Class 9th Chemistry Chapter 3 Notes
Important Notes of Class 9th Chemistry Chapter 3 Notes Urdu Medium written by Honorable Sir M. Shahid Suib. These notes are very helpful in the preparation of Periodic Table and Periodicity of Properties Notes for students of the Class 9th Chemistry Chapter 3 Notes Urdu Medium and these are according to the paper patterns of all Punjab boards.
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- PERIODIC TABLE: With the discovery of the periodic table the study of individual properties of the known elements is reduced to study of a few groups. We will describe various attempts which were made to classify the elements into a tabular form.
- Dobereiner's Triads: A German chemist Dobereiner observed relationship between atomic masses of several groups of three elements called triads. In these groups, the central or middle element had atomic mass average of the other two elements. One triad group example is that of calcium (40), strontium(88) and barium (137). The atomic mass of strontium is the average of the atomic masses of calcium and barium. Only a few elements could be arranged in this way. This classification did not get wide acceptance.
- Newlands Octaves: After successful determination of correct atomic masses of elements by Cannizzaro in 1860, attempts were again initiated to organize elements. In 1864 British chemist Newlands put forward his observations in the form of 'law of octaves'. He noted that there was a repetition in chemical properties of every eighth element if they were arranged by their increasing atomic masses. He compared it with musical notes. His work could not get much recognition as no space was left for undiscovered element. The noble gases were also not known at that time.
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- 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"
- 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.
- Salient Features of Long Form of Periodic Table: This table consists of seven horizontal rows called periods.
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- First period consists of only two elements. Second and third periods consist of 8 elements each. Fourth and fifth periods consist of 18 elements each. Sixth period has 32 elements while seventh period has 23 elements and is incomplete.
- Elements of a period show different properties.
- There are 18 vertical columns in the periodic table numbered 1 to 18 from left to right, which are called groups.
- The elements of a group show similar chemical properties.
- Elements are classified into four blocks depending upon the type of the subshell which gets the last electron.
- Shielding Effect: The electrons present between the nucleus and the outer most shell of an atom, reduce the nuclear charge felt by the electrons present in the outer most shell. The attractions of outer electrons towards nucleus is partially reduced because of presence of inner electrons.
- As a result valance electron experiences less nuclear charge than that of the actual charge, which is called effective nuclear charge (Z). It means that the electrons present in the inner shells screen or shield the force of attraction of nucleus felt by the valence shell electrons. This is called shielding effect. With increase of atomic number, the number of electrons in an atom also increases, that results in increase of shielding effect.
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