev’s periodic table.
Therefore, the placing of isotopes in distinguishable positions in the periodic table become almost impossible.
$(3)$ Irregularities of atomic masses of elements:
Atomic masses do not increase in a regular manner in moving from one element to next element.
So it was difficult to predict that how many element would be discovered between two elements.

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Question 53 Marks

Explain the irregularities (anomalies) of Mendeleev’s periodic table.

Answer

In Mendeleev’s periodic table, the elements are arranged in the increasing order of atomic masses.
But there are few instances where Mendeleev had to place an element with a slightly higher atomic mass before an element with a slightly lower atomic mass.
Hence, the sequence was inverted so that elements with similar properties could be grouped together.
For example, cobalt $(Co)$ (atomic mass $58.9 \ u$) placed before nickel $(Ni)$ (atomic mass $58.7 \ u$).
Similarly, tellurium $(Te)$ with higher atomic mass has been placed before iodine $(I)$ with lower atomic mass.
Mendeleev left some gaps in his periodic table for those elements which were not discovered at that time.
Instead of considering gaps as defects, Mendeleev boldly predicted the existence of some elements which would be discovered in due course of time and would fill the gaps.
Mendeleev named these elements by prefixing a Sanskrit word eka (one) to the name of preceding element in the same group.
For example,

Elements discovered after classification	Suggested name by Mendeleev
Scandium $(Sc)$	Eka-boron
Gallium $(Ga)$	Eka-aluminium
Germanium $(Ge)$	Eka-silicon

The properties of eka-aluminium predicted by Mendeleev and the element gallium which was discovered later are listed in Table.

Table : Properties of eka-aluminium and gallium

Property	Eka-aluminium	Gallium
Atomic Mass	$68$	$69.7$
Formula of oxide	$\mathrm{E}_2 \mathrm{O}_3$	$\mathrm{Ga}_2 \mathrm{O}_3$
Formula of chloride	$\mathrm{ECl}_3$	$\mathrm{GaCl}_3$

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Question 63 Marks

Explain the Newlands’ law of octaves.

Answer

The attempts of Dobereiner encouraged other chemists to correlate the properties of elements with their atomic masses.
In $1866$, John Newlands, an English scientist, arranged the then known elements in the order of increasing atomic masses.
He started with hydrogen, the element having the lowest atomic mass and ended at thorium which was the $56th$ element.
Newlands found that every eighth element had properties similar to the properties of the first element.
Newlands observe this analogy with the octaves found in musical scale and proposed the law of octaves which is given as below:
Law of octaves:
“When elements are arranged in order of their increasing atomic masses, then the properties of every eighth element are similar to the properties of the first element.”
A part of the original form of Newlands’ octaves is given below in Table $2$.

Table : Newlands’ octaves

Serial Number	1	2	3	4	5	6	7
Notes as per Indian music	sa	Re	ga	ma	pa	dha	ni
Notes as per Western music	do	Re	mi	fa	so	la	ti
Element Atomic mass Mass upto $40 \ u$	H F Cl	Li Na K	Be Mg Ca	B Al	C Si	N P	O S
Element Atomic mass Mass greater than $40 \ u$	Co and Ni Br	Cu Rb	Zn Sr	Cr Y Ce, La	Ti In Zr	Mn As -	Fe Se

In Newlands’ octaves, the properties of lithium and sodium and sodium are same, Sodium is the eighth element after lithium.
Similarly, beryllium and magnesium possess similar properties.

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Question 73 Marks

Some groups of three elements are given below:

Group $A$ element	Atomic mass	Group $B$ element	Atomic mass	Group $C$ element	Atomic mass
$N$	$14.0$	$Ca$	$40.1$	$Cl$	$35.5$
$P$	$31.0$	$Sr$	$87.6$	$Br$	$79.9$
$As$	$74.9$	$Ba$	$137.3$	$I$	$126.9$

Which of these groups is not an example of Dobereiner’s triad? Explain it.

Answer

For group $'A'$:
Average of atomic masses of $N$ and $As$
$=\frac{14+74.9}{2}=44.45 u$
This average value is not equal to the actual atomic mass of the middle element phosphorus ($P$).
Therefore, these three elements do not form a Dobereiner's triad.
For group ' $B$ ':
Average of atomic masses of $Ca$ and $Ba$
$=\frac{40.1+137.3}{2}$
$=88.7 u$
$=88.7 u$
This average value is approximately equal to the atomic mass of the middle element strontium ( $Sr$ ). Therefore, these three elements form a Dobereiner's triad.
For group ' $C$ ':
Average of atomic masses of $Cl$ and $l$
$=\frac{35.5+126.9}{2}$
$=81.2 u$
This average value is approximately equal to the atomic mass of the middle element bromine ( $Br$ ). Therefore, these three elements form a Dobereiner's triad.

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Question 83 Marks

What is meant by Dobereiner’s triads? Explain.

Answer

Johann Wolfgang Dobereiner, a German chemist tried to arrange the elements with similar properties into groups.
He identified some groups having three elements each.
These groups of three elements were called triads.
Law of triads:
When the three elements in a triad were arranged in the order of increasing atomic masses, the atomic mass of the middle element is roughly the average of the atomic masses of the other two elements.
For example, lithium $(Li)$, sodium $(Na)$ and potassium $(K)$ constitute a triad.
The atomic masses of $Li,$ Na and $K$ are $6.9 u$, $23.0 u$ and $39.0 u$ respectively.
The atomic mass of middle element, sodium $(Na)$ is roughly the average of the atomic masses of lithium $(Li)$ and potassium $(K).$

Elements	Symbol	Atomic mass
Lithium	$Li$	$6.9 \ u$
Sodium	$Na$	$23.0 \ u$
Potassium	$K$	$39.0 \ u$

Average of atomic masses of
$Li$ and $K =\frac{6.9+39}{2}=22.95 \cong 23.0 u$
Dobereiner's triads are as follows:

Table 1: Dobereiner’s triads

$Li$	$Ca$	$Cl$
$Na$	$Sr$	$Br$
$K$	$Ba$	$I$

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