CHAPTER 4 :
PERIODIC TABLE OF ELEMENTS
4.1 The Periodic Table of Elements
Prepared by:
KUMARI PARONJODI
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Historical Development
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PERIODIC TABLE
OF ELEMENTS
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Historical Development of
The Periodic Table
Antoine Lavoisier (1743-1794)
Johann W. Dobereiner (1780-1849)
John Newlands (1837-1898)
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Lothar Meyer (1830-1895)
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Dmitri Mendeleev (1839-1907)
H. J. G. Moseley (1887-1915)
Antoine Lavoisier (1743-1794)
A French chemist, was the first person to classify
elements into groups.
In the year 1789, the known elements at that time were
classified into four groups as shown in table 1.
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In his table, elements were classified into metals and
non-metals.
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Classification by Lavoisier was unsuccessful because his
table consisted of many wrong information. For
example, light, heat and a few compound which were
unable to be decomposed at that time such as lime,
silica, alumina, barita and magnesia were considered as
elements.
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Group 1
Oxygen
Nitrogen
Hydrogen
Light
Heat
Group 2
Sulphur
Phosphorus
Carbon
Chlorine
Flourine
Group 3
Arsenic
Bismuth
Cobalt
Lead
Zinc
Nickel
Tin
Silver
Group 4
Lime
Silica
Alumina
Barita
Magnesia
Table 1 Classification of the elements by Lavoisier
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Johann W. Dobereiner (1780-1849)
In the year 1829, Johann W. Dobereiner, a German
chemist, divided the elements into groups. Each group
consisted of three elements with similar chemical
properties. He named each of these groups as triad.
In each triad, the relative atomic mass of the middle
element was approximately the average relative atomic
mass of the other two elements. Two examples of
triads are shown in table 2.
Classification of the elements into triads by
Dobereiner
was
unsuccessful
because
this
classification was limited to a few elements only.
However, this Triad Law had awakened other chemist
to realise that there was a relationship between the
chemical properties and the atomic mass of the
elements.
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Elements in
triad
Relative atomic
mass
Elements in
triad
Relative atomic
mass
Lithium
Sodium
7
23
29
Chlorine
Bromine
35
80
Table 2
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Potassium Average relative atomic of
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lithium and potassium
= ( 7+39 ) / 2
= 23
Iodine Average relative atomic of
127
chlorine and iodine
= ( 35+127 ) / 2
= 81
Triad
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John Newlands (1837-1898)
In the year 1863, John Newlands, a British chemist,
arranged all the known elements according to the
ascending order of their atomic masses.
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Table 3 shows part of the arrangement of the elements
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suggested by Newlands.
1
2
3
4
5
6
7
H
Li
Be
B
C
N
O
F
Na
Mg
Al
Si
P
S
Cl
K
Ca
Cr
Ti
Mn
Fe
Co,Ni
Cu
Zn
Y
In
As
Se
Br
Rb
Sr
Ce,Ia
Pd
Ag
Cd
U
Zr Di,Mo Ro,Ru
Sn
Sb
Te
Table 3 Arrangement of the elements by Newlands
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He found that the same properties were repeated at
every eight element in his arrangement. This pattern
was similar to the octave notes in music. This
arrangement of elements was known as the Law of
Octaves.
Contribution by Newlands failed because his Law of
Octaves was obeyed by the first 17 elements only.
His contribution is important because he is the first
chemist to show the existence of a periodic pattern
for the properties of elements.
The periodic repetition of the properties of elements
was used as a basis for further developments of the
Periodic Table.
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Lothar Meyer (1830-1895)
In the year 1870, Lothar Meyer, a German chemist,
plotted a graph of the atomic volume against the atomic
mass for all the known elements as in figure 1. (The
atomic volume of an element is the volume of one mole
atom of that element).
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He realised that elements with similar chemical
properties occupied the same relative positions on the
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atomic volume curve.
For example:
Li, Na, K, Rb and Cs (alkali metals) located at the
maximum points of the curve have similar chemical Advantages
properties.
F, Cl, Br and I (halogens) located at the slopes of the Questions
curve also have similar chemical properties.
Meyer was successful in showing that the properties of
the elements were in a periodic pattern with their
atomic masses.
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Figure 2
Atomic volume curve
Dmitri mendeleev (1839-1907)
A Russian chemistry professor, had shown that the properties
of elements changed periodically with their atomic masses.
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In the year 1869, he arranged the elements in the same way
as Newlands but made a few changes as below.
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a) elements with similar chemical properties were placed
in the same column called group.
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b) empty spaces were left in the Periodic Table for those
undiscovered elements at the time.
c) he made use of the positions of elements in the Periodic
Table to predict the properties of undiscovered elements at
that time. Table 4 shows the comparison of the properties
of ‘Ekasilicon’ as predicted by Mendeleev with the properties
of germanium that was discovered in the year 1886.
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Table 4 Prediction of properties of germanium by Mendeleev
Property
Atomic mass
Colour of metal
Density
Formula of
oxide
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Ekasilicon (Es)
Germanium (Ge)
72
72.6
Grey
Grey
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5.5 g/cm³
5.47 g/cm³
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EsO2
GeO2
d) He mutually changed the positions of two pairs of
elements, that were nickel (atomic mass = 58.7) with
cobalt (atomic mass = 58.9) and iodine (atomic mass =
126.9) with tellurium (atomic mass = 127.6) so that the
elements with similar chemical properties were placed
under the same group.
