Sunday, September 19, 2021

5. Periodic Classification of Elements | Class 10 CBSE | Web Notes | Part 1 | Early Attempts of classification

 5. PERIODIC CLASSIFICATION OF ELEMENTS


The elements are the substances that contain only one type of atoms.


At present, 118 elements are known. Of these, only 94 are naturally occurring.


MAKING ORDER OUT OF CHAOS – EARLY ATTEMPTS AT THE CLASSIFICATION OF ELEMENTS


Elements have earlier grouped as metals & non-metals. Other attempts are given below:


Döbereiner’s Triads


A German chemist, Johann Wolfgang Döbereiner (1817) identified some groups having three elements each. So he called these groups triads.


He showed that when the three elements in a triad are written in the order of increasing atomic masses; the atomic mass of middle element was roughly the average of the atomic masses of the other two elements. E.g.


Triad 1

Atomic mass

Li

6.9

Na

23.0

K

39.0


Triad 2

Atomic mass

Ca

40.1

Sr

87.6

Ba

137.3


Triad 3

Atomic mass

Cl

35.5

Br

79.9

I

126.9


But he could identify only 3 triads. Hence, system of classification into triads was not useful. E.g.


Elements

Atomic mass

N
P
As

14.0
31.0
74.9

Here, average of atomic mass of N & As is 44.45. So, it is not a Döbereiner triad.


Newlands’ Law of Octaves


An English scientist, John Newlands (1866) arranged the elements in the order of increasing atomic masses. He started with hydrogen (lowest atomic mass) and ended at thorium which was the 56th element.

He found that every 8th element had properties similar to that of the first. He compared this to the octaves found in music. Therefore, he called it ‘Law of Octaves’.

E.g. Sodium is 8th element after lithium. So they resemble each other. Similarly, Be & Mg resemble each other.


A part of Newlands’ Octaves


Notes of music:

sa

(do)

re

(re)

ga

(mi)

ma

(fa)

pa

(so)

da

(la)

ni

(ti)

 

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 & La

Zr


Drawbacks of Newland’s Octaves:


o  It is applicable only up to calcium. After this, every 8th element had no properties similar to that of the first.


o  Newlands assumed that only 56 elements existed in nature. But new elements were discovered which did not obey Law of Octaves. To fit elements into his Table, he adjusted two elements in the same slot, and put some unlike elements under the same note. 

    E.g. Cobalt & nickel are in the same slot and are placed in the column of F, Cl & Br. But their properties are very different. Iron resembles Co & Ni. But it was placed far away from them.


o  Discovery of noble gases made Law of Octave irrelevant. Thus, this law worked well with lighter elements only.


Select Your Next Topic 👇

👉 Part 1: Early Attempts of classification
👉 Part 2: Mendeleev's Periodic Table
👉 Part 3: Modern Periodic Table



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