Dr Inam Ul Haq Jazbi Periodic Classification
1.1. Historical Background of Periodic Classification
Need and Search for Classification
With the discovery of more and more new elements, it was necessary to organize these elements
systematically and need arose for a frame work in which these elements could be classified and arranged in in
order to facilitate their study and make their study simple and systematic. The classification of elements
enabled the chemists to understand and interpret the properties of elements in a better way.
There could be many ways of arranging the elements; firstly they could be classified by their states (solids,
liquids or gases) at a particular temperature, secondly they could be arranged as metals, non-metals and
metalloids and thirdly one might find patterns in their reactions with oxygen or water or other chemicals. Would
one consider trying to link these properties to the relative atomic masses of the elements?
Previously scientist tried to arrange the elements in a scientific, systematic and an organized manner on
the basis of their atomic weight (atomic masses) as it was thought that the properties of elements
depended upon their atomic masses (the thought was grounded on Dalton‟s atomic theory). But recently,
the basis of classification has been changed and elements are arranged on the basis of their atomic
numbers instead of their atomic masses.
Different attempts of Classification
Following attempts were made to classify the known elements:
1. Al-Razi Classification
2. Origin of Classification; Dalton‟s Atomic Theory
3. Dumas Work
4. Prout‟s Attempt
5. Dobereiner‟s Triads
6. Newland‟s Law of Octave
7. Lother Meyer‟s Classification
8. Mendeleev‟s Classification
9. Modern Periodic table
1. Origin of Classification
The basis of classification of elements was grounded on the Daltons‟ atomic theory put forward by an English
scientist, John Dalton in 1808, according to which:
“Atoms of different elements have different atomic masses.”
Thus it was concluded that there is a regular relationship between atomic masses and properties of elements.
“This relationship proves to be the corner stone for the future classification of elements”
2. Dumas Work
Dumas (1800-1884), a French chemist arranged the elements on their combining power with chlorine.
For example, elements that combined with 1 chlorine atom could be arranged in vertical columns in increasing
order of their atomic weights and so on.
Reasons for Failure
Dumas attempt of classification did not gain success as all elements do not combine with chlorine and few
show variable valency.
3. Prout‟s Attempt
Prout, an English chemist considered the atomic weight of hydrogen as the basis of his classification. He
considered that:
“Atomic weights of all elements are simple multiple of the atomic weight of hydrogen”
Reason for Failure
It could not explain the fractional atomic weights of elements.
,Dr Inam Ul Haq Jazbi Periodic Classification
3. Dobereiner‟s Triads
A German chemist, Johann Wolfgang Dobereiner in 1817 noticed an interesting pattern in certain sets of
three similar elements and classified the similar elements in the groups of three elements (in the sequence of
increasing atomic mass) known as triad. He found that the atomic mass of the middle element lay (fall) roughly
half way (midway) between the other two (i.e. the lightest and the heaviest) elements of a triad and the
elements of a triad also resemble in properties. He also noticed that the middle elements had properties that
were an average of the other two members of a triad when arranged by the atomic weights. e.g. He found that
the density of the middle element in most triad is roughly equal to the average of the densities of the other two
elements. The density of strontium (2.6 g/cm3) for example is close to the average of the densities of calcium
(1.55 g/cm3) and barium (3.51g/cm3).
He put forward Law or rule of Triads, according to which;
“Central atom of each set of triad has an atomic mass equal to the arithmetic mean of
the atomic masses of the other two elements.”
OR
Each set of triad (group of three elements ordered by increasing atomic weights) has
similar properties and atomic weight of the middle element of a triad was approximately
equal to arithmetic mean (average) of the atomic weights of other two elements of a
triad”.
He arranged the elements in triads. The elements of triad resemble in properties.
He first found alkaline earth metal triad of Ca, Sr and Ba. He further noticed the same pattern for the alkali
metal triad (Li, Na, K), the halogen triad (Cl, BR, I), Chalcogen (S, Se, Te), metalloid triad (P, As, Sb) and
transitional metal triad (Mn, Cr, Fe).
