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 Page 1


16
1. What are the types of matter?  
2. What are the types of elements?
3. What are the smallest particles of matter called?
4. What is the difference between the molecules of elements and   
compounds?
2.Periodic Classification of Elements
Ø Elements and their classification    Ø Dobereiner’s Triads
Ø Newlands Law of Octaves         Ø Mendeleev’s Periodic Table
Ø Modern Periodic Table
Classification of elements
 We have learnt in the previous standards that all the atoms of an element are of only 
one type. Today 118 elements are known to the scientific world. However, around year 
1800 only about 30 elements were known. More number of elements were discovered in 
the course of time. More and more information about the properties of these elements was 
gathered. To ease the study of such a large number of elements, scientists started studying 
the pattern if any, in the vast information about them. You know that in the initial 
classification elements were classified into the groups of metals and nonmetals. Later  on 
another class of elements called metalloids was noticed. As the knowledge about elements 
and their properties went on increasing different scientists started trying out different 
methods of classification.
Dobereiner’s Triads
 In the year 1817 a German scientist Dobereiner suggested that properties of elements 
are related to their atomic masses. He made groups of three elements each, having similar 
chemical properties and called them triads. He arranged the three elements in a triad in an 
increasing order of atomic mass and showed that the atomic mass of the middle element 
was approximately equal to the mean of the atomic masses of the other two elements. 
However, all the known elements could not be classified into the Dobereiner’s triads. 
2.1 Dobereiner’s Triads
Identify Dobereiner’s triads from the following groups of 
elements having similar chemical properties.
Can you recall?
Can you tell?
1. Mg (24.3), Ca (40.1), Sr (87.6)        2. S (32.1), Se (79.0), Te (127.6)         
3. Be (9.0), Mg (24.3), Ca (40.1)
Sr. 
No.
Triad Element -1 
Actual atomic 
mass(a)
Element - 2 Element - 3 
Actual atomic
mass (c) 
                
Actual 
atomic mass
1 Li, Na, 
K
 Lithium (Li)
6.9
Sodium (Na)
23.0
Potassium (K)
39.1
2 Ca, Sr, 
Ba
Calcium (Ca)
40.1
Strontium (Sr)
87.6
Barium (Ba)
137.3
3 Cl, Br, I Chlorine (Cl)
35.5
Bromine (Br)
79.9
Iodine (I)
126.9
= 23.0 
6.9 + 39.1
      2
40.1+ 137.3
        2
= 88.7
35.5 + 126.9
         2
= 81.2
 a+c
   2
Mean =
Page 2


16
1. What are the types of matter?  
2. What are the types of elements?
3. What are the smallest particles of matter called?
4. What is the difference between the molecules of elements and   
compounds?
2.Periodic Classification of Elements
Ø Elements and their classification    Ø Dobereiner’s Triads
Ø Newlands Law of Octaves         Ø Mendeleev’s Periodic Table
Ø Modern Periodic Table
Classification of elements
 We have learnt in the previous standards that all the atoms of an element are of only 
one type. Today 118 elements are known to the scientific world. However, around year 
1800 only about 30 elements were known. More number of elements were discovered in 
the course of time. More and more information about the properties of these elements was 
gathered. To ease the study of such a large number of elements, scientists started studying 
the pattern if any, in the vast information about them. You know that in the initial 
classification elements were classified into the groups of metals and nonmetals. Later  on 
another class of elements called metalloids was noticed. As the knowledge about elements 
and their properties went on increasing different scientists started trying out different 
methods of classification.
Dobereiner’s Triads
 In the year 1817 a German scientist Dobereiner suggested that properties of elements 
are related to their atomic masses. He made groups of three elements each, having similar 
chemical properties and called them triads. He arranged the three elements in a triad in an 
increasing order of atomic mass and showed that the atomic mass of the middle element 
was approximately equal to the mean of the atomic masses of the other two elements. 
However, all the known elements could not be classified into the Dobereiner’s triads. 
2.1 Dobereiner’s Triads
Identify Dobereiner’s triads from the following groups of 
elements having similar chemical properties.
Can you recall?
Can you tell?
1. Mg (24.3), Ca (40.1), Sr (87.6)        2. S (32.1), Se (79.0), Te (127.6)         
3. Be (9.0), Mg (24.3), Ca (40.1)
Sr. 
No.
Triad Element -1 
Actual atomic 
mass(a)
Element - 2 Element - 3 
Actual atomic
mass (c) 
                
Actual 
atomic mass
1 Li, Na, 
K
 Lithium (Li)
6.9
Sodium (Na)
23.0
Potassium (K)
39.1
2 Ca, Sr, 
Ba
Calcium (Ca)
40.1
Strontium (Sr)
87.6
Barium (Ba)
137.3
3 Cl, Br, I Chlorine (Cl)
35.5
Bromine (Br)
79.9
Iodine (I)
126.9
= 23.0 
6.9 + 39.1
      2
40.1+ 137.3
        2
= 88.7
35.5 + 126.9
         2
= 81.2
 a+c
   2
Mean =
17
The English scientist John Newlands 
correlated the atomic masses of elements 
to their properties  in a different way. In the 
year 1866 Newlands arranged the elements 
known at that  time in an increasing order 
of their atomic masses. It started with the 
lightest element hydrogen and ended up 
with thorium. He found that every eighth 
element had properties similar to those of 
the first. For example, sodium is the eighth 
element from lithium and both have similar 
properties. Also, magnesium shows 
similarity to beryllium and chlorine shows 
similarity with fluorine. Newlands 
compared this similarity with the octaves 
in music. He called the similarity observed 
in the eighth and the first element as the 
Law of octaves.
