In Chemistry when we talk about an important
topic.
So, Periodic classification is important in
such a way,
Like, if in this you properly study about
the elements and their classification then
it has its vast applications.
By knowing the properties of sodium,
We know the properties of potassium,
We know the properties of other element lying
in the same groups.
So the first thing is that, the periodic classification
we are talking about,
For the study of elements
That will be modern periodic table.
But, from the beginning,
What were the criteria to classify the elements
and how do we reached the modern periodic
table?
In short, we should also cover these things
up.
So, let us start with the point that when
elements were being discovered,
When more and more number of elements started
coming in front of us.
Then it was felt necessary to classify the
elements in an organized and systematic manner.
When various methods were followed to classify
them,
So there were only the criteria of atomic
mass.
Because at that time, only the known properties
of elements were the atomic masses.
In the beginning all the scientists who gave
the classification of elements, the basis
for this was atomic mass.
I am just explaining and outline of all the
old classifications.
That, how do we reached to the modern periodic
table.
Likewise, a classification was given by Dobernier, in the form of triads.
Dobernier Triads in this term the meaning
of Triads is Dobernier made Groups of 3-3 elements.
The groups were consisting of three elements.
The groups of 1-1, likewise a group was made
of lithium sodium and potassium elements.
Like this, many triads were made means grouping
of 3-3 elements were done.
Specialty wise, the three elements which were
placed in the group
They were in the increasing order of their
atomic mass.
So lithium sodium and potassium, the atomic
mass is in the increasing order in the three  elements.
Number 2, the atomic mass of the middle element
is the average atomic mass of other two elements.
Third specialty was that, the members which
consisted triads there was gradation in their
physical properties Means gradual change.
And the fourth characteristic of triad was
that their chemical nature was similar.
I am repeating
What were the specialties regarding the triads,
number 1 firstly all the three elements are
in the increasing order of their atomic mass.
Number 2, the atomic mass of the middle element
in the triad was the average of rest of the
two elements.
Number 3, there was a gradation in the physical
property.
Number 4, there was similarity in the chemical
nature.
This kind of a concept was introduced for
triads to classify the elements.
But there was a problem
Problem number 1, the size of the group was very small.
Because there were only 3 elements, the groups
were very small.
Problem number 2 was that, it was not possible
to fit all the elements in the form of triads.
Any of the elements fits anywhere in any of
the triads, so the grouping would be of only
3-3 elements this was not found to be valid.
Like this few more classification came into
existence.
Like a classification given by Newland in
the form of Newland's Octaves.
The term octaves stand for the eight elements.
Here the classification was made by relating
any eight elements.
Now what was the concept of those eight elements?
Concept was that If you will arrange the elements
in their increasing order of atomic mass,
Then every eighth element will resemble the
first element in the properties.
It will resemble the first element in the
property.
As an example for octaves lithium, beryllium,
boron, carbon, nitrogen, oxygen and fluorine
talking about these,
Then comes the sodium, and then comes the magnesium,
aluminum, silicon, phosphorous, sulfur, chlorine,
Then after this comes potassium then comes
calcium.
These all elements are in the order of their
increasing atomic mass.
All are in the increasing order of the atomic
mass.
And every eight element, as if I start with
the lithium then eight element will be sodium,
If start with beryllium then the eight element
will be magnesium,
Starting from boron then eight element is
aluminum,
They are resembling in properties.
Lithium sodium are similar, beryllium magnesium
are similar.
So in property wise every eight element resemble
the first element.
One more thing is there.
If you are thinking that, why I have not written
the noble gasses here?
So just keep in mind this is that classification
when noble gasses were not even discovered.
After the discovery of noble gasses of course
octaves were also disturbed.
Now we won't be able to resemble the eight
elements as per the arrangement of the mass.
So when the noble gasses were discovered the
octaves were disturbed.
The second problem occurred with octaves was
that
All the elements that come after calcium were
not valid.
Like I will talk about the scandium, scandium
comes after calcium.
So it does not resemble boron or aluminum
in the properties.
So in this way the limitations of octaves
remained in existence.
