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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