Elements Atomic Mass Mean Atomic Mass Elements Atomic Mass Mean Atomic Mass
Lithium 7 7 + 39 Chlorine 35.5 35.5 + 127
= 23 2 = 81.25
Sodium 23 2 Bromine 80
Potassium 39 Iodine 127
Elements Atomic Mass Mean Atomic Mass Elements Atomic Mass Mean Atomic Mass
Calcium 40 40 + 137.3 Sulphur 32 32 + 127.5
Strontium 87.6 = 88.65 Selenium 79 = 79.8
2 2
Barium 137.3 Tellurium 127.5
Elements Atomic Mass Mean Atomic Mass Elements Atomic Mass Mean Atomic Mass
Manganese 55 55 + 56 Phosphorus 31 31+ 121.75
Chromium 52 = 55.5 Arsenic 75 = 76.37
2 2
Iron 56 Antimony 121.75
Triad Name Elements Atomic Mass Mean Atomic Mass
Alkaline Calcium 40 40 + 137.5
Atomic mass of Sr = 2 = 88.65
Earths Strontium 87.6
Triads Barium 137.3
Alkali Lithium 7
7 + 39
Metal Sodium 23 Atomic mass of Na = = 23
2
Triad Potassium 39
Chlorine 35.5
Halogen 35.5 + 127
Bromine 80 (79.9) Atomic mass of Br = = 81.25
Triad 2
Iodine 127 (126.9)
Sulphur 32 32 127.6
Chalcogen 79.8
Selenium 79
Triad Atomic mass of Se = 2
Tellurium 127.6
Transition Manganese 55 55 56
Metal Chromium 52 55.5
Atomic mass of Cr = 2
Triad Iron 56
Phosphorus 31 31 121.75
Metalloid 76.37
Arsenic 74.92
Triad Atomic mass of As = 2
Antimony 121.75
,Dr Inam Ul Haq Jazbi Periodic Classification
4. Newland‟s Law of Octave
In 1864, an English (London) industrial chemist John Alexander Newland arranged the 56 (60 or 62) known
elements by order of increasing atomic weights into a table along horizontal rows seven element long with
seven vertical columns and proposed has law of octave accordingly:
“If elements are arranged in the ascending order of their atomic weights, the eighth (8th)
element following any given element in the series has nearly same physical and
chemical properties as first one” which means that starting from any element, the
properties of every eighth element were similar to those of first i.e. its properties are a
kind of repetition of the first (like the eight notes of an octave of music or by the
analogy with the seven intervals of the musical scale).
Group I II III IV V VI VII
Element/Atomic mass Li-7 Be -9 B -11 C-12 N-14 O-16 F-19
Element/Atomic mass Na-23 Mg-24 Al-27 Si-28 P-31 S-32 Cl-35.5
Element/Atomic mass K-39 Ca -40 Cr-52 Ti-48 Mn-55 Fe-56 Co-58.7, Ni-59
Element/Atomic mass Cu-63.5 Zn-65.4 Y-88.9 In-114 As-75 Se-79 Br-80
Element/Atomic mass Rb-85.46 Sr-87.6 Ce/La Zr-91 Di/Mo Ro/Ru Pd-106.4
Element/Atomic mass Ag-107 Cd-112.4 -------- Sn-118.7 Sb-121.76 Te I-127
Element/Atomic mass Cs-132.9 Ba/V Ta W Nb Au Pt/Ir
Element/Atomic mass Os-190 Hg-200 Ti- Pb-207 Bi Th -------
It was compared to octaves (Sa, Re, Ga, Ma, Pa, Da, Ni, Sa) in musical scale and thus the name Newland‟s
law of octaves (notes of music)
Merits
1. It arranges all 56 elements into tabular form.
2. It arranges all elements with identical properties into same group.
3. Newland‟s classification of elements for the first time showed the existence of periodicity i.e.
recurrence of chemical and physical properties of elements at regular intervals.
4. It also provided a great idea towards the development of modern periodic table.
Objections
1. The Law of Octave holds up well for the first 17 elements, but it failed rather badly beyond calcium in
predicting a consistent trend.
2. The heavier elements could not be accommodated by this arrangement.
3. Moreover hydrogen as not included in his table.
, Dr Inam Ul Haq Jazbi Periodic Classification
4. Lother Meyer‟s Classification
In 1869, a German Physicist Julius Lother Meyer (a contemporary of Mendeleev) classified the known 56
elements on the basis of their increasing atomic weights in graphical form in nine vertical columns or
groups from I to IX. Meyer‟s work was based on physical properties of elements like atomic volume. He put
forward his periodic law, which states that
„„Physical properties of elements are periodic function of their atomic weights‟‟.