Musical 
Note
Do
(Sa)
Re
(Re)
Mi
(Ga)
Fa
(Ma)
So
(Pa)
La
(Dha) 
Ti
(Ni)
Elements
H
F
Cl
Co &Ni
Br
Li
Na
K
Cu
Rb
Be
Mg
Ca
Zn
Sr
B
Al
Cr
Y
Ce & La
C
Si
Ti
In
Zr
N
P
Mn
As
O
S
Fe
Se
2.2 Newlands’ Octaves
Many limitation were found in Newlands’ octaves. This law was found to be applicable 
only up to calcium. Newlands fitted all the known elements in a table of 7 X 8 that is 56 
boxes. Newlands placed two elements each in some boxes to accommodate all the known 
elements in the table. For example, Co and Ni, Ce and La. Moreover, he placed some 
elements with different properties under the same note in the octave. For example, 
Newlands placed the metals Co and Ni under the note ‘Do’ along with halogens, while Fe, 
having similarity with Co and Ni, away from them along with the nonmetals O and S 
under the note ‘Ti’. Also, Newlands’ octaves did not have provision to accommodate the 
newly discovered elements. The properties of the new elements discovered later on did not 
fit in the Newlands’ law of octaves. 
In the Indian music system there 
are seven main notes, namely, Sa, Re, 
Ga, Ma, Pa, Dha, Ni, and their collection 
is called ‘Saptak’. The frequency of the 
notes goes on increasing from ‘Sa’ to 
‘Ni’. Then comes, the ‘Sa’ of the upper 
‘Saptak’ at the double the frequency of 
the original ‘Sa’. It means that notes 
repeat after completion of one ‘Saptak’. 
The seven notes in the western music 
are Do, Re, Mi, Fa, So, La, Ti.
The note ‘Do’ having double the 
original frequency comes again at the 
eighth place. This is the octave of 
western notes. Music is created by the 
variety in the use of these notes.
Newlands’ Law of Octaves
Mendeleev’s Periodic table
The Russian scientist Dmitri Mendeleev developed the periodic table of elements  
during the period 1869 to 1872 A.D. Mendeleev’s periodic table is the most important step 
in the classification of elements. Mendeleev considered the fundamental property of 
elements, namely, the atomic mass, as standard and arranged 63 elements  known at that 
time in an increasing order of their atomic masses. Then he transformed this into the 
periodic table of elements in accordance with the physical and chemical properties of 
these elements. 
Do you know ?
Page 3


16
1. What are the types of matter?  
2. What are the types of elements?
3. What are the smallest particles of matter called?
4. What is the difference between the molecules of elements and   
compounds?
2.Periodic Classification of Elements
Ø Elements and their classification    Ø Dobereiner’s Triads
Ø Newlands Law of Octaves         Ø Mendeleev’s Periodic Table
Ø Modern Periodic Table
Classification of elements
 We have learnt in the previous standards that all the atoms of an element are of only 
one type. Today 118 elements are known to the scientific world. However, around year 
1800 only about 30 elements were known. More number of elements were discovered in 
the course of time. More and more information about the properties of these elements was 
gathered. To ease the study of such a large number of elements, scientists started studying 
the pattern if any, in the vast information about them. You know that in the initial 
classification elements were classified into the groups of metals and nonmetals. Later  on 
another class of elements called metalloids was noticed. As the knowledge about elements 
and their properties went on increasing different scientists started trying out different 
methods of classification.
Dobereiner’s Triads
 In the year 1817 a German scientist Dobereiner suggested that properties of elements 
are related to their atomic masses. He made groups of three elements each, having similar 
chemical properties and called them triads. He arranged the three elements in a triad in an 
increasing order of atomic mass and showed that the atomic mass of the middle element 
was approximately equal to the mean of the atomic masses of the other two elements. 
However, all the known elements could not be classified into the Dobereiner’s triads. 
2.1 Dobereiner’s Triads
Identify Dobereiner’s triads from the following groups of 
elements having similar chemical properties.
Can you recall?
Can you tell?
1. Mg (24.3), Ca (40.1), Sr (87.6)        2. S (32.1), Se (79.0), Te (127.6)         
3. Be (9.0), Mg (24.3), Ca (40.1)
Sr. 
No.
Triad Element -1 
Actual atomic 
mass(a)
Element - 2 Element - 3 
Actual atomic
mass (c) 
                
Actual 
atomic mass
1 Li, Na, 
K
 Lithium (Li)
6.9
Sodium (Na)
23.0
Potassium (K)
39.1
2 Ca, Sr, 
Ba
Calcium (Ca)
40.1
Strontium (Sr)
87.6
Barium (Ba)
137.3
3 Cl, Br, I Chlorine (Cl)
35.5
Bromine (Br)
79.9
Iodine (I)
126.9
= 23.0 
6.9 + 39.1
      2
40.1+ 137.3
        2
= 88.7
35.5 + 126.9
         2
= 81.2
 a+c
   2
Mean =
17
The English scientist John Newlands 
correlated the atomic masses of elements 
to their properties  in a different way. In the 
year 1866 Newlands arranged the elements 
known at that  time in an increasing order 
of their atomic masses. It started with the 
lightest element hydrogen and ended up 
with thorium. He found that every eighth 
element had properties similar to those of 
the first. For example, sodium is the eighth 
element from lithium and both have similar 
properties. Also, magnesium shows 
similarity to beryllium and chlorine shows 
similarity with fluorine. Newlands 
compared this similarity with the octaves 
in music. He called the similarity observed 
in the eighth and the first element as the 
Law of octaves.
