Modern Periodic Classification of Elements




Written by Ziyyara 29-04-2020

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Modern Periodic Classification of Elements


As we all know that whole universe constitute chemistry in it. In the form of smallest unit that is atom, and after it comes to molecules and modern periodic classification of elements. The entire things which are there in universe are made up of element. So to understand the chemistry around us first there is a need to understand its basic unit that is element.

Anything can be understood well if there is a proper system. And to understand the Classification of Elements there physical and chemical properties there is a need of classification.

So, to understand the properties of elements we have classified modern periodic classification of elements. Various scientists have done their contribution in this regard with the elements of their time. As there is on-going discovery of element taking place till now as well. That’s why modern periodic classification of elements has important stand in chemistry.

Now let us begin the journey with the ancient to recent one classification.


Dobereiner's triads in Periodic Classification of Elements

In Dobereiner's triads:- In 1817, a chemist decided to manage element in a table of a triad in which he arranged elements in the order of atomic masses that the atomic mass of the middle element is the average of first and third element in the triad.

Group A element Atomic masses
Nitrogen 14.0
Phosphorus 31.0
Arsenic 74.09
Group B element Atomic masses
Calcium 40.1
Strontium 87.6
Barium 137.3

(In first triad phosphorus mass is average of nitrogen and arsenic mass; in second strontium has atomic mass which is a average of calcium and barium)

Limitation of Dobereiner Triads in Periodic Classification of Elements Lesson

The limitations of Dobereiner’s triads are as follows:-
(i) The discovery of new element has ruled out the triads law
(ii) Newly discovered elements are not satisfying the rule.
(iii) Only 5 Dobereiner’s triads were identified.
(iv) Several of the known elements were not fit into the triad.
After knowing these limitations, the new classification development had occurred.


Newland’s Octaves law in Periodic Classification of Elements 10th Class

This idea of the periodic table consists of the idea of the law of octaves of the music of West like (do, re, mi, Fa, so, la, ti. Comparatively sa, re, Ga, ma, pa, Dha, ni (in Indian style way), according to his idea like the musical notes repeat itself in the same way in classification every eighth element in the order of their increasing atomic masses have its property repetition with the first element.


Indian and Western Musical Octaves for Element’s Table

SA Re Ga Ma Pa Dha Ni
Do Re Mi Fa So La Ti
H Le Bi 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


Limitations of Octaves Law of Newland While Studying Periodic Classification of Elements

1. After calcium, there is no element that seems to have the same property as per the conditions of the law.

2. He imagined that only 56 elements are present in the whole world which had been included in the table as well.

3. Placement of some elements like Nickel and cobalt in the same slot where both are different in chemical properties. It created a fuss here to understand the table.

4. Discovered some noble gases in the environment like helium, beryllium, etc. This also affected the table because there is no place for place them in the table.



Mendeleev Periodic Table

One of the important names when it comes to modern periodic classification of elements, Mendeleev contribution is enormous in terms of classification.

In 1869, when octave law was rejected. Then one of the great scientist’s “Mendeleev”, his periodic table was came into effect. In his periodic table atoms were arranged based on their fundamental property, atomic mass, and chemical property. In his time of formulation only 63 elements were known, so he made his periodic table with those 63 elements. Mendeleev’s table was divided into horizontal rows and vertical columns. For his arrangement of elements in the periodic table he made its formulation with oxygen and hydrogen (oxides and hydrides) then he wrote all his observation on the cards (formula with oxides, hydrides and atomic mass) he grouped together the element which had the similar properties he found that element with similar properties were occupying same vertical columns and that similar trend were observed in horizontal rows too;

In his observations he formulated the periodic law which states that

“The property of elements is a periodic function of their atomic masses”

He named the vertical column as “groups” and horizontal rows as” period.



Standards of Mendeleev Periodic Table of Elements

Some gaps were left in the table for the element which were not discovered at that time so he said that the element which will discover later will occupy this position in the table without disturbing the existing sequence.

(a) The table was found in the sequence of atomic masses as we can see in the table.


