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PERIODIC CLASSIFICATION
INTRODUCTION
Till to-day, 117 elements have been discovered. It is quite difficult to remember the properties of all
the elements and their compounds. To solve this problem, attempts were made from time to time to
group the elements with similar properties. Thus, knowing the properties of one element, the properties
of the other elements in a group can be known.
MENDELEEV’S CLASSIFICATION OF ELEMENTS
Mandeleef, a Russian chemist gave the periodic table in the more complete form. He even predicted the
properties and existence of the elements which were not discovered then. At that time, only 63
elements were known. Mandeleef gave the Periodic Law.
“The properties of the elements are the periodic function of their atomic masses”. He arranged all
these elements in order of their increasing atomic masses at the spaces where their properties suited
them.
The properties of an element is periodic function of its atomic number.
ADVANTAGES OF THE MENDELEEV’S PERIODIC TABLE
When Mandeleef published his periodic table, only 63 elements were known. He arranged the
known elements in the periodic table in order of their increasing atomic masses.
Some of the advantages of this table are :
(i) Prediction of New Elements – In Mandeleeff’s table, there were many vacant spaces. Mandeleef
even predicted the properties of the elements that would occupy vacant places as and when
discovered. When a few elements were later discovered, then they were found to have exactly
similar properties as told by Mandeleeff according to their positions in the table.
(ii) Correction of Atomic Mass – We know that the elements present in a particular group (vertical
column) are identical in chemical properties. On this basis, Mandeleef placed Be in the second
group along with Mg, Ca, Sr and Ba. These are all divalent. The atomic mass of Beryllium was
found to be 13.5 and it should be placed after carbon whose atomic mass is 12. Later, Mandeleef
determined its equivalent mass. Thus, he determined the exact atomic mass by multiplying the
equivalent mass by 2 since it was found to be a divalent metal. The exact atomic mass of Be was
found to be 9.
(iii)Atomic Structure – It helped in making comprehensive study of the electronic configurations of
the elements.
Defects in the Mendeleev’s Periodic Table – Some of the defects are as under:
(i) Anomalous Position – When elements are arranged in order of ascending atomic masses,
Potassium should precede Argon, Nickel should precede Cobalt and Iodine should precede
Tellurium. These elements, however, are placed in the most suitable positions in the table as far
as their chemical properties go.
(ii) Position of Triads – The elements in the triads like Fe, Co, Ni, are so similar in properties that
these should be placed in one group. In Mendeleev’s table, the space for one element is occupied
by three elements. This is against the periodic law.
(iii)Dissimilar Elements Placed Together – Alkali metals and coinage metals differ much in their
properties. But these are placed in group 1. This difficulty was of course, overcome by dividing
the group into two sub-groups.
(iv)Position of Lanthanides and Actinides – Lanthanides and Actinides consist of a group of 14
elements each. 14 elements have been assigned one place in the periodic table which goes
against the periodic law.
(v) Atomic Mass – Mendeleev’s table formed on the basis of atomic mass does not give an idea of
the structure of an atom.
(vi)Position of Isotopes – Mendeleev’s periodic table is silent about the positions of the Isotopes.
MODEN PERIODC LAW
In order to explain the drawbacks in Mendeleev’s periodic table, two possibilities strike our mind :
(i) Perhaps the arrangement of Mendeleev’s table is not a satisfactory one.
(ii) Perhaps the atomic mass is not a fundamental property upon which this table was based.
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PERIODIC CLASSIFICATION
Modern Periodic Law :
“The physical and chemical properties of elements are the periodic function of their atomic
numbers.” i.e., when elements are arranged in the increasing order of their atomic number, similar
elements are repeated after regular intervals.
Modified Mendeleev’s Periodic Table – On the basis of this law, the Mendeleev’s periodic table was
modified. The elements were arranged in order of their increasing atomic numbers instead of atomic
masses. This periodic table removed anomalous cases of original Mendeleev’s table. For example –
(a) Argon (at. no. 18, at. mass = 40) precedes potassium (at. no. =19; at mass = 39)
(b) Cobalt (at. no. = 27;at mass = 58.9) precedes Nickel (at. no. = 28 at. mass = 59)
(c) Tellurium (at. no. = 52, at mass = 127.6) precedes Iodine (at. no. =53, at. mass = 127)
LONG AND EXTENDED FORM OF PERIODIC TABLE
The long form of the periodic table is based on the Modern periodic law. This table has the following
features :
1. All the elements have been arranged in the increasing order of their atomic number. The
arrangement consist of :
(i) Seven horizontal rows, called periods or series.
(ii) Eighteen vertical columns, called groups or families.