Musical 
Note
Do
(Sa)
Re
(Re)
Mi
(Ga)
Fa
(Ma)
So
(Pa)
La
(Dha) 
Ti
(Ni)
Elements
H
F
Cl
Co &Ni
Br
Li
Na
K
Cu
Rb
Be
Mg
Ca
Zn
Sr
B
Al
Cr
Y
Ce & La
C
Si
Ti
In
Zr
N
P
Mn
As
O
S
Fe
Se
2.2 Newlands’ Octaves
Many limitation were found in Newlands’ octaves. This law was found to be applicable 
only up to calcium. Newlands fitted all the known elements in a table of 7 X 8 that is 56 
boxes. Newlands placed two elements each in some boxes to accommodate all the known 
elements in the table. For example, Co and Ni, Ce and La. Moreover, he placed some 
elements with different properties under the same note in the octave. For example, 
Newlands placed the metals Co and Ni under the note ‘Do’ along with halogens, while Fe, 
having similarity with Co and Ni, away from them along with the nonmetals O and S 
under the note ‘Ti’. Also, Newlands’ octaves did not have provision to accommodate the 
newly discovered elements. The properties of the new elements discovered later on did not 
fit in the Newlands’ law of octaves. 
In the Indian music system there 
are seven main notes, namely, Sa, Re, 
Ga, Ma, Pa, Dha, Ni, and their collection 
is called ‘Saptak’. The frequency of the 
notes goes on increasing from ‘Sa’ to 
‘Ni’. Then comes, the ‘Sa’ of the upper 
‘Saptak’ at the double the frequency of 
the original ‘Sa’. It means that notes 
repeat after completion of one ‘Saptak’. 
The seven notes in the western music 
are Do, Re, Mi, Fa, So, La, Ti.
The note ‘Do’ having double the 
original frequency comes again at the 
eighth place. This is the octave of 
western notes. Music is created by the 
variety in the use of these notes.
Newlands’ Law of Octaves
Mendeleev’s Periodic table
The Russian scientist Dmitri Mendeleev developed the periodic table of elements  
during the period 1869 to 1872 A.D. Mendeleev’s periodic table is the most important step 
in the classification of elements. Mendeleev considered the fundamental property of 
elements, namely, the atomic mass, as standard and arranged 63 elements  known at that 
time in an increasing order of their atomic masses. Then he transformed this into the 
periodic table of elements in accordance with the physical and chemical properties of 
these elements. 
Do you know ?
18
2.3 Mendeleev’s Periodic Table
Se-
ries
Group I
-
R
2
O
Group II
-
RO 
Group  III
-
R
2
O
3
Group  IV
RH
4
RO
2
Group  V
RH
3
R
2
O
5
Group  
VI
RH
2
RO
3
Group VII
RH
R
2
O
7
Group VIII
-
RO
4
1 H=1
2 Li=7 Be=9.4 B=11 C=12 N=14 O=16 F=19
3 Na=23 Mg=24 Al=27.3 Si=28 P=31 S=32 Cl= 35.5
4 K=39 Ca=40 - = 44 Ti= 48 V=51 Cr= 52 Mn=55 Fe=56, Co=59
Ni=59, Cu=63
5 (Cu=63) Zn=65 -=68 -=72 As=75 Se=78 Br=80
6 Rb=85 Sr=87 ?Yt=88 Zr=90 Nb=94 Mo=96 -=100 Ru=104,Rh=104
Pd=106,Ag=108
7 (Ag=108) Cd=112 In=113 Sn=118 Sb=122 Te=125 J=127
8 Cs=133 Ba=137 ?Di=138 ?Ce=140 - - - ----
9 (-) - - - - - -
10 - - ?Er=178 ?La=180 Ta=182 W=184 - Os=195, Ir=197
Pt=198, Au=199
11 (Au=199) Hg=200 Ti=204 Pb=207 Bi= 208 - -
12 - - - Th=231 - U=240 - ---
Introduction to scientist
 Dmitri Mendeleev (1834-1907) was a professor in the St. 
Petersburg University. He made separate card for every known 
element showing its atomic mass. He arranged the cards in 
accordance with the atomic masses and properties of the 
elements which resulted in the invention of the periodic table 
of elements.
     Dmitri Mendeleev
 Mendeleev organized the periodic table on the basis of the chemical and physical 
properties of the elements. These were the molecular formulae of hydrides and oxides of 
the elements, melting points, boiling points and densities of the elements and their hydrides 
and oxides. Mendeleev found that the elements with similar physical and chemical 
properties repeat after a definite interval. On the basis of this finding Mendeleev stated 
the following periodic law.
 Properties of elements are periodic function of their atomic masses.   
The vertical columns in the Mendeleev’s periodic table are called groups while the 
horizontal rows are called periods.
