chemistry class 9 chapter 3 topic: introduction to classification of elements in periodic table

1. CLASSIFICATION OF
ELEMENTS AND
PERIODICITY IN
PROPERTIES
CHAPTER # 03

2. In early ages there were only 9 elements.
At the end of 19th century there were 63
elements discovered.

3. After the discovery of elements chemists try
to arrange these element in proper way.
Dobereiner was first chemist in this race.

4. In 1829 Dobereiner introduced a law to
arrange 3 element in specific order and this
law was known as DOBEREINER’S LAW OF
TRIADS.

5. LAW OF TRIAD

6. Dobereiner’s law of triad states that the
atomic mass of middle element of triad is
arithmetic mean of atomic masses of other
two elements.
For example in the triad of lithium sodium
and potassium.

7. Atomic mass of Na=
7+39
2
= 23

8. Why did dobereiner’s law of
triad fail?

9. DEMERITS OF LAW OF TRIAD:
Dobereiner’s law of triad was failed because
all elements can not arrange in triads and
many elements were not discovered at that
time

10. NEWLAND’S LAW OF
OCTAVES

11. The elements were placed in increasing order
of their atomic masses and it was noted
that every eighth element had properties
similar to the first element.
Just like MUSICAL NOTES.

13. DEMERITS OF NEWLAND
OCTAVES:
There was no space for undiscovered
elements.
All the elements can not be arrange by this
law.

14. MENDELEEV’S PERIODIC
LAW
MENDELEEV was Russian
Chemist. He arranged 63
Elements in periods.

15. He presented a law known as periodic law.
PERIODIC LAW:
According to this law,
Properties of elements are periodic
functions of their atomic masses.

16. DEMERITS OF MENDELEEV
PERODIC LAW:
This law does not explain the position of
elements.
Wrong order of atomic masses of some
elements.