1. The document discusses the historical development of the periodic table from Dobereiner's classification of elements into triads to Moseley's ordering of all elements by their atomic number. 2. Mendeleev's periodic table was a significant improvement as it predicted future elements and organized elements into periods and groups according to chemical properties. 3. The modern periodic table is based on Moseley's work classifying elements by atomic number, with elements in the same group having the same number of valence electrons and similar chemical properties.

1. CHAPTER 5
PERIODIC CLASSIFICATION
OF ELEMENTS
CLASS 10 - CHEMISTRY

2. Elements : Simple Substances Which cannot be simplified further Eg: Water
can be simplified into Hydrogen and Oxygen.But, Hydrogen cannot be divided
further. So, Hydrogen is an element.
Classification : We Classify the elements so that it would be easier for us to
study/ Understand them.
Periodicity : Repetition of properties after certain intervals is called periodicity
Definition for the Title:

3. DOBEREINER’S CLASSIFICATION OF
ELEMENTS
● Dobereiner is a German Scientist
● In 1817, he started his classification of elements.
● He grouped the elements into triads which had 3 elements in it.
● He is grouping the elements with increasing Atomic masses (A)
● Another criteria was that, the atomic mass of the centre element is
approximately the average of the other two elements in the triad.
● He classified 3 triades in total i.e, 9 elements. They are: Li, Na, K; Ca, Sr, Ba: Cl,
Br, I.

4. LIMITATIONS
OF
DOBEREINER’S
CLASSIFICATIO
N
At the time when he started his
classification, there were around 35
elements found. But Dobereiner
could only classify 3 triades, i.e., 9
elements. This was his main
Disadvantage.

5. He inspired many chemists and
scientists to start their own
classification of elements.
MERITS OF
DOBEREINER’S
CLASSIFICATIO
N

6. NEWLAND’S CLASSIFICATION OF
ELEMENTS:
● In the year of 1866, he started his own classification of elements.
● At the time when he started his classification, there were totally 56 elements
found.
● The 1st element in his table was Hydrogen and the 56th element in his table
was Thorium.
● He arranged the elements in such a way that the chemical properties of the
1st and the 8th element are similar. This was called as the ‘LAW OF OCTAVES’
● There are totally 7 vertical columns called groups
● In one group, the chemical properties of the elements would be similar.

7. DEMERITS OF
NEWLAND’S
CLASSIFICATIO
N
● The law of octaves worked only till
Calcium.
● He assumed that, there would be
only 56 elements in the future. So
he didn't leave a slot for the future
elements in his periodic table.
● He tried to adjust 2 elements in a
single slot.
Totally, we can say that
Newland’s classification is
only applicable for elements
with less atomic mass.

8. MENDELEEV’S CLASSIFICATION OF
ELEMENTS:
● In 1882, He published his classification in a magazine.
● At the time when he started his classification, There were 63 elements.
● He is also arranging the elements in their increasing atomic mass.
● Another criteria he used was the formation of Hydride and formation of Oxide.
● Formation of Hydride means the reactivity of an element towards Hydrogen.
● Formation of Oxide means the reactivity of an element towards Oxygen.
● He uses Hydrogen and Oxygen to check the reactivity of an element because,
they are the highly reactive.
● He checks the reactivity of each element , and writes it in a separate card. He
had 63 of them.

9. Cont...
● He then arranged those cards according to their reactivity and if elements had
the same reactivity, he made them as a group and created a periodic table.
● He makes 8 vertical columns called groups and 6 horizontal rows called
period.

10. MERITS OF
MENDELEEV’S
CLASSIFICATIO
N
● He predicts the future elements and
then leaves some space for them.
He predicts it with the reactivity of
the element above the gap, i.e., the
element in the row above the gap.
With this, he left three gaps. They
are: eka - aluminium, eka- boron and
eka- silicon. For this invention, he
got the nobel prize award
● Noble gas were not found at the
time he did his classification, but it
was easier to add them to his table
after they were found.

11. DEMERITS OF
MENDELEEV’S
CLASSIFICATION
● He didn't leave a fixed position
for hydrogen as it possess both
alkaline metal’s property and
halogens property.
● He didn't leave any gap for
isotopes
● Some elements were not
arranged with increasing atomic
mass

12. HENRY MOSELEY'S CLASSIFICATION:
● He started his classification in 1913.
● Totally, it had 18th group and 7 periods.
● He was the 1st scientist / chemist to classify elements with atomic number.
● The elements in a group behave similar, i.e., they all have the similar chemical
properties because they have the same valence electron.
● If the valence electron is 2, then it is placed in the second group; valence
electron is 3, in third group, and the same for the other valence electrons
● The groups 3-12 are vacant, i.e, they have multiple valence electrons in a
group.

13. “The properties of elements are the
periodic function of their atomic
number.”
MODERN PERIODIC LAW

14. METALLIC CHARACTER:
Metals are the electropositive in
nature. It’s also called as electro
positivity.

15. NON - METALLIC
CHARACTER:
Non - metallic character means that
non- metal have electronegative in
nature, It’s also called as
electronegativity.

16. TRENDS IN MODERN PERIODIC TABLE:
● Valence electrons:
i) Down the group - same
ii) Across the period - Increases one by one [ 1-8 ]
● Valency:
i) Down the group - same [ because valence electrons are similar]
ii) Across the period - Increases from 1-4 then decreases from 4-0

17. TRENDS IN MODERN PERIODIC TABLE:
● Atomic Size:
i) Down the group - Increases
ii) Across the period - Decreases [ due to effective nuclear charge ]
● Metallic Character:
i) Down the group - Increases [ Atomic size increases ]
ii) Across the group - Decreases [ Atomic size decreases ]
Therefore, atomic size is proportional to electro positivity

18. TRENDS IN MODERN PERIODIC TABLE:
● Non - metallic character:
i) Down the group - Decreases
ii) Across the period - Increases
Therefore, Atomic size is inversely proportional to electronegativity