(The general molecular formulae of compounds shown as R
2
O, R
2
O
3
, etc. in the upper 
part of Mendeleev’s periodic table, are written as R
2
O, R
2
O
3
,etc. in the present system.)
Page 4


16
1. What are the types of matter?  
2. What are the types of elements?
3. What are the smallest particles of matter called?
4. What is the difference between the molecules of elements and   
compounds?
2.Periodic Classification of Elements
Ø Elements and their classification    Ø Dobereiner’s Triads
Ø Newlands Law of Octaves         Ø Mendeleev’s Periodic Table
Ø Modern Periodic Table
Classification of elements
 We have learnt in the previous standards that all the atoms of an element are of only 
one type. Today 118 elements are known to the scientific world. However, around year 
1800 only about 30 elements were known. More number of elements were discovered in 
the course of time. More and more information about the properties of these elements was 
gathered. To ease the study of such a large number of elements, scientists started studying 
the pattern if any, in the vast information about them. You know that in the initial 
classification elements were classified into the groups of metals and nonmetals. Later  on 
another class of elements called metalloids was noticed. As the knowledge about elements 
and their properties went on increasing different scientists started trying out different 
methods of classification.
Dobereiner’s Triads
 In the year 1817 a German scientist Dobereiner suggested that properties of elements 
are related to their atomic masses. He made groups of three elements each, having similar 
chemical properties and called them triads. He arranged the three elements in a triad in an 
increasing order of atomic mass and showed that the atomic mass of the middle element 
was approximately equal to the mean of the atomic masses of the other two elements. 
However, all the known elements could not be classified into the Dobereiner’s triads. 
2.1 Dobereiner’s Triads
Identify Dobereiner’s triads from the following groups of 
elements having similar chemical properties.
Can you recall?
Can you tell?
1. Mg (24.3), Ca (40.1), Sr (87.6)        2. S (32.1), Se (79.0), Te (127.6)         
3. Be (9.0), Mg (24.3), Ca (40.1)
Sr. 
No.
Triad Element -1 
Actual atomic 
mass(a)
Element - 2 Element - 3 
Actual atomic
mass (c) 
                
Actual 
atomic mass
1 Li, Na, 
K
 Lithium (Li)
6.9
Sodium (Na)
23.0
Potassium (K)
39.1
2 Ca, Sr, 
Ba
Calcium (Ca)
40.1
Strontium (Sr)
87.6
Barium (Ba)
137.3
3 Cl, Br, I Chlorine (Cl)
35.5
Bromine (Br)
79.9
Iodine (I)
126.9
= 23.0 
6.9 + 39.1
      2
40.1+ 137.3
        2
= 88.7
35.5 + 126.9
         2
= 81.2
 a+c
   2
Mean =
17
The English scientist John Newlands 
correlated the atomic masses of elements 
to their properties  in a different way. In the 
year 1866 Newlands arranged the elements 
known at that  time in an increasing order 
of their atomic masses. It started with the 
lightest element hydrogen and ended up 
with thorium. He found that every eighth 
element had properties similar to those of 
the first. For example, sodium is the eighth 
element from lithium and both have similar 
properties. Also, magnesium shows 
similarity to beryllium and chlorine shows 
similarity with fluorine. Newlands 
compared this similarity with the octaves 
in music. He called the similarity observed 
in the eighth and the first element as the 
Law of octaves.
Musical 
Note
Do
(Sa)
Re
(Re)
Mi
(Ga)
Fa
(Ma)
So
(Pa)
La
(Dha) 
Ti
(Ni)
Elements
H
F
Cl
Co &Ni
Br
Li
Na
K
Cu
Rb
Be
Mg
Ca
Zn
Sr
B
Al
Cr
Y
Ce & La
C
Si
Ti
In
Zr
N
P
Mn
As
O
S
Fe
Se
2.2 Newlands’ Octaves
Many limitation were found in Newlands’ octaves. This law was found to be applicable 
only up to calcium. Newlands fitted all the known elements in a table of 7 X 8 that is 56 
boxes. Newlands placed two elements each in some boxes to accommodate all the known 
elements in the table. For example, Co and Ni, Ce and La. Moreover, he placed some 
elements with different properties under the same note in the octave. For example, 
Newlands placed the metals Co and Ni under the note ‘Do’ along with halogens, while Fe, 
having similarity with Co and Ni, away from them along with the nonmetals O and S 
under the note ‘Ti’. Also, Newlands’ octaves did not have provision to accommodate the 
newly discovered elements. The properties of the new elements discovered later on did not 
fit in the Newlands’ law of octaves. 
In the Indian music system there 
are seven main notes, namely, Sa, Re, 
Ga, Ma, Pa, Dha, Ni, and their collection 
is called ‘Saptak’. The frequency of the 
notes goes on increasing from ‘Sa’ to 
‘Ni’. Then comes, the ‘Sa’ of the upper 
‘Saptak’ at the double the frequency of 
the original ‘Sa’. It means that notes 
repeat after completion of one ‘Saptak’. 
The seven notes in the western music 
are Do, Re, Mi, Fa, So, La, Ti.
The note ‘Do’ having double the 
original frequency comes again at the 
eighth place. This is the octave of 
western notes. Music is created by the 
variety in the use of these notes.
