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CBSE Periodic Classification of Elements Subject Notes

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Periodic Classification of Elements

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Periodic Classification of Elements

In the Modern Periodic Table, a zigzag line separates the metals from the nonmetals.

  • Metals are found on left hand side of the periodic table while nonmetals are found on the right hand side of the periodic table
  • Oxides of metals are basic while oxides of nonmetals are acidic in nature.
  • Maximum number of electrons that can be accommodated in a shell are calculated using the formula 2n2 where n is number of shell.
  • The position of an element in the periodic table tells us about its reactivity.
  • Need for classification of elements. It is very difficult to study the properties of all the elements and their compounds. So, to study the properties of elements and their compounds easily and conveniently scientists classified them.
  • Besides gallium, germanium (Ge) of group IV A, polonium (Po) of group VI A and astatine (At) of group VII A have been discovered that fill the gaps left by Mendeleev in his periodic table.
  • Out of all the elements known, noble gases are most unreactive. They are present in atmosphere in very low concentration. So, they are grouped together in separate group (zero group) in Mendeleev’s periodic table.
  • Atomic number of helium is 2. Its first shell is the last shell which can accommodate only 2 electrons. So, outermost shell of helium is completely filled. This is why helium is placed in the 18th group of the periodic table.

Trends in the Modern Periodic Table

Characteristics

In Period (Left to Right)

In Group ( Top to Bottom)

Valence electrons

Increase due to the increase in atomic number

Remains the same

Valency

First increases from 1 to 4 then decreases to 0

Remains the same

Atomic size or atomic radius

Decreases due to increase in nuclear charge which tends to pull the electrons closer to the nucleus

Increases due to the increase in the new shells. This increases the distance between the nucleus and the outermost shell. So the atomic size increases in spite of increase in nuclear charge.

Metallic character

Decreases. When the effective nuclear charge acting on the valence electrons increases, the tendency to lose electrons decreases.

Increases because the effective nuclear charge on valence electrons decreases due to increase in atomic size.

Nonmetallic character

Increases. When the effective nuclear charge acting on the valence electrons increases, the tendency to gain electrons increases.

Decreases because the effective nuclear charge on valence electrons decreases due to increase in atomic size.

Reactivity

First decreases and then increases

Reactivity of metals increases because the tendency to lose electrons increases but the reactivity of nonmetals decrease because the tendency to gain electrons decreases.

 

Mendeleev’s Periodic Table

Modern Periodic Table

  1. Elements are arranged in increasing order of their atomic masses.
  1. Elements are arranged in increasing order of their atomic numbers.
  1. There are no separate positions for isotopes of an element as their atomic masses are different.
  1. Separate positions are not required as they have same atomic numbers.
  1. Some elements of higher atomic masses have been placed before elements of lower atomic masses.
  1. No such problems occur in this table. All elements are in proper order.
  1. Some dissimilar elements are grouped together while some similar elements are not grouped together.
  1. All similar elements are grouped together.
  1. Electronic configuration of an element cannot be calculated from its position.
  1. Electronic configuration of an element can be easily calculated from its position.

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