(b) But some of the elements were placed before than any other slightly light element of the table. For example: cobalt (along with an atomic mass of 58.9) was placed in slightly higher than a nickel (along with the atomic mass of 58.7) and even we can see such anomaly in the periodic table classification of elements.


(c) Eka - word of Sanskrit was emphasized by the Mendeleev to that undiscovered element in that time. It means that the undiscovered element would be mentioned in the hollow cards place or the unfilled slots of the periodic table of elements.
For example:- gallium, scandium, and germanium and other element were discovered later, they would add in the classification of the periodic table of the element in the Sanskrit style of Eka-silicon, Eka-boron, Eka-aluminium and other further discovered elements like this. Further discovered element of aluminium would be discovered mentioned as Eka-aluminium and later discovered element as the -gallium in the table of the elements of the periodic table.


Property Eka- aluminium Gallium
Atomic mass 68 69.7
Formula of oxides E2O3 Ga2O3
Formula of chlorides ECl3 GaCl3

Limitations of Mendeleev Modern Periodic Classification of Elements Table

• He was not able to assign proper position for hydrogen

• The gradation in atomic masses was not regular there was no number prediction that how many elements will occupy the empty spaces.

• After the discovery of isotopes there was violation in Mendeleev’s periodic law


Modern Periodic Table of Elements

To make the classification system easier Henry Mosley gave the concept of “ATOMIC NUMBER”

Hence, atomic number became the fundamental property of the element in the modern periodic table.


Fundamental things to be noted for the Modern Mendeleev Periodic Table of Elements

1. Horizontal columns as “periods” total 7 in the list of the table in which five are at the top and two lanthanides and actinides at the bottom.

2. Vertical columns are known as “groups” they are total 18 in number.

3. Group 18 contains noble gases.

4.Each group has the same valency or valence electron in the outermost shells.


Till now we have discovered the 118 elements in modern periodic table in 2016 oganesson was discovered and added in the modern periodic classification of elements table.


Position of Elements in the Modern Periodic Classification Table

There are 18 groups and 7 horizontal columns in the modern periodic classification of elements table.

(i) The element with same number of valence electrons is there in a same group and as we down to the group the number of shells goes on increasing.

(ii) The elements with different number of electrons but same number of valence shell (that means number of electrons in the valence shell is increasing but the number of shell remains same) were occupied in the same group.


Trends in the Modern Periodic Classification of Elements Table

1. As we move top to bottom in a group the atomic radius (the distance of outermost shell from the nucleus) will go on increasing because the numbers of shells are increasing, and as we move left to right in a period the atomic size goes on decreasing because nuclear force is increasing.

2. As the left extreme there are a metals (group1&2) on the right side there are a non-metals a zig gag line separating the metalloid which has the property of both metals and non- metals.

3. The effective nuclear charge is increasing form left to right so the tendency to lose electron is decreasing because they experience more nuclear pull as we move down the group the valence electron remain same but the number of shells are increasing, so the effective nuclear charge is decreasing and the tendency to loose electrons is increasing so tendency to behave as a metal is increasing along a group and decreasing across a period.



Some Important Questions From the Modern Periodic Classification of Elements

Mendeleev separated the elements according to their atoms. He tried to put elements of similar qualities in a group.


At that time the Dobereiner could only identify the triad of three elements and all the elements could not be classified according to their triad.

1. In the modern periodic table, elements are separated according to their atomic numbers. Hydrogen was ranked in the first group.
2. The position of the elements in the periodic table shows their chemical reactivity.
3. Elements were placed in different orders of heavy and light according to their weight.

Elements are grouped according to their electronic configuration. The electron present in the outermost shell of an element is known to have its valency and the total number of cells is its periodic number.

1.Modern Periodic Table: -
(i) It is ranked by the atomic number.
(ii) It has 18 squares.
(iii) In these, inert gases are placed in the 18th class.

2.Mendeleev's Periodic Table: -
(i) It is based on atomic mass.
(ii) It has 8 groups.
(iii) Inert gases have no place in them.

Thank you for reading!!!


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