Classification of elements and periodicity of properties
Tuesday 29 August 2017
Classification of elements :-
As the number of elements are very much in counting and many resembles and exhibit same properties so studying all without any classification will be a difficult task so to make study easier classification was done.
many attempts were made for classifying the elements some are given below:-
Prout's Hypothesis (1815) :-
It is also known as unitary method according to it hydrogen atom was considered as the fundamental unit from which all other atoms were made.
Dobereiner's Triads (1829) :-
Dobereiner classified elements into group of 3 of similar properties in which atomic weight of middle was arithmetic mean of other two.
Dobereiner could not able to arrange all elements he had found only three triads so this theory failed.
Newland Octaves (1864 law of octaves) :-
It states that when elements are arranged in order of increasing atomic masses,every eight element has properties similar to the first just like in the musical note.
Limitations:-
This classification was not successfull beyond calcium
After the discovery of noble gases their arrangement disturbed the whole configuration
Lother Meyer's Atomic volume curve (1869) :-
It classified the elements in the form of a curve between atomic volume and atomic masses and state that properties of elements are the periodic functions of their atomic masses & concluded that elements with similar properties occupy similar position in the curve.
Mendeleef Periodic Table :-
The physical and chemical properties of the elements are a periodic function of their atomic masses and their were only 63 elements discovered at the time of Mendeleev. The periodic table is suppose to consist of seven horizontal periods and eight vertical columns,zero group added later in the Mendeleev periodic table.
Importance of Mendeleev periodic law :-
a)He has done a proper and systematic arrangement of elements.
b)He left the space for some undicovered elements e.g: he left the space for Ga and Ge and named these elements as ERa-Aluminium(Ga) and EKa-silicon (Ge)
c)He has also done atomic mass correction of doubtful elements on the basis of their expected positions and properties.
Defects of Mendeleev periodic Table :-
a)Position of hydrogen :-
Position of hydrogen is doubtfull as it is placed in group IA(Alkali metal) but it also shows resemblance with halogens of group VIIA,so its position in Mendleev periodic table is doubtfull
b)Position of Isotopes :-
As Mendeleev periodic table was based on atomic weight so isotopes of an element should occupy different positions which is not true.
c)Anomalous positions of some elements :-
In some situation elements with higher atomic mass precedes the element with lower atomic mass like AL atomic weight=39.9 precedes K which is having atomic weight 39.1
d)Position of Lanthanoids and actinoids :-
They both are not placed in main periodic table
odern Periodic Law :- It states that " Physical and chemical properties of elements are the periodic function of their atomic numbers " He also observed that when elements are arranged in order of increasing atomic numbers their properties repeats after some regular intervals like 0,1,2,8,8,18 and 32.These numbers are called magic numbers and cause perodicicty due to repetition of similar electronic configuration. Features of long form of Periodic table :- a)It has 18 groups and 7 periods .
Features of long form of Periodic table :-
It has 18 groups and 7 periods
First period(H-He) conatins 2 elements and it is the smallest period.
Second Period(Li-Ne) contains 8 elements
Third Period(Na-Ar) contains 8 elements
Fourth Period(K-Kr) contains 18 elements
Fifth Period(Rb-Xe) contains 18 elements
Sixth Period(Cs-Rn) contains 32 elements
Seventh Period(Fr) contains 19 elements,it is incomplete
elements of group 1 are called alkali metals ,group 2 are called alkaline earth metals ,elements of group 16 are called chalcogens,group 17 are called halogens and group 18 are called noble gases
As the number of elements are very much in counting and many resembles and exhibit same properties so studying all without any classification will be a difficult task so to make study easier classification was done.
many attempts were made for classifying the elements some are given below:-
Prout's Hypothesis (1815) :-
It is also known as unitary method according to it hydrogen atom was considered as the fundamental unit from which all other atoms were made.
Dobereiner's Triads (1829) :-
Dobereiner classified elements into group of 3 of similar properties in which atomic weight of middle was arithmetic mean of other two.
Dobereiner could not able to arrange all elements he had found only three triads so this theory failed.
Newland Octaves (1864 law of octaves) :-
It states that when elements are arranged in order of increasing atomic masses,every eight element has properties similar to the first just like in the musical note.
Limitations:-
This classification was not successfull beyond calcium
After the discovery of noble gases their arrangement disturbed the whole configuration
Lother Meyer's Atomic volume curve (1869) :-
It classified the elements in the form of a curve between atomic volume and atomic masses and state that properties of elements are the periodic functions of their atomic masses & concluded that elements with similar properties occupy similar position in the curve.
Mendeleef Periodic Table :-
The physical and chemical properties of the elements are a periodic function of their atomic masses and their were only 63 elements discovered at the time of Mendeleev. The periodic table is suppose to consist of seven horizontal periods and eight vertical columns,zero group added later in the Mendeleev periodic table.
Importance of Mendeleev periodic law :-
a)He has done a proper and systematic arrangement of elements.
b)He left the space for some undicovered elements e.g: he left the space for Ga and Ge and named these elements as ERa-Aluminium(Ga) and EKa-silicon (Ge)
c)He has also done atomic mass correction of doubtful elements on the basis of their expected positions and properties.
Defects of Mendeleev periodic Table :-
a)Position of hydrogen :-
Position of hydrogen is doubtfull as it is placed in group IA(Alkali metal) but it also shows resemblance with halogens of group VIIA,so its position in Mendleev periodic table is doubtfull
b)Position of Isotopes :-
As Mendeleev periodic table was based on atomic weight so isotopes of an element should occupy different positions which is not true.
c)Anomalous positions of some elements :-
In some situation elements with higher atomic mass precedes the element with lower atomic mass like AL atomic weight=39.9 precedes K which is having atomic weight 39.1
d)Position of Lanthanoids and actinoids :-
They both are not placed in main periodic table
odern Periodic Law :- It states that " Physical and chemical properties of elements are the periodic function of their atomic numbers " He also observed that when elements are arranged in order of increasing atomic numbers their properties repeats after some regular intervals like 0,1,2,8,8,18 and 32.These numbers are called magic numbers and cause perodicicty due to repetition of similar electronic configuration. Features of long form of Periodic table :- a)It has 18 groups and 7 periods .
Features of long form of Periodic table :-
It has 18 groups and 7 periods
First period(H-He) conatins 2 elements and it is the smallest period.
Second Period(Li-Ne) contains 8 elements
Third Period(Na-Ar) contains 8 elements
Fourth Period(K-Kr) contains 18 elements
Fifth Period(Rb-Xe) contains 18 elements
Sixth Period(Cs-Rn) contains 32 elements
Seventh Period(Fr) contains 19 elements,it is incomplete
elements of group 1 are called alkali metals ,group 2 are called alkaline earth metals ,elements of group 16 are called chalcogens,group 17 are called halogens and group 18 are called noble gases
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