The volume occupied by 1 gram atomic weight or 1 gram atom or 1 gram mole (i.e. 6.02 x 1023 atoms) of
any element in solid state is called atomic volume which is a rough measure of the relative sizes of
atoms.
atomic mass
Atomic volume = [d = m / V )]
density
Lother Meyer‟s Atomic Volume Curve
Meyer arranged the elements by plotting a graph between atomic volumes of elements (on y-axis) against
their increasing atomic masses (on x-axis).
The plot gave a curve called Atomic Volume Curve, consisted of sharp peak (crests) and broad minima
(troughs). The curve exhibits periodicity as similar elements occupy same positions on the curve. For
example, the highly reactive alkali metals (Li, Na, K, Rb, Cs) occupy the peak of the curve thereby showing
that these elements have largest atomic volumes.
According to Meyer, the occupying of similar elements on same positions on the curve was called
periodicity. The regular spacing of the highest points and occupying of similar elements on the same positions
on the curve confirmed the idea of periodicity, suggested by Newland. [Meyer was the first scientist who
considered valency as a period property.]
Meyer‟s curve showed the following characteristics and periodicity:
1. Chemically similar elements occupy similar position on the curves. For example; Alkali Metals like
Li, Na, K etc. occupy the peaks of the curve indicating that they have largest atomic volumes than those
of neighbouring elements while ascending portion of the curve just before the peak is occupied by
halogens showing their smallest atomic volumes. The crest of each wave is occupied by an alkali metal
and trough by an element of small chemical affinity.
2. Alkali metals occupy the peaks or crests of the curves.
3. Weak metals or elements of small chemical affinity or transition metals occupy the troughs or minima
of the curve.
4. Electronegative and gaseous volatile elements or acidic oxides forming elements are located on the
ascending portions of the curve.
5. Electropositive or transition elements or elements with high melting points are found on the
descending portions of the curve.
6. Midway of ascending portions of curve is occupied by halogens.
7. Midway of descending portions of curve is occupied by alkaline earth metals.
1.1. Historical Background of Periodic Classification
Need and Search for Classification
With the discovery of more and more new elements, it was necessary to organize these elements
systematically and need arose for a frame work in which these elements could be classified and arranged in in
order to facilitate their study and make their study simple and systematic. The classification of elements
enabled the chemists to understand and interpret the properties of elements in a better way.
There could be many ways of arranging the elements; firstly they could be classified by their states (solids,
liquids or gases) at a particular temperature, secondly they could be arranged as metals, non-metals and
metalloids and thirdly one might find patterns in their reactions with oxygen or water or other chemicals. Would
one consider trying to link these properties to the relative atomic masses of the elements?
Previously scientist tried to arrange the elements in a scientific, systematic and an organized manner on
the basis of their atomic weight (atomic masses) as it was thought that the properties of elements
depended upon their atomic masses (the thought was grounded on Dalton‟s atomic theory). But recently,
the basis of classification has been changed and elements are arranged on the basis of their atomic
numbers instead of their atomic masses.
Different attempts of Classification
Following attempts were made to classify the known elements:
1. Al-Razi Classification
2. Origin of Classification; Dalton‟s Atomic Theory
3. Dumas Work
4. Prout‟s Attempt
5. Dobereiner‟s Triads
6. Newland‟s Law of Octave
7. Lother Meyer‟s Classification
8. Mendeleev‟s Classification
9. Modern Periodic table
1. Origin of Classification
The basis of classification of elements was grounded on the Daltons‟ atomic theory put forward by an English
scientist, John Dalton in 1808, according to which:
“Atoms of different elements have different atomic masses.”
Thus it was concluded that there is a regular relationship between atomic masses and properties of elements.
“This relationship proves to be the corner stone for the future classification of elements”
2. Dumas Work
Dumas (1800-1884), a French chemist arranged the elements on their combining power with chlorine.
For example, elements that combined with 1 chlorine atom could be arranged in vertical columns in increasing
order of their atomic weights and so on.
Reasons for Failure
Dumas attempt of classification did not gain success as all elements do not combine with chlorine and few
show variable valency.