Newlands’ Law of Octaves
Mendeleev’s Periodic table
The Russian scientist Dmitri Mendeleev developed the periodic table of elements  
during the period 1869 to 1872 A.D. Mendeleev’s periodic table is the most important step 
in the classification of elements. Mendeleev considered the fundamental property of 
elements, namely, the atomic mass, as standard and arranged 63 elements  known at that 
time in an increasing order of their atomic masses. Then he transformed this into the 
periodic table of elements in accordance with the physical and chemical properties of 
these elements. 
Do you know ?
18
2.3 Mendeleev’s Periodic Table
Se-
ries
Group I
-
R
2
O
Group II
-
RO 
Group  III
-
R
2
O
3
Group  IV
RH
4
RO
2
Group  V
RH
3
R
2
O
5
Group  
VI
RH
2
RO
3
Group VII
RH
R
2
O
7
Group VIII
-
RO
4
1 H=1
2 Li=7 Be=9.4 B=11 C=12 N=14 O=16 F=19
3 Na=23 Mg=24 Al=27.3 Si=28 P=31 S=32 Cl= 35.5
4 K=39 Ca=40 - = 44 Ti= 48 V=51 Cr= 52 Mn=55 Fe=56, Co=59
Ni=59, Cu=63
5 (Cu=63) Zn=65 -=68 -=72 As=75 Se=78 Br=80
6 Rb=85 Sr=87 ?Yt=88 Zr=90 Nb=94 Mo=96 -=100 Ru=104,Rh=104
Pd=106,Ag=108
7 (Ag=108) Cd=112 In=113 Sn=118 Sb=122 Te=125 J=127
8 Cs=133 Ba=137 ?Di=138 ?Ce=140 - - - ----
9 (-) - - - - - -
10 - - ?Er=178 ?La=180 Ta=182 W=184 - Os=195, Ir=197
Pt=198, Au=199
11 (Au=199) Hg=200 Ti=204 Pb=207 Bi= 208 - -
12 - - - Th=231 - U=240 - ---
Introduction to scientist
 Dmitri Mendeleev (1834-1907) was a professor in the St. 
Petersburg University. He made separate card for every known 
element showing its atomic mass. He arranged the cards in 
accordance with the atomic masses and properties of the 
elements which resulted in the invention of the periodic table 
of elements.
     Dmitri Mendeleev
 Mendeleev organized the periodic table on the basis of the chemical and physical 
properties of the elements. These were the molecular formulae of hydrides and oxides of 
the elements, melting points, boiling points and densities of the elements and their hydrides 
and oxides. Mendeleev found that the elements with similar physical and chemical 
properties repeat after a definite interval. On the basis of this finding Mendeleev stated 
the following periodic law.
 Properties of elements are periodic function of their atomic masses.   
The vertical columns in the Mendeleev’s periodic table are called groups while the 
horizontal rows are called periods.
(The general molecular formulae of compounds shown as R
2
O, R
2
O
3
, etc. in the upper 
part of Mendeleev’s periodic table, are written as R
2
O, R
2
O
3
,etc. in the present system.)
19
1.  There are some vacant places in the Mendeleev’s periodic table. 
In some of these places the atomic masses are seen to be 
predicted. Enlist three of these predicted atomic masses along 
with their group and period.
2. Due to uncertainty in the names of some of the elements, a 
question mark is indicated before the symbol in the Mendeleev’s 
periodic table. What are such symbols?
Merits of Mendeleev’s periodic table 
Science is progressive. There is a freedom in science to revise the old inference by 
using more advanced means and methods of doing experiments. These characteristics of 
science are clearly seen in the Mendeleev’s periodic table.
While applying the law that the properties of elements are a periodic function of their 
atomic masses, to all the known elements, Mendeleev arranged the elements with a thought 
that the information available till then was not final but it could change. As a result of this, 
Mendeleev’s periodic table demonstrates the following merits.
1.  Atomic masses of some elements were revised so as to give them proper place in the 
periodic table in accordance with their properties. For example, the previously determined 
atomic mass of beryllium, 14.09, was changed to the correct value 9.4, and beryllium was 
placed before boron.
2.  Mendeleev kept vacant places in the periodic table for elements not discovered till 
then. Three of these unknown elements were given the names eka-boron, eka-aluminum 
and  eka-silicon from the known neighbours and their atomic masses were indicated as 
44,  68 and 72, respectively. Not only this but their properties were also predicted. Later 
on these elements were discovered and named as scandium (Sc), gallium (Ga) and 
germanium (Ge) respectively. The properties of these elements matched well with those 
predicted by  Mendeleev. See table 2.4. Due to this success all were convinced about the 
importance of Mendeleev’s periodic table and this method of classification of elements 
was accepted immediately.
Property eka- aluminum(E) (Mendeleev’s prediction) Gallium (Ga)(actual)
1. Atomic mass 68 69.7
2. Density (g/cm
3
) 5.9 5.94 
3. Melting point(
0
C) Low 30.2 
4. Formula of chloride ECl
3
GaCl
3
5. Formula of oxide E
2
O
3
Ga
2
O
3
6. Nature of oxide Amphoteric oxide Amphoteric oxide
2.4  Actual and predicted properties of gallium.
3.  There was no place reserved for noble 
gases in Mendeleev’s original periodic 
table. However, when noble gases such 
as helium, neon and argon were 
discovered towards the end of 
nineteenth century, Mendeleev created 
the ‘ zero’ group without disturbing the 
original periodic table in which the 
noble gases were fitted very well.
