Chapter classification of elements and periodicity in properties

CLASSIFICATION OF ELEMENTS AND PERIODICITY IN  PROPERTIES

Brief Historical Development Of Periodic Table

1.Johann Dobereiner’s (Law of triads)

In between 1815 and 1829 he gave law of triads.According to which group of three elements which posses similar chemical properties, then the central element mass is equal to the arithmetic mean of atomic masses of the other two elements.

Predicted vs actual atomic mass of the central atom of each triad
Triad name Elements and atomic masses
Element 1
Mass
Element 2
Mean of 1 and 3
Actual mass
Element 3
Mass
Alkali-forming elements Lithium
6.94
Sodium
23.02
22.99
Potassium
39.10
Alkaline-earth-forming elements Calcium
40.1
Strontium
88.7
87.6
Barium
137.3
Salt-forming elements Chlorine
35.470
Bromine
80.470
78.383
Iodine
126.470
Acid-forming elements Sulphur
32.239
Selenium
80.741
79.263
Tellurium
129.243

The main drawback of this classification is that the concept is not applicable to all the known elements but only to a limited numbers of elements

2.De-Chancourtois (telluric helix)

A French geologist , he arranged the elements in the order of their atomic weights along a helix which was traced on the surface of a vertical cylinder, with an angle of 45 degrees to its axis. The base of the cylinder was divided into 16 equal parts (the atomic weight of oxygen), and the lengths of the spiral corresponding to the weights of the elements were found by taking the one-sixteenth part of a complete turn as a unit. It is seen that analogous elements fall on approximately the same perpendicular. Tellurium was situated at the centre, prompting the name “vis tellurique”, or “telluric screw”.This classification is accepted because it was ignored by chemists as it was written in terms of geology.He is the first person who try to arrange elements on the basis of their atomic masses.

3.Newland (Law of octaves)

He is the first one who tries to arrange all known elements in 1864.According to his law of octaves, the elements with similar physical and chemical properties repeat after each interval of seven elements when arranged in increasing order of their atomic masses.

Newland’s Periodic Table
H
F
Cl
Co & Ni
Br
Pd
I
Pt & Ir
Li
Na
K
Cu
Rb
Ag
Cs
Os
G
Mg
Ca
Zn
Sr
Cd
Ba & V
Hg
Bo
Al
Cr
Y
Ce & La
U
Ta
Tl
C
Si
Ti
In
Zr
Sn
W
Pb
N
P
Mn
As
Di & Mo
Sb
Nb
Bi
O
S
Fe
Se
Ro & Ru
Te
Au
Th

This system worked well for the lighter elements example H, F and Cl show similar properties and similarly Li,Na and K show similar properties.But the system fails in case of heavy elements like manganese placed with nitrogen and phosphorus or iron is placed with oxygen and sulphur (dissimilar elements have been grouped together which is against the system of classification)

Also, no space is left for the unknown elements which is not discovered at that time.

Many elements placed at wrong position on the account of wrong values of atomic masses in this classification.

Thus , his work is not accepted as an appropriate arrangement.

4.Lother Meyer’s curve

Lother Meyer presented the classification in the form of curves.He plotted atomic volumes against atomic masses, a curve taking the form of sharp peaks and broad minima is obtained.

Atomic volume =​\( \dfrac{Atomic~mass}{Density} \)

Properties of Lother Meyer’s curve

(i) Elements having similar properties occupy similar positions on the curve.

(ii) Alkaline metals,[ lithium (Li), sodium (Na), potassium (K),rubidium (Rb), cesium (Cs), and francium (Fr) ] having the largest atomic volumes, occupy  the maxima  of the curve.

(iii) The alkaline earth metal (Mg , Ca , Sr, Ba) occupy positions of about the mid points on the descending portions of the curve.

(iv) The halogens (F, Cl , Br , I) occupy positions on the ascending portions of the curve before inert gases.

(v) The transition elements occupy minima of the curve.

[Note: The same graph is obtained if we replace atomic mass with atomic number ]

Although it is an ideal classification but lacked practical utility as it is very hard to learn exact position of the elements in the graph.

 

5.Mendeleev’s Periodic Law And Original Periodic Table

Mendeleev periodic law is  “the properties of the elements are periodic functions of their atomic numbers. Also called Mendeleev’s law. (originally) the statement that the chemical and physical properties of the elements recur periodically when the elements are arranged in the order of their atomic weights.

Main points from  the Mendeleev’s periodic law

(i) The elements, if arranged according to their atomic weights, exhibit an evident periodicity of properties.

(ii) Elements which are similar as regards their chemical properties have atomic weights which either are of nearly same value e.g., Fe, Co, Ni , or increasing regularly e.g., K , Rb , Cs.

(iii) The arrangement of elements or groups of elements in the order of their atomic weights corresponds to their so called “valencies” as well as their distinctive properties.

(iv) The elements which are most widely distributed in nature have small atomic weights and posses sharply defined properties.

(v) The magnitude of atomic weights determines the character of the elements.

(vi) Many yet unknown elements may be discovered.

(vii) The atomic weight of an element may sometimes be corrected with the aid of the knowledge of the atomic weights of the adjacent elements.

(viii) Certain characteristic properties of elements can be foretold from their atomic weights.

 

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