3. Prout‟s Attempt
Prout, an English chemist considered the atomic weight of hydrogen as the basis of his classification. He
considered that:
“Atomic weights of all elements are simple multiple of the atomic weight of hydrogen”
Reason for Failure
It could not explain the fractional atomic weights of elements.
,Dr Inam Ul Haq Jazbi Periodic Classification
3. Dobereiner‟s Triads
A German chemist, Johann Wolfgang Dobereiner in 1817 noticed an interesting pattern in certain sets of
three similar elements and classified the similar elements in the groups of three elements (in the sequence of
increasing atomic mass) known as triad. He found that the atomic mass of the middle element lay (fall) roughly
half way (midway) between the other two (i.e. the lightest and the heaviest) elements of a triad and the
elements of a triad also resemble in properties. He also noticed that the middle elements had properties that
were an average of the other two members of a triad when arranged by the atomic weights. e.g. He found that
the density of the middle element in most triad is roughly equal to the average of the densities of the other two
elements. The density of strontium (2.6 g/cm3) for example is close to the average of the densities of calcium
(1.55 g/cm3) and barium (3.51g/cm3).
He put forward Law or rule of Triads, according to which;
“Central atom of each set of triad has an atomic mass equal to the arithmetic mean of
the atomic masses of the other two elements.”
OR
Each set of triad (group of three elements ordered by increasing atomic weights) has
similar properties and atomic weight of the middle element of a triad was approximately
equal to arithmetic mean (average) of the atomic weights of other two elements of a
triad”.
He arranged the elements in triads. The elements of triad resemble in properties.
He first found alkaline earth metal triad of Ca, Sr and Ba. He further noticed the same pattern for the alkali
metal triad (Li, Na, K), the halogen triad (Cl, BR, I), Chalcogen (S, Se, Te), metalloid triad (P, As, Sb) and
transitional metal triad (Mn, Cr, Fe).
Elements Atomic Mass Mean Atomic Mass Elements Atomic Mass Mean Atomic Mass
Lithium 7 7 + 39 Chlorine 35.5 35.5 + 127
= 23 2 = 81.25
Sodium 23 2 Bromine 80
Potassium 39 Iodine 127
Elements Atomic Mass Mean Atomic Mass Elements Atomic Mass Mean Atomic Mass
Calcium 40 40 + 137.3 Sulphur 32 32 + 127.5
Strontium 87.6 = 88.65 Selenium 79 = 79.8
2 2
Barium 137.3 Tellurium 127.5
Elements Atomic Mass Mean Atomic Mass Elements Atomic Mass Mean Atomic Mass
Manganese 55 55 + 56 Phosphorus 31 31+ 121.75
Chromium 52 = 55.5 Arsenic 75 = 76.37
2 2
Iron 56 Antimony 121.75
Triad Name Elements Atomic Mass Mean Atomic Mass
Alkaline Calcium 40 40 + 137.5
Atomic mass of Sr = 2 = 88.65
Earths Strontium 87.6
Triads Barium 137.3
Alkali Lithium 7
7 + 39
Metal Sodium 23 Atomic mass of Na = = 23
2
Triad Potassium 39
Chlorine 35.5
Halogen 35.5 + 127
Bromine 80 (79.9) Atomic mass of Br = = 81.25
Triad 2
Iodine 127 (126.9)
Sulphur 32 32 127.6
Chalcogen 79.8
Selenium 79
Triad Atomic mass of Se = 2
Tellurium 127.6
Transition Manganese 55 55 56
Metal Chromium 52 55.5
Atomic mass of Cr = 2
Triad Iron 56
Phosphorus 31 31 121.75
Metalloid 76.37
Arsenic 74.92
Triad Atomic mass of As = 2
Antimony 121.75
,Dr Inam Ul Haq Jazbi Periodic Classification
4. Newland‟s Law of Octave
In 1864, an English (London) industrial chemist John Alexander Newland arranged the 56 (60 or 62) known
elements by order of increasing atomic weights into a table along horizontal rows seven element long with
seven vertical columns and proposed has law of octave accordingly:
“If elements are arranged in the ascending order of their atomic weights, the eighth (8th)
element following any given element in the series has nearly same physical and
chemical properties as first one” which means that starting from any element, the
properties of every eighth element were similar to those of first i.e. its properties are a
kind of repetition of the first (like the eight notes of an octave of music or by the
analogy with the seven intervals of the musical scale).