  Chlorine has two isotopes,viz, C1-35 and C1-
37. Their atomic masses are 35 and 37  respectively. 
Their chemical properties are same. Where should 
these be placed in Mendeleev’s periodic table? In 
different places or in the same place?
Think about it
Use your brain power !
Page 5


16
1. What are the types of matter?  
2. What are the types of elements?
3. What are the smallest particles of matter called?
4. What is the difference between the molecules of elements and   
compounds?
2.Periodic Classification of Elements
Ø Elements and their classification    Ø Dobereiner’s Triads
Ø Newlands Law of Octaves         Ø Mendeleev’s Periodic Table
Ø Modern Periodic Table
Classification of elements
 We have learnt in the previous standards that all the atoms of an element are of only 
one type. Today 118 elements are known to the scientific world. However, around year 
1800 only about 30 elements were known. More number of elements were discovered in 
the course of time. More and more information about the properties of these elements was 
gathered. To ease the study of such a large number of elements, scientists started studying 
the pattern if any, in the vast information about them. You know that in the initial 
classification elements were classified into the groups of metals and nonmetals. Later  on 
another class of elements called metalloids was noticed. As the knowledge about elements 
and their properties went on increasing different scientists started trying out different 
methods of classification.
Dobereiner’s Triads
 In the year 1817 a German scientist Dobereiner suggested that properties of elements 
are related to their atomic masses. He made groups of three elements each, having similar 
chemical properties and called them triads. He arranged the three elements in a triad in an 
increasing order of atomic mass and showed that the atomic mass of the middle element 
was approximately equal to the mean of the atomic masses of the other two elements. 
However, all the known elements could not be classified into the Dobereiner’s triads. 
2.1 Dobereiner’s Triads
Identify Dobereiner’s triads from the following groups of 
elements having similar chemical properties.
Can you recall?
Can you tell?
1. Mg (24.3), Ca (40.1), Sr (87.6)        2. S (32.1), Se (79.0), Te (127.6)         
3. Be (9.0), Mg (24.3), Ca (40.1)
Sr. 
No.
Triad Element -1 
Actual atomic 
mass(a)
Element - 2 Element - 3 
Actual atomic
mass (c) 
                
Actual 
atomic mass
1 Li, Na, 
K
 Lithium (Li)
6.9
Sodium (Na)
23.0
Potassium (K)
39.1
2 Ca, Sr, 
Ba
Calcium (Ca)
40.1
Strontium (Sr)
87.6
Barium (Ba)
137.3
3 Cl, Br, I Chlorine (Cl)
35.5
Bromine (Br)
79.9
Iodine (I)
126.9
= 23.0 
6.9 + 39.1
      2
40.1+ 137.3
        2
= 88.7
35.5 + 126.9
         2
= 81.2
 a+c
   2
Mean =
17
The English scientist John Newlands 
correlated the atomic masses of elements 
to their properties  in a different way. In the 
year 1866 Newlands arranged the elements 
known at that  time in an increasing order 
of their atomic masses. It started with the 
lightest element hydrogen and ended up 
with thorium. He found that every eighth 
element had properties similar to those of 
the first. For example, sodium is the eighth 
element from lithium and both have similar 
properties. Also, magnesium shows 
similarity to beryllium and chlorine shows 
similarity with fluorine. Newlands 
compared this similarity with the octaves 
in music. He called the similarity observed 
in the eighth and the first element as the 
Law of octaves.
Musical 
Note
Do
(Sa)
Re
(Re)
Mi
(Ga)
Fa
(Ma)
So
(Pa)
La
(Dha) 
Ti
(Ni)
Elements
H
F
Cl
Co &Ni
Br
Li
Na
K
Cu
Rb
Be
Mg
Ca
Zn
Sr
B
Al
Cr
Y
Ce & La
C
Si
Ti
In
Zr
N
P
Mn
As
O
S
Fe
Se
2.2 Newlands’ Octaves
Many limitation were found in Newlands’ octaves. This law was found to be applicable 
only up to calcium. Newlands fitted all the known elements in a table of 7 X 8 that is 56 
boxes. Newlands placed two elements each in some boxes to accommodate all the known 
elements in the table. For example, Co and Ni, Ce and La. Moreover, he placed some 
elements with different properties under the same note in the octave. For example, 
Newlands placed the metals Co and Ni under the note ‘Do’ along with halogens, while Fe, 
having similarity with Co and Ni, away from them along with the nonmetals O and S 
under the note ‘Ti’. Also, Newlands’ octaves did not have provision to accommodate the 
newly discovered elements. The properties of the new elements discovered later on did not 
fit in the Newlands’ law of octaves. 
In the Indian music system there 
are seven main notes, namely, Sa, Re, 
Ga, Ma, Pa, Dha, Ni, and their collection 
is called ‘Saptak’. The frequency of the 
notes goes on increasing from ‘Sa’ to 
‘Ni’. Then comes, the ‘Sa’ of the upper 
‘Saptak’ at the double the frequency of 
the original ‘Sa’. It means that notes 
repeat after completion of one ‘Saptak’. 
The seven notes in the western music 
are Do, Re, Mi, Fa, So, La, Ti.