So, later on the most organized classification
which came into existence,
That was in the form of Mendeleev's Periodic
Table
Although in between Telluric Helix or
By relating atomic volume with atomic mass,
the one which resembles the properties Lother
Meyer Curves, these concepts were also introduced.
So the Lother Meyer Curves in this atomic
volume was mass wise related to properties of elements.
But all these classification as I have said,
If it is the matter of Telluric Helix or the
classification of Lother Meyer or it is Newlands
Octaves or if it Dobernier Triads.
There was a common problem with all of these that it was not possible to include lots of things together.
When we are going to concentrate on atomic
mass then the properties are being disturbed,
When we are going to relate the properties
then the grouping is not properly done.
So there were number of limitations.
But the most accepted classification came
in the form of Mendeleev's Periodic Table.
It was Mendeleev's Periodic Table.
See although I will not make you concentrate
on Mendeleev's Periodic Table.
Because, you have to study in detail about
Modern Periodic Table,
But still, we should know the journey to modern
periodic table,
How do we reach there?
So if we see like that then the whole idea
of the basis of modern periodic table was of Mendeleev's.
Mendeleev for the first time gave the classification
in the form of table.
He classified it in a tabular form and Mendeleev's
was the scientist who gave the organized classification.
The table which was made in that there was
total 7 periods,
In this there were 7 periods.
And the vertical columns whose numbering would
be changed while going from left to right.
Those were called as groups.
See, these are the horizontal rows, so how
will their number change?
When you will move vertically, then horizontal
rows will change.
Right?
So, periods are the horizontal rows,
And period numbers change when you move top
to bottom.
Groups are the vertical columns.
And the group's number will change when we
move left to right in the table.
In Mendeleev periodic table there were total
eight groups.
The groups were eight, from1 to 8.
In Mendeleev periodic table group's number
are given in roman numbers.
First to eight groups,
In the Mendeleev's classification after eight
groups one more group was been added.
Because at the time when the Mendeleev period
table was given the noble gases were not discovered.
And when the noble gasses were discovered
as per their valences.
You know that the noble gasses are monatomic.
So according to the valence they were zero
valiant elements and that is why it was not
possible to fit them in any group,
So based on that one zeroth group was also
added,
By giving group number zero one more group
was added in the Mendeleev periodic table.
So like this tell me how many groups are there
in Mendeleev periodic table.
9!
Eight groups from 1 to 8 and then a zero group
after the discovery of noble gas was added.
This was the whole format of Mendeleev's periodic
table.
But this periodic table also had limitations.
By overcoming the limitation of Mendeleev's
periodic table modern periodic table came
into existence.
But the point is what were the problems and
limitations in Mendeleev's classification.
First of all the very important talk now a
days is that what is the basis of all the classifications?
Atomic mass!
Because it was the time,
Till this classification was given at the
time of Mendeleev atomic number was not discovered.
So even in Mendeleev's periodic table the
basis of classification was atomic mass.
This classification was given on the basis
of atomic mass.
So the first problem was occurred that,
There was no separate position for the isotopes in the periodic table.
We know it well that isotope are those which are the atom of different mass of a single element.
So if mass is different, then the demand was
that they should be given different positions.
But there was only one position for every
element in the classification of Mendeleev.
There was no separate position for the isotopes.
Position of hydrogen was the second problem.
Second problem was hydrogen's position.
Hydrogen is a non metal.
In Mendeleev's classification it was placed
with the metals.
So that was again an issue.
It was a limitation, it was a question.
That why do you place non metallic elements
with metal in the group.
Other then these points, means not giving
separate position to the isotopes, placing hydrogen with metals.
Along this few in proper grouping was also
made.
Lithium, sodium and potassium are highly reactive
metals.
And with this in the first group copper silver
and gold were placed.
In this silver is count in a semi noble metal.
And gold is a noble metal.
So very less reactive copper, silver and gold
were placed with highly reactive lithium,
sodium and potassium,
It was considered to be improper grouping.
When things were observed according to the
mass of elements,
So it was found that the position of argon
and potassium, in this potassium is placed after argon.
