PERIODIC CLASSIFICATION OF ELEMENTS

PERIODIC CLASSIFICATION OF ELEMENTS

Firstly what is element , an element is a substance containing atoms of only one type e.g Na , Au , Ca , etc . Up till now there are around 118 elements are found . Which is such a large amount and is not possible to learn about each and every single element. So, to make things easier we classified elements of same chemical property together which make study easier. In this chapter we study about the attempts to classify the elements made time to time.

DOBEREINER’S TRAID

Johann Dobereiner’s a German chemist in 1829 given “Law of Traids” where Traids refers to group of three elements.

According to the law ,When elements are arranged in the order of increasing atomic masses , groups of three elements (known as traids) , having similar chemical properties are obtained and the atomic mass of the middle element is equal to the arithmetic mean(average) of the other two elements.

1. The alkali metal group : The element Lithium , Sodium and Potassium have similar chemical properties and form a traid. e.g

(i) All these elements are metals.

(ii) All of them react with water to form alkalis and hydrogen gas.

(iii) All of them have the valency of 1 .(They are monovalent)

Alkali metals group (Dobereiner’s triads)
Elements of traids symbols Atomic masses
1.  Lithium Li 7
2.  Sodium Na 23
3.  Potassium K 39

So, according to law, the arithmetic mean of atomic masses of (Li) and (K) is equal to atomic mass of (Na).

Atomic mass of lithium = 7

Atomic mass of potassium = 39

There Arithmetic mean ​\( \begin{align*}&=\dfrac{7+39}{2}\\&=23\end{align*} \)

Hence, proved

 

2. Alkali earth metals group : The elements are calcium , strontium and barium have similar chemical properties and form a triad. e.g

(i) All these elements are metal 

(ii) Their oxides are alkaline(base) in nature.

(iii) All of these elements have a valency of 2. (They are divalent.)

Alkali earth metals group (Dobereiner’s triads)
Elements of traids symbols Atomic masses
1.  Calcium Ca 40
2.  Strontium Sr 88
3.  Barium Ba 137

So, according to law,the arithmetic mean of atomic masses of (Ca) and (Ba) is equal to atomic mass of (Sr).

Atomic mass of Calcium = 40

Atomic mass of Barium = 137

There Arithmetic mean ​\( \begin{align*}&=\dfrac{40+137}{2}\\&=88.5\end{align*} \)

Hence, proved

[Note: Ca,Sr and Ba are called alkaline earth metals because their oxides are alkaline(base) in nature and are existed in earth.]

 

3. The Halogen group : The elements are chlorine, bromine and iodine have similar chemical properties and form a triad. e.g

(i) All these elements are non-metal .

(ii) All these elements reacts with water to form acid.

(iii) All of these elements have a valency of 1. (They are monovalent.)

The halogen group (Dobereiner’s triads)
Elements of traids symbols Atomic masses
1.  Chlorine Cl 35.5
2.  Bromine Br 80
3.  Iodine I 127

So, according to law,the arithmetic mean of atomic masses of (Cl) and (I) is equal to atomic mass of (Br).

Atomic mass of Chlorine = 35.5

Atomic mass of Iodine = 127

There Arithmetic mean ​\( \begin{align*}&=\dfrac{35.5+127}{2}\\&=81.2\end{align*} \)

Which is approximately equal to actual atomic mass of Bromine.

[Note: Cl, Br, I are called Halogen group because they reacts with metal to form salts .(halo=salt and gene=producer)]

 

LIMITATIONS OF DOBEREINER’S LAW OF TRAIDS 

Only three traids were identified from all of the elements. Which means it fails to arrange all the elements having similar chemical properties.

 

NEWLAND’S LAW OF OCTAVES

In 1864 , Newland gives his “Law of octaves” . According to Newland’s law of octaves : When elements are arranged in order of their increasing atomic masses, the properties of the every eighth element similar to that of the first element just like the repetition of eighth note in octaves of music.

Newland divided the elements in horizontal rows and vertical columns known at that time.

Classification of elements based on Newland’s Law of octaves
H Li Be B C N O
F Na Mg Al Si P S
Cl K Ca Cr Ti Mn Fe
Co and Ni Cu Zn Y In As Se
Br Ru Sr Ce and La Zr _ _

For illustration , if we take Li as first element then Na is eighth element and if we take Na as first element then K is the eighth element which means Li , Na and K are having similar chemical properties.

Another example is, if we take Be as first element then Mg is eighth element and if we take Be as first element then Ca is the eighth element which means Be , Mg and Ca are having similar chemical properties.

 

LIMITATIONS OF NEWLANDS LAW OF OCTAVES

1. Newland’s law of octaves only applicable up to lighter elements which means it is only applicable up to calcium only.

2. Newland assumed that only 56 elements existed in nature and no more elements would be discovered in future. But later on more elements are discovered which does not followed the property of law of octaves.

3. Iron(Fe) is placed far away from Cobalt(Co) and Nickle(Ni) elements, even if they both possess same property.

 

PERIODIC TABLE : A periodic table is a chart of elements placed in such a way that the elements with similar chemical properties occur in same vertical column or group.

A periodic table consist of Horizontal rows of elements called periods and a vertical column called group.

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