The note ‘Do’ having double the 
original frequency comes again at the 
eighth place. This is the octave of 
western notes. Music is created by the 
variety in the use of these notes.
Newlands’ Law of Octaves
Mendeleev’s Periodic table
The Russian scientist Dmitri Mendeleev developed the periodic table of elements  
during the period 1869 to 1872 A.D. Mendeleev’s periodic table is the most important step 
in the classification of elements. Mendeleev considered the fundamental property of 
elements, namely, the atomic mass, as standard and arranged 63 elements  known at that 
time in an increasing order of their atomic masses. Then he transformed this into the 
periodic table of elements in accordance with the physical and chemical properties of 
these elements. 
Do you know ?
18
2.3 Mendeleev’s Periodic Table
Se-
ries
Group I
-
R
2
O
Group II
-
RO 
Group  III
-
R
2
O
3
Group  IV
RH
4
RO
2
Group  V
RH
3
R
2
O
5
Group  
VI
RH
2
RO
3
Group VII
RH
R
2
O
7
Group VIII
-
RO
4
1 H=1
2 Li=7 Be=9.4 B=11 C=12 N=14 O=16 F=19
3 Na=23 Mg=24 Al=27.3 Si=28 P=31 S=32 Cl= 35.5
4 K=39 Ca=40 - = 44 Ti= 48 V=51 Cr= 52 Mn=55 Fe=56, Co=59
Ni=59, Cu=63
5 (Cu=63) Zn=65 -=68 -=72 As=75 Se=78 Br=80
6 Rb=85 Sr=87 ?Yt=88 Zr=90 Nb=94 Mo=96 -=100 Ru=104,Rh=104
Pd=106,Ag=108
7 (Ag=108) Cd=112 In=113 Sn=118 Sb=122 Te=125 J=127
8 Cs=133 Ba=137 ?Di=138 ?Ce=140 - - - ----
9 (-) - - - - - -
10 - - ?Er=178 ?La=180 Ta=182 W=184 - Os=195, Ir=197
Pt=198, Au=199
11 (Au=199) Hg=200 Ti=204 Pb=207 Bi= 208 - -
12 - - - Th=231 - U=240 - ---
Introduction to scientist
 Dmitri Mendeleev (1834-1907) was a professor in the St. 
Petersburg University. He made separate card for every known 
element showing its atomic mass. He arranged the cards in 
accordance with the atomic masses and properties of the 
elements which resulted in the invention of the periodic table 
of elements.
     Dmitri Mendeleev
 Mendeleev organized the periodic table on the basis of the chemical and physical 
properties of the elements. These were the molecular formulae of hydrides and oxides of 
the elements, melting points, boiling points and densities of the elements and their hydrides 
and oxides. Mendeleev found that the elements with similar physical and chemical 
properties repeat after a definite interval. On the basis of this finding Mendeleev stated 
the following periodic law.
 Properties of elements are periodic function of their atomic masses.   
The vertical columns in the Mendeleev’s periodic table are called groups while the 
horizontal rows are called periods.
(The general molecular formulae of compounds shown as R
2
O, R
2
O
3
, etc. in the upper 
part of Mendeleev’s periodic table, are written as R
2
O, R
2
O
3
,etc. in the present system.)
19
1.  There are some vacant places in the Mendeleev’s periodic table. 
In some of these places the atomic masses are seen to be 
predicted. Enlist three of these predicted atomic masses along 
with their group and period.
2. Due to uncertainty in the names of some of the elements, a 
question mark is indicated before the symbol in the Mendeleev’s 
periodic table. What are such symbols?
Merits of Mendeleev’s periodic table 
Science is progressive. There is a freedom in science to revise the old inference by 
using more advanced means and methods of doing experiments. These characteristics of 
science are clearly seen in the Mendeleev’s periodic table.
While applying the law that the properties of elements are a periodic function of their 
atomic masses, to all the known elements, Mendeleev arranged the elements with a thought 
that the information available till then was not final but it could change. As a result of this, 
Mendeleev’s periodic table demonstrates the following merits.
1.  Atomic masses of some elements were revised so as to give them proper place in the 
periodic table in accordance with their properties. For example, the previously determined 
atomic mass of beryllium, 14.09, was changed to the correct value 9.4, and beryllium was 
placed before boron.
2.  Mendeleev kept vacant places in the periodic table for elements not discovered till 
then. Three of these unknown elements were given the names eka-boron, eka-aluminum 
and  eka-silicon from the known neighbours and their atomic masses were indicated as 
44,  68 and 72, respectively. Not only this but their properties were also predicted. Later 
on these elements were discovered and named as scandium (Sc), gallium (Ga) and 
germanium (Ge) respectively. The properties of these elements matched well with those 
predicted by  Mendeleev. See table 2.4. Due to this success all were convinced about the 
importance of Mendeleev’s periodic table and this method of classification of elements 
was accepted immediately.
Property eka- aluminum(E) (Mendeleev’s prediction) Gallium (Ga)(actual)
1. Atomic mass 68 69.7
2. Density (g/cm
3
) 5.9 5.94 
3. Melting point(
0
C) Low 30.2 
4. Formula of chloride ECl
3
GaCl
3
5. Formula of oxide E
2
O
3
Ga
2
O
3
6. Nature of oxide Amphoteric oxide Amphoteric oxide
2.4  Actual and predicted properties of gallium.
3.  There was no place reserved for noble 
gases in Mendeleev’s original periodic 
table. However, when noble gases such 
as helium, neon and argon were 
discovered towards the end of 
nineteenth century, Mendeleev created 
the ‘ zero’ group without disturbing the 
original periodic table in which the 
noble gases were fitted very well.