Group I II III IV V VI VII
Element/Atomic mass Li-7 Be -9 B -11 C-12 N-14 O-16 F-19
Element/Atomic mass Na-23 Mg-24 Al-27 Si-28 P-31 S-32 Cl-35.5
Element/Atomic mass K-39 Ca -40 Cr-52 Ti-48 Mn-55 Fe-56 Co-58.7, Ni-59
Element/Atomic mass Cu-63.5 Zn-65.4 Y-88.9 In-114 As-75 Se-79 Br-80
Element/Atomic mass Rb-85.46 Sr-87.6 Ce/La Zr-91 Di/Mo Ro/Ru Pd-106.4
Element/Atomic mass Ag-107 Cd-112.4 -------- Sn-118.7 Sb-121.76 Te I-127
Element/Atomic mass Cs-132.9 Ba/V Ta W Nb Au Pt/Ir
Element/Atomic mass Os-190 Hg-200 Ti- Pb-207 Bi Th -------
It was compared to octaves (Sa, Re, Ga, Ma, Pa, Da, Ni, Sa) in musical scale and thus the name Newland‟s
law of octaves (notes of music)
Merits
1. It arranges all 56 elements into tabular form.
2. It arranges all elements with identical properties into same group.
3. Newland‟s classification of elements for the first time showed the existence of periodicity i.e.
recurrence of chemical and physical properties of elements at regular intervals.
4. It also provided a great idea towards the development of modern periodic table.
Objections
1. The Law of Octave holds up well for the first 17 elements, but it failed rather badly beyond calcium in
predicting a consistent trend.
2. The heavier elements could not be accommodated by this arrangement.
3. Moreover hydrogen as not included in his table.
, Dr Inam Ul Haq Jazbi Periodic Classification
4. Lother Meyer‟s Classification
In 1869, a German Physicist Julius Lother Meyer (a contemporary of Mendeleev) classified the known 56
elements on the basis of their increasing atomic weights in graphical form in nine vertical columns or
groups from I to IX. Meyer‟s work was based on physical properties of elements like atomic volume. He put
forward his periodic law, which states that
„„Physical properties of elements are periodic function of their atomic weights‟‟.
The volume occupied by 1 gram atomic weight or 1 gram atom or 1 gram mole (i.e. 6.02 x 1023 atoms) of
any element in solid state is called atomic volume which is a rough measure of the relative sizes of
atoms.
atomic mass
Atomic volume = [d = m / V )]
density
Lother Meyer‟s Atomic Volume Curve
Meyer arranged the elements by plotting a graph between atomic volumes of elements (on y-axis) against
their increasing atomic masses (on x-axis).
The plot gave a curve called Atomic Volume Curve, consisted of sharp peak (crests) and broad minima
(troughs). The curve exhibits periodicity as similar elements occupy same positions on the curve. For
example, the highly reactive alkali metals (Li, Na, K, Rb, Cs) occupy the peak of the curve thereby showing
that these elements have largest atomic volumes.
According to Meyer, the occupying of similar elements on same positions on the curve was called
periodicity. The regular spacing of the highest points and occupying of similar elements on the same positions
on the curve confirmed the idea of periodicity, suggested by Newland. [Meyer was the first scientist who
considered valency as a period property.]
Meyer‟s curve showed the following characteristics and periodicity:
1. Chemically similar elements occupy similar position on the curves. For example; Alkali Metals like
Li, Na, K etc. occupy the peaks of the curve indicating that they have largest atomic volumes than those
of neighbouring elements while ascending portion of the curve just before the peak is occupied by
halogens showing their smallest atomic volumes. The crest of each wave is occupied by an alkali metal
and trough by an element of small chemical affinity.
2. Alkali metals occupy the peaks or crests of the curves.
3. Weak metals or elements of small chemical affinity or transition metals occupy the troughs or minima
of the curve.
4. Electronegative and gaseous volatile elements or acidic oxides forming elements are located on the
ascending portions of the curve.
5. Electropositive or transition elements or elements with high melting points are found on the
descending portions of the curve.
6. Midway of ascending portions of curve is occupied by halogens.
7. Midway of descending portions of curve is occupied by alkaline earth metals.