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e) Arranged certain elements such as manganese,
iron, cobalt and nickel in separate groups.
These groups of elements were known as
transition elements.
His Periodic Table was used as the basis for the
formation of the Periodic Table today.
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H. J. G. Moseley (1887-1915)
In the year 1914, H. J. G. Moseley, a British physicist,
investigated the X-ray spectrum of elements.
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He plotted a graph of the square root of the frequency
of X-ray from the elements against their proton Relationship
numbers. A straight line was obtained.
The proton numbers should be used as a basis for the
periodic changes in the chemical properties of elements.
Therefore, Moseley arranged the elements in increasing
order of their proton numbers. He produced a Periodic
Table similar to Mendeleev’s Periodic Table.
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In Moseley’s Periodic Table, he confirmed that tellurium
(Te) must be placed before iodine (I) and cobalt (Co)
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must be placed before nickel (Ni) as predicted by
Mendeleev.
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Hence, Moseley was successful in developing the
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Periodic Table based on the arrangement of elements in
ascending order of their proton numbers.
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Arrangement of Elements in
the Periodic Table
Figure 3 Shows the Periodic Table.
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Elements are arranged horizontally in ascending order
of their proton numbers in the periodic table.
Each vertical column of elements in the periodic table
is known as a group. Elements with the same number of
valence electrons are arranged in the same group.
There are 18 vertical columns of elements in the
periodic table, known as Group 1, Group 2, Group 3 until
Group 18.
Group 1 elements are known as alkali metals. Group 2
elements are known as alkali earth metals. Group 3 to
Group 12 elements are known as transition elements.
Group 17 elements are known as halogens. Group 18
elements are known as noble gases.
Each horizontal row of elements in the periodic table is
known as a period.
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There are 7 horizontal rows in the periodic table,
known as Period 1, Period 2, Period 3 until Period 7.
Period 1 – 2 elements,
Periods 2 & 3 – 8 elements respectively.
Periods 4 & 5 – 18 elements each.
Periods 6 – 32 elements.
Period 7 – 23 elements.
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Periods 1 to 3 are short periods while Periods 4 to 7 are
long periods.
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Although Period 6 contains 32 elements, elements with
proton numbers 58 to 71 are listed separately at the
bottom of the periodic table. This series of elements is
known as lantanides.
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Similarly, elements with proton numbers 90 to 103 in
Period 7 are listed separately at the bottom of the
Periodic Table. This series of elements is known as
actinides.
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Elements in Groups 1, 2 and 13 are metals.
Transition elements in Groups 3 to 12 are also metals.
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Non-metals usually have 5, 6 or 7 valence electrons.
They are placed in Groups 15, 16 and 17 respectively in Relationship
the Periodic Table. Carbon and silicon from Group 14 are
also non-metals.
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Relationship Between the electron
Arrangement and the Position of the
Element in the Periodic Table
Based on Figure 4, we can deduce that the number of
valence electrons in an atom of an element determines
the position of the group of that element in the Periodic
Table.
Group
Period
1
18
1
1H
2
13
14
15
16
17
2He
2
3Li
4Be
5B
6C
7N
8O
9F
10Ne
3
11Na
12Mg
13Al
14Si
15P
16S
17Cl
18Ar
19K
20Ca
1
2
3
4
4
Figure 4
Elements with proton numbers 1 to 20
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Remember
For elements with 1 or 2 valence electrons, the group
number of that element is equal to the number of
valence electrons.
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For elements with 3 to 8 valence electrons, the group
number of that element is equal to the number of
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valence electrons plus 10.
Number of
valence
electrons
1
2
3
4
5
6
7
8
(Except
Helium)
Group
1
2
13
14
15
16
17
18
Table 5 Relationship between the number of valence electrons
and the position of the group of an element
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Examples
Element R has a proton number of 16. In which group
is element R located in the periodic Table?
SOLUTION:
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Number of electrons in atom R
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= number of proton
= proton number
= 16
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 Electron arrangement of atom R = 2.8.6
 Number of valence electrons = 6
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Hence, element R is located in Group 16 of the Periodic
Table.
The numbers of shells occupied with electrons in an
atom determines the position of the period of that
element in the Periodic Table.
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Number of shells
occupied with
electrons
1
2
3
4
5
6
7
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Period
1
2
13
14
15
16
17
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Table 5 Relationship between the number of shells occupied
with electrons and the position of the period of an
element.
Hence, the position of the period of an element is equal
to the number of shells occupied with electrons in the
atom of that element.
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Examples
An atom of element T has 16 neutrons and a nucleon
number of 31. In which period is element T located in
the periodic table?
SOLUTION:
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Number of electrons in atom T
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= number of protons in atom T
= nucleon number – number of neutrons
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= 31 – 16
= 15
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 Electron arrangement of atom T = 2.8.5
Atom T has 3 shells occupied with electrons.
Hence, element T is located in period 3 of the Periodic
table.
Elements with the same number of valence electron will
exhibit similar chemical properties.
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Example:
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Atom X with an electron arrangement of 2.8.2 and atom Relationship
Y with an electron arrangement of 2.8.8.2 exhibit
similar chemical properties because both atoms have 2
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valence electrons.
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Advantages Of Classifying Elements
in the Periodic Table
Enables chemist to learn and understand physical
properties and chemical properties of the elements
and compounds more systematically, orderly and
easily.
The properties of an element and its compounds can
be predicted based on its position in the Periodic
Table.
It becomes easier to study and understand the
relationship among the elements from different
groups.
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