Periods – The number of elements in each period are shown in the following table :
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PERIODIC CLASSIFICATION
Groups – The number of different groups are based upon the original groups assigned by
Mendeleev. These are :
(i) IA, IIA, IIIA, IVA, VA, VIA, VIIA
(ii) IB, IIB, IIIB, IVB, VB, VIB, VIIB
(iii)VIIIB. It consists of three vertical columns
(iv)Zero group. It consists of inert gases.
Horizontal Rows at the Bottom of the Periodic Table – In the sixth period, the elements with atomic
numbers 58 to 71 have been grouped with Lanthanum (At. No. 57) and are called Lanthanides or
Rare earths. Similarly in the seventh period, elements with atomic numbers 90 to 103, have been
grouped with Actinium (At. no. 89) and these are called Actinides.
It is due to two reasons :
(i) Elements with At. No 57 resemble elements with At. No. 58 to 71 in their properties. Also
elements with At. No. 89 resemble elements with At. No. 90 to 103 in their properties.
(ii) The undue expansion of the periodic table is avoided.
PERIODICITY
The word periodicity means recurring at regular interval. In modern periodic table, similar elements
get repeated after regular intervals. These regular intervals are 2, 8, 8, 18, 18, 32, and are called
“magic numbers.”
The elements which are repeated after regular intervals have similar electronic configurations. They
have same number of electrons in the valence shell. The properties of an element depend upon its
atomic number and hence on its electronic configuration. Thus, the cause of periodicity of elements
is the repetition of similar electronic configurations after certain regular intervals.
1. Find elements with similar properties.
Example : Let us find elements resembling hydrogen (At. No. = 1)
Explanation : All alkali metals have exactly similar electronic configuration properties. Hence placed in the
same group. Each element has one electron in the ns-subshell.
n = The period to which the element belongs
2 1
Li = 1s 2s ;
3
2 2 6 1
Na = 1s 2s 2p 3s
19
K = 1s2 2s2 2p6 3s2 3p6 4s1 and so on.
1. Determine the group to which the element belongs.
2. Determine the period to which the element belongs.
Example : The element having atomic number 24 belongs to which group and period?
Consider the number series 1 3 1 19 37 55 87
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PERIODIC CLASSIFICATION
I II III IV V VI VII
The asking atomic number occurs between which pair of numbers in the series indicate its belonging
th
period. The atomic number 24 occurs between 19-37, hence belong to 4 period.
th
The atomic number 19 belong to first group. So atomic number 24 occurs in 6 group.
STRUCTURAL FEATURES OF LONG FORM OF THE PERIODIC TABLE
The table is based on the Modern Periodic Law. It is an improved form of the Mendeleev’s periodic table.
This table has the following features.
1. All the elements have been arranged in the increasing order of atomic numbers.
2. Elements with similar electronic configurations have similar properties and hence have been placed
together at one place. Elements with different electronic configurations have different properties and
hence have been placed at different places in the periodic table.
The periodic table consists of :
(i) Seven horizontal rows called periods.
(ii) Eighteen vertical columns called groups or families.
(iii) Four blocks.
Moreover, the modern periodic table is considered to be an expanding spiral from top to bottom.
PERIODS
The horizontal rows from left to right in the periodic table are called periods.
(a) First period contains only two elements ( H to He) : This period corresponds to first main energy
1 2
level whose capacity is only of two electrons. Hence, only two different elements in which one and two
1 2
electrons are present in first energy level are possible. H(1s ), He(1s ).
(b) Second period contains eight elements Li to Ne : It corresponds to the second main energy level
G 10
2 6
(2s 2p ) whose capacity is of eight electrons and hence, only eight elements occur in this period.
(c) Third Period contains eight elements ( Na to Ar) : It corresponds to third shell (3s2 3p6 3d10). It is known
11 18
that 3d orbitals are higher in energy than 4s orbital. As a result, these 3d orbitals are filed after filling
4s orbitals. Hence, this period involves the filling of only 3s and 3p-orbitals. Therefore, it contains eight
elements and not eighteen elements.
(d) Fourth period contains eighteen elements (19K to 36Kr) : This period corresponds to fourth main
energy level. It starts with the element which receives electrons in 4s orbital. After filling 4s orbital,
the filling of 3d and then 4p takes place. It is so because energy of 3d orbital is in-between the 4s and
4p-orbitals. As 4s, 3d and 4p can have 2, 10 and 6 electrons respectively, therefore, eighteen electrons
are present in this period. The 4d and 4f orbitals are higher in energy than 5s and are filled up in the next
periods.
(e) Seventh period contains 31 elements (87 – 117) : These elements are radioactive. Upto U, the
92
elements are naturally occurring and the remaining elements are artificially prepared. Hence, the
elements having atomic number higher than 92 are known as synthetic elements. The
relationship between electron filling of orbitals and the number of elements in a period is
summarized on next page table.
GROUPS
Vertical columns in the periodic table starting from top to bottom are called groups or families.
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