Classification of elements
There is need to classify the elements present in the nature so that we can easily study the properties of the elements and predict the new elements
The arrangement of elements in a systematic way such that similar elements falls within the same group is called the classification of elements
History of Classification Of elements
DOBEREINER’S TRAIDS
He arranged elements with similar properties in groups of three in such a way that mass of middle element is the average mass of 1
st and 3
rd element.
Triad
|
Atomics Masses
|
Arithmetic mean of first and second elements
|
Cl, Br, I
|
35.5,80,127
|
(35.5+127)/2=81.25
|
Ca, Sr, Ba
|
40,88,137
|
(40+137)/2=88.5
|
However, this law of triads can be applied to limited number of elements so it was dismissed
The Law of Octaves
In 1865, Newlands arrange the elements in increasing order of atomic masses and found that eight elements have similar properties to the first element. The relationship was just like the eighth note in an octave of music
Element
|
Li
|
Be
|
B
|
C
|
N
|
O
|
F
|
Atomic Mass
|
7
|
9
|
11
|
12
|
14
|
16
|
19
|
Elements
|
Na
|
Mg
|
Al
|
Si
|
P
|
S
|
Cl
|
Atomic Mass
|
23
|
24
|
27
|
29
|
31
|
32
|
35.5
|
Element
|
K
|
Ca
|
|
|
|
|
|
Atomic Mass
|
39
|
40
|
|
|
|
|
|
However, this law cannot be applied beyond Calcium so it was also dismissed
Mendeleev’s Periodic Law
It states that the properties of the elements are the periodic function of their atomic masses.
Based on this, Mendeleev’s constructed a periodic table. In the table, elements are arranged in increasing order of the atomic masses in horizontal rows and vertical columns such that elements with similar properties occupies the same vertical column. Mendeleev’s relies of broad range of physical and chemical properties to classify the elements
He also found if atomic mass order is strictly followed , then some of elements does not fit in with this scheme of classification. He ignored the order of atomic masses thinking it could be wrong because of measurement and arrange the similar elements in the same group
Advantages of Mendeleev Periodic Table
Mendeleev left few gaps for the elements yet to be discovered. Thus, if a certain new element is discovered, it can be placed in a new group without disturbing any existing group.
Disadvantages of Mendeleev Periodic Table
a) He was unable to locate hydrogen in the periodic table.
b) Increase in atomic mass was not regular while moving from one element to another. Hence, the number of elements yet to be discovered was not predictable.
c) isotopes of elements which violated Mendeleev’s periodic law.
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