Chemistry Chapter 1 | 2nd Year

 Chapter 1 Notes | Chemistry 2nd Year

“Periodic Classification Of Elements And Periodicity”

  1. Although a number of chemists attempted to classify the elements but Dmitri Mendeleev gave the most the most useful and comprehensive classification.
  2. In Mendeleev’s periodic table the elements were arranged according to ascending order of their atomic masses
  3. The modern periodic law states “if the elements are arranged in ascending order of their atomic numbers, their chemical properties repeat in a periodic manner”.
  4. In modern periodic table elements similar properties are placed in eight vertical columns called groups. Each group is divided into two subgroups A and B. Normal or typical elements are placed in subgroups A and transition elements ae placed in subgroups B.
  5. The seven horizontal rows of the periodic table are called “periods”.
  6. Metals of sub-groups IA and IIA are called alkali metals and alkaline-earth metals, respectively. Members of subgroups VIIA are called halogens.
  7. Due to their less reactivity the elements shown in subgroups VIIIA are called noble gases.
  8. Elements of periodic table can also be classified into s-block, p-block, d-block and f-block elements depending upon the valence orbital which is in the process of completion.
  9. Elements of periodic table can also be divided into metals, non-metals and metalloids depending upon their properties.
  10. Atomic radii increase from top to bottom in a group and decrease along a period.
  11. Positive ions are always smaller than their parent’s atoms while the negative ions are usually larger than the atoms from which they formed.
  12. Electron affinities generally increase with increasing atomic number within period and decrease from lighter to heavier elements in a given group.
  13. Metallic character of elements increase down the group decreases along a period.
  14. The oxidation state of a typical element is directly or indirectly related to the group number of which the element belongs in the periodic table.
  15. The electrical conductance of an element depends upon the number of free movable electrons.
  16. There are three types of halides: ionic, polymeric and covalent. Halides of group IA are ionic in nature, have three dimensional lattices with high melting boiling points.
  17. There are three types of hydrides formed by the elements of periodic table: ionic, intermediate and covalent.
  18. Highly polar hydrides show hydrogen bonding in them
  19. Oxides may be divided on the basis of their acidic, basic or amphoteric character.
  20. Metallic oxides are basic in character, nonmetallic oxides are acidic in character and oxides of less electropositive elements like Zn Pb are amphoteric.
  21. Hydrogen is unique element of the periodic table. Due to similarities in properties it can be placed at the top of group IA or IVA or VIIA

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