Lesson no :-2 Periodic classification of elements important

                                                      SCIENCE PART :- 1

                             LESSON 2. PERIODIC CLASSIFICATION OF ELEMENT
                                                         IMPORTANT

1. Dobereiner's Traids:- He made groups of three elements each, showing similar chemical properties and called them traids. He arranged the three elements in a traid in a increasing order of atomic mass and showed that the atomic mass of the  middle element was approximately equal to the mean of the atomic masses of the other two elements.

      2.Newlands' law of octaves:- Newlands arranged the elements known at the time an increasing             order of their atomic masses, he found that every eighth element had properties similar those of           the first. For example, sodium.
      3.Limitations of Newlands' octaves:- 1.Arrange 56 elements only up to calcium in  an                           increasing order of their masses.                                                                                                               2. Arranged lightest element hydrogen and end up thorium.                                                                     3. Newlands placed the metals Co and Ni under the note Do along with halogens.                                 4. The properties of the new elements discovered later did not fit in the Newlands' law of                         octaves.
      4.Mendeleev's periodic law :- Mendeleev found that the elements with similar physical and                    chemical properties repeat after a definite interval.
          Mendeleev's periodic law :- The elements with physical and chemical properties are a periodic            function of their atomic masses.
      5.Merits of Mendeleev's :- 1. To give the proper place in the periodic table, atomic masses of               some elements were revised in accordance with their properties.                                                             2. Mendeleev had kept some vacant places in the periodic table for elements that yet to be                   discovered. Three of these unknown elements were given the names eka-boron, eka-silicon and           eka-aluminium. After discovered their names is scandium(Sc), germinium(Ge) and                               gallium(Ga)  respectively.                                                                                                                          3.When noble gases such as helium, neon and argon were discovered,"Mendeleev created the               'zero group' without disturbing the original periodic table in which the noble gases were                        placed  very well.
      6.Demerits of Mendeleev's :- 1. The elements cobalt(Co) and nickel(Ni) have the same whole               number atomic mass.                                                                                                                                 2.Isotopes were discovered long time after Mendeleev put forth the periodic table.                               3.The rise in atomic mass does not appear to be uniform when elements are arranged in an                      increasing order of atomic masses.                                                                                                        4.Position of hydrogen :- Hydrogen shows similarity with halogens (group VII ). It is difficult to             decide the correct position of hydrogen whether it is the group of alkali metals (group I) or in               the group of halogens (group VII).
      7.Modern periodic law :- 1. Henry Moseley showed that the atomic number of an element is the           most fundamental property and not in atomic mass.                                                                                 2.The chemical and physical properties of elements are a periodic function of their atomic                      numbers.                                                                                                                                                 3.There are seven horizontal rows called periods 1 to 7.                                                                           4. There are eighteen vertical columns called groups 1 to 18.                                                                   5. Each box corresponds to the place for one element. There are two series of elements placed                 separately at the bottom of the periodic table. These are called lanthanide series and actinide               series .                                                                                                                                                     6. There are 118 boxes in the periodic table including the two series.                                                     7.The elements in the modern periodic table are divided into four blocks : the s-block, the p-                  block, the d-block and f-block.
      8.Groups and electronic configuration :- The number of valence electrons in all these elements             from the group 1, i.e the family of alkali metals, is the same. For example, the elements                        beryllium (Be).                                                                                                                                       In the modern periodic table: 1. Elements are arranged in an increasing order of their atomic           numbers.                                                                                                                                                     2.Vertical columns are called groups .There are 18 groups .The chemical properties of the                   elements in the same group show similarity and gradation.
      9.Period and electronic configuration :- 1. In the modern periodic table , there are seven                     horizontal rows are called periods.                                                                                                            2. In a perod, while going from left to right , the atomic number increasing by one at a time  and              number of valence electrons also increased by one at a time.                                                             3. The elements with the same number of shells occupied by electrons belong to the same                       period.                                                                                                                                                   10. Periodic trends in the modern periodic table :- When the properties of elements in a period             or a group of the modern periodic table are compared, certain regularity is observed in their                   variations. It is called as the modern periodic trends in the modern periodic table.
     11.Valency:- The valency of an element is determined by the number of electrons present in the              outermost shell of its atoms, that is, the valence electrons.
     12.Atomic size :- The size of an atom is indicated by its radius. Atomic radius is the distance                    between the nucleus of the atom and its outermost shell.  (1pm=10-12m).

     13.Metallic and non metallic character:- It is observed that the metallic elements like sodium,               magnesium are towards the left. The nonmetallic elements such as sulphur, chlorine are                       towards the right.
     14.Gradation in halogen family :-The group 17 contains the members of halogen family. All of              them have the general formula X2.
     15.Reaction of alkaline earth metal with water :- A general chemical equation indicating the                reaction of alkaline earth metals is M+2H2O----->M(OH)2+H2.                                                                                                                                                                                                                                                                                                                                         THANKS FOR SEE MY SITE.