What is important to know about Periodic Classification of Elements?

In 1800 there were only 31 known elements; by 1865 their number became 63, and now 118 elements have been discovered.

What is important to know about Periodic Classification of Elements?

In 1817, the German chemist Johann Wolfgang Dobereiner suggested grouping elements based on their relative atomic masses, in groups of three called triads.

What is important to know about Periodic Classification of Elements?

Dobereiner's law of triads: the atomic mass of the middle element of a triad is nearly the same as the average of the atomic masses of the other two elements.

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Periodic Classification of Elements

20 Key Points

10 MCQs

Study Material

Substances are formed by the combination of various elements, and every element is unique in its nature and property. In 1800 there were only 31 known elements; by 1865 the number became 63; now 118 elements have been discovered. As more elements were found, scientists looked for a pattern in their properties so that such a large number of elements could be studied with ease.

1. Early Attempts at Classification – Timeline

Year	Scientist	Contribution	Basis
1817	Johann Wolfgang Dobereiner (German chemist)	Law of Triads	Relative atomic masses
1866	John Newlands	Law of Octaves (56 known elements)	Increasing order of atomic mass
1869	Dmitri Mendeleev (Russian chemist)	Law of Periodicity; short form of periodic table	Atomic masses
1913	Henry Moseley (English physicist)	Proved properties depend on atomic number (X-ray diffraction experiments); led to the modern periodic table	Atomic number

Key Points (20)

1
In 1800 there were only 31 known elements; by 1865 their number became 63, and now 118 elements have been discovered.
2
In 1817, the German chemist Johann Wolfgang Dobereiner suggested grouping elements based on their relative atomic masses, in groups of three called triads.
3
Dobereiner's law of triads: the atomic mass of the middle element of a triad is nearly the same as the average of the atomic masses of the other two elements.

Practice MCQs (10)

Test your knowledge with these practice questions.

Q1. Which of the following sets is NOT one of the Dobereiner's triads given in the chapter?

A. Li, Na, K

B. Cl, Br, I

C. Ca, Sr, Ba

D. Co, Ni, Pd

Q2. According to Mendeleev's law of periodicity, the physical and chemical properties of the elements are the periodic functions of their:

A. atomic numbers

B. atomic masses

C. number of shells

D. electronic configuration

Q3. Which block of the periodic table is the only one that contains all three types of elements: metals, nonmetals and metalloids?

A. s-block

B. d-block

C. p-block

D. f-block

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