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Classification Of Elements- Class Xi

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Published in: Chemistry
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Here is the ppt for classification of elements covering the topics Dobereineir triads, Newland Law of Octaves, Mendeleev Periodic table- merits and demerits

Oruganti H / Visakhapatnam

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Qualification: MSc. in Applied Chemistry, B.Ed

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  1. Classification of elements and periodicity in properties
  2. Dobereiner's Triads • Dobereiner arranged certain elements with similar properties in groups of three in such a way that the atomic mass of the middle element was nearly the same as the average atomic masses of the first and the third elements. Table 3.1 Dobereiner's Triads Element Atomic weight 7 23 39 Element ca sr Atomic weight 88 137 Element 1 Atomic weight 35.5 80 127
  3. • Limitations of Dobereiner's Trids The triads given by Dobereiner were helpful in grouping some elements with similar characteristics together, but he could not arrange all the elements known at that time into triads. • Newlands' Law of Octaves John Newlands proposed the law of octaves by stating that when elements are arranged in order of increasing atomic masses, every eighth element has properties similar to the first.
  4. Newlands' Octaves Element At. wt. Element At. wt. Element At. wt. 23 39 Table 3.2 24 Ca 40 11 27 12 29 14 31 16 32 19 35.5 Limitations of Newlands' Law of Octaves (i) This classification was successful only up to the element calcium. After that, every eighth element did not possess the same properties as the element lying above it in the same group (ii) When noble gas elements were discovered at a later stage, their inclusion in the table disturbed the entire arrangement.
  5. Mendeleev's Periodic Table • Mendeleev arranged the elements known at that time in order of increasing atomic masses and this arrangement was called periodic table. • Elements with similar characteristics were present in vertical rows called groups. The horizontal rows were known as periods. • Mendeleev's Periodic Law: The physical and chemical properties of the elements are a periodic function of their atomic masses.
  6. Merits of Mendeleev's Periodic Table: This made the study of the elements quite systematic in the sense that if the properties of one element in a particular group are known, those of others can be pridicted. • He left several gaps in the table. For example, both gallium and germanium were unknown at the time Mendeleev published his Periodic Table. He left the gap under aluminium and a gap under silicon, and called these elements Eka- Aluminium and Eka-Silicon. • Mendeleev predicted not only the existence of gallium and germanium, but also described some of their general physical properties. These elements were discovered later.
  7. N/A
  8. Demerits: • Hydrogen has been placed in group IA along with alkali metals. But it also resembles halogens of group VII A in many properties • Isotopes of an element have different atomic masses but same atomic number. Since, periodic table has been framed on the basis of increasing atomic masses of the elements, different positions must have been allotted to all the isotopes of a particular element. • No proper explanation has been offered for the fact that why the elements placed in group show resemblance in their properties.

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