In Mendeleev's periodic table potassium was
placed after argon.
Whereas while comparing mass wise potassium's
mass is more the argon's mass.
What is the meaning?
It was anomalous pair.
While looking at the mass it was an anomalous
pair.
Argon was of less mass element still it was
given the first position.
The problem with Mendeleev's was that if the
positions were changed then their grouping is not proper.
If we will shift the element with the higher
mass and interchange potassium and argon,
Then the problem occurs with the properties
and grouping.
Right now Mendeleev had kept argon and potassium
with neon and krypton.
So that way some improper grouping some improper
grouping some anomalous pairs they started existing.
One more problem with Mendeleev's periodic
table was that few specific elements they
are called as lanthanides and actinides, they
were separate from the main body of the periodic table.
They were separated from the main body of
the periodic table.
Mendeleev had no reason that on which basis
he separated all the things, why the grouping
is not proper and in few elements how the
anomalous pairs are made.
If we will go to depth of the actual problem
then problem was not in the classification.
Today also the modern periodic table which
we follow that is having exactly the similar situation.
By similar situation I mean that still potassium
is placed after argon in the modern periodic table.
Hydrogen is still paced with the metals in
the first group.
Copper, silver, gold is still placed in the
group of lithium, sodium and potassium.
But today we have answers for this.
That why have we placed it and how have we
placed it,
The biggest answer we got with this thing
that the modern classification is based on
the atomic number and not the atomic mass.
Most of the problems are eliminated by this
only.
Then the modern periodic table you will see
in that the number of groups,
Like in the Mendeleev's periodic table there
are 9 groups.
In modern periodic table there are total 18
groups.
So this way many problems are rectified.
Firstly long form of periodic table was given
and secondly its basis was changed.
Atomic number is now the basis.
So many of the problems they are automatically
resolved.
Simply with these changes,
There were no changes in the classification.
Only their criterion was changed and periodic
table was converted into a long form.
Now by this we have got lots of the answers.
Still Mendeleev's classification always appraised,
the reason is
It gets the appreciation the reason is that
in Mendeleev's periodic classification Mendeleev
was that much organized, that much confident
that this classification helped to develop
the new elements, did correction in atomic
mass.
Many elements atomic mass was not correctly
known.
Based on the position in periodic table atomic
mass was been corrected.
Mendeleev has also confidently said about
the undiscovered elements.
In Mendeleev's periodic table place was left
vacant for the undiscovered elements.
Like I will give you an example that boron
was discovered,
Aluminum was discovered.
But gallium which you know today it was not
discovered at that time.
So a position of an element which was to be
discovered after aluminum was vacant.
Elements carbon and silicon were known at
that time,
But today which we call as germanium it was
not been developed at that time.
But just think that Mendeleev was that much
confident in his classification that he even
told about the position of the undiscovered
element,
That in nature still there are few elements
to be discovered,
And you have to place those elements here
according to the properties.
He mentioned even those undiscovered elements
in the form of eka aluminum and eka silver.
That term eka stand for the element following.
So eka aluminum means the element following
aluminum.
Eka silicon that means the element following
silicon,
Not only the position,
The important thing is that he also had predicted the properties with positions.
Whenever in nature this element would be discovered
with which property it would be discovered.
What will be its physical state, what will
be its melting point, its boiling point, its
density even the physical constants were predicted.
Just imagine a person you had predicted about
the physical constants of the undiscovered element.
It was a great achievement.
And not only this really when these elements were discovered then even the physical constants
were observed as Mendeleev's has said.
And also got help for the discovery.
Because we started knowing that the elements
which are to be placed on the vacant place
of the periodic table what is the property
of it.
So in nature which in which combined state
will it be found.
What will be the characteristics?
What will be the physical state?
With this it was helpful in discoveries.
Ultimately today we are actually following
Mendeleev's classification in the form of
Modern periodic classification.
Just a slight difference that Modern classification
is given by Moseley,
Its criterion is atomic number.
The criterion of modern classification is
atomic number.