  Chlorine has two isotopes,viz, C1-35 and C1-
37. Their atomic masses are 35 and 37  respectively. 
Their chemical properties are same. Where should 
these be placed in Mendeleev’s periodic table? In 
different places or in the same place?
Think about it
Use your brain power !
20
Demerits of Mendeleev’s periodic table
1. The whole number atomic mass of the elements cobalt (Co) and nickel (Ni) is the same. 
Therefore there was an ambiguity regarding their sequence in Mendeleev’s periodic 
table. 
2. Isotopes were discovered long time after Mendeleev put forth the periodic table. As 
isotopes have the same chemical properties but different atomic masses, a challenge 
was posed in placing them in Mendeleev’s periodic table.  
3.  When elements are arranged in an increasing order of atomic masses, the rise in atomic 
mass does not appear to be uniform. It was not possible, therefore, to predict how many 
elements could be discovered between two heavy elements. 
 Element 
(Molecular 
formula)
Compounds 
with metals
Compounds 
with nonmetals
H
2
Cl
2
NaH
NaCl
CH
4
CCl
4
4.  Position of hydrogen : Hydrogen shows 
similarity with halogens (group VII). For 
example, the molecular formula of 
hydrogen is H
2
 while the molecular 
formulae of fluorine and chlorine are F
2 
and Cl
2
, respectively. In the same way, 
there is a similarity in the chemical 
properties of hydrogen and alkali metals 
(group I). There is a similarity in the 
molecular formulae of the compounds of 
hydrogen alkali metals (Na, K, etc.) 
formed with chlorine and oxygen. On 
considering the above properties it can not 
be decided whether the correct position of 
hydrogen is in the group of alkali metals 
 Compounds of  H Compounds of  Na
HCl
H
2
O
H
2
S
NaCl
Na
2
O
Na
2
S
2.5  Similarity in hydrogen and alkali metals
1. Write the molecular formulae of oxides of the following elements by referring to the 
Mendeleev’s periodic table.  Na, Si, Ca, C, Rb, P, Ba, Cl, Sn.
2.  Write the molecular formulae of the compounds of the following elements with hydrogen 
by referring to the Mendeleev’s periodic table.     C, S, Br, As, F, O, N, Cl
Modern Periodic Law
The scientific world did not know anything about the interior of the atom when 
Mendeleev put forth the periodic table. After the discovery of electron, scientists started 
exploring the relation between the electron number of an atom and the atomic number. The 
atomic number in Mendeleev’s periodic table only indicated the serial number of the element.
In 1913 A.D. the English scientist Henry Moseley demonstrated, with the help of the 
experiments done using X-ray tube, that the atomic number (Z) of an element corresponds 
to the positive charge on the nucleus or the number of the protons in the nucleus of the atom 
of that element. This revealed that ‘atomic number’ is a more fundamental property of an 
element than its atomic mass. Accordingly the statement of the modern periodic law was 
stated as follows: 
Properties of elements are a periodic function of their atomic numbers.
2.6 : Similarity in hydrogen and halogens
Use your brain power !
(group I) or in the group of halogens (group VII).
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FAQs on Textbook: Periodic Classification of Element - Science and Technology Class 10 (Maharashtra SSC Board)

1. What is the significance of the periodic table in chemistry?
Ans. The periodic table is a crucial tool in chemistry as it organizes all known chemical elements based on their atomic number, electron configuration, and recurring chemical properties. This systematic arrangement helps scientists predict the characteristics and behaviors of elements, understand their relationships, and study trends such as electronegativity, atomic radius, and ionization energy.
2. Who developed the first periodic table, and what was the basis of its arrangement?
Ans. The first periodic table was developed by Dmitri Mendeleev in 1869. Mendeleev arranged the elements in order of increasing atomic mass and grouped them based on similar chemical properties. This organization allowed him to identify gaps for undiscovered elements and predict their properties, demonstrating the periodic nature of element characteristics.
3. How are elements classified in the modern periodic table?
Ans. In the modern periodic table, elements are classified into three main categories: metals, nonmetals, and metalloids. Metals are typically found on the left side of the table and are characterized by their conductivity, malleability, and ductility. Nonmetals are located on the right side and are usually poor conductors. Metalloids exhibit properties of both metals and nonmetals and are found along the zig-zag line that separates the two groups.
4. What are periods and groups in the context of the periodic table?
Ans. In the periodic table, periods are the horizontal rows that indicate the number of electron shells an atom has. Each period corresponds to a principal energy level. Groups, or families, are the vertical columns that categorize elements with similar properties and the same number of valence electrons. Elements in the same group tend to exhibit similar chemical behavior.
5. How did the discovery of noble gases impact the periodic table?
Ans. The discovery of noble gases in the late 19th century led to significant changes in the periodic table. These elements, which are chemically inert due to their full valence electron shells, were added to a new group (Group 18). This discovery helped refine the understanding of element classification and reinforced the periodic law, highlighting the importance of electron configuration in determining chemical properties.
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