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3.Periodic Table and Periodicity of Properties eLearn.Punjab

Mendeleev’s Periodic Table

Russian chemist, Mendeleev arranged the known elements (only 63) in order of increasing
atomic masses, in horizontal rows called periods, so that elements with similar properties were in
the same vertical columns. This arrangement of elements was called Periodic Table. He put forward
the results of his work in the form of periodic law, which is stated as “properties of the elements are
periodic functions of their atomic masses”

Although, Mendeleev periodic table was the first ever attempt to arrange the elements, yet it has
a few demerits in it. His failure to explain the position of isotopes and wrong order of the atomic
masses of some elements suggested that atomic mass of an element cannot serve as the basis for
the arrangement of elements.

Periodic Law

In 1913 H. Moseley discovered a new property of the elements i.e. atomic number. He observed that
atomic number instead of atomic mass should determine the position of element in the periodic
table and accordingly the periodic law was amended as “properties of the elements are periodic
function of their atomic numbers”. Atomic number of an element is equal to the number of
electrons in a neutral atom. So atomic number provides the basis of electronic configurations as
well.

Atomic number is a more fundamental property than atomic mass because
atomic number of every element is fixed and it increases regularly by 1 from
element to element. No two elements can have the same atomic number.

1. What was the contribution of Dobereiner towards classification of elements?
2. How did Newlands arrange the elements?
3. Who introduced the name Periodic Table ?
4. Why improvement in Mendeleev’s periodic table was made?
5. State Mendeleev’s periodic law.
6. Why and how elements are arranged in a period?

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