Class 10 Periodic Classification of Elements Notes

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Periodic Classification of Elements Notes

Döbereiner’s triads:-

Dobereiner arranged elements with similar properties into groups having three elements each and called those groups triads.
When the three elements in a triad were written in the order of increasing atomic masses, the atomic mass of middle element was roughly equal to the average atomic masses of other two elements.

Limitation of Döbereiner’s triads:-

Döbereiner could identify only 3 triads from the elements known at that time.

Newlands’ Law of Octaves:-

Newlands’ arranged the then known 56 elements in the increasing order of atomic masses. He found that every eighth element had properties similar to that of the first. He compared this to the octaves of music and called it the law of octaves.

Limitations of Newlands’ Law of Octaves:-

Law of octaves was applicable only up to calcium.
He told that only 56 elements existed in nature and no more elements will be discovered in future.
In order to fit elements into his table, Newlands’ adjusted two elements in the same slot.
He also put some unlike elements under the same note.

Mendeleev’s periodic table:-

Mendeleev’s periodic law:-

The properties of the elements are the periodic function of their atomic masses.

Achievements of Mendeleev’s periodic table:-

Mendeleev left some gaps in his periodic table and boldly predicted the existence of some elements that had not been discovered at that time.
He named them by prefixing “eka” to the name of preceding element in the same group.
He also predicted the chemical properties and atomic masses of some undiscovered elements which were similar to the later discovered elements.
Noble gases which were discovered very late, could be placed in a new group without disturbing existing order.

Limitations of Mendeleev’s periodic table:-

Hydrogen shows the properties of both alkali metals and halogens. So, no fixed position can be given to the hydrogen.
Isotopes (which were discovered very late) posed a challenge to Mendeleev’s periodic law.
He was not able to predict actually how many elements could be discovered between two elements.

Modern periodic table:-

Modern periodic law:-

The properties of the elements are the periodic function of their atomic number.

In modern periodic table, there are 18 groups and 7 periods.

Trends in the modern periodic table:-

Number of shells:-

In a group, number of shells increases from top to bottom.
In a period, number of shells remains same.

Valence electron:-

In a group, number of valence electrons remains same.
In a period, number of valence electrons increases from left to right.

Valency:-

In a group, valency remains same.
In a period, valency first increases and then decreases from left to right.

Atomic size (radius):-

In a group, atomic size increases from top to bottom as the number of shells increases.
In a period, atomic size decreases from left to right as the number of valence electrons in the outermost shell and the number of protons in the nucleus increase and so the force of attraction between the nucleus and the outermost shell increases.

Electropositivity:-

In a group, electropositivity increases from top to bottom.
In a period, electropositivity decreases from left to right.

Electronegativity:-

In a group, electronegativity decreases from top to bottom.
In a period, electronegativity increases from left to right.

Metallic properties:-

In a group, metallic properties increase from top to bottom.
In a period, metallic properties decrease from left to right.

Non-metallic properties:-

In a group, non metallic properties decreases from top to bottom.
In a period, non-metallic properties increases from left to right.

Nature of oxides:-

In a group, acidic properties of oxides of elements decrease from top to bottom and basic properties increase from top to bottom.
In a period, acidic properties of oxides of elements increase from left to right while the basic properties decrease from left to right.

Chemistry