Present basis for the classification of elements modern periodic table

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05 Periodic table

HISTORY OF THE PERIODIC TABLE

  • Scientists assumed that the properties of elements are periodic functions of their atomic masses and based on this assumption, Mendeleev placed 63 elements in a vertical column called groups and in horizontal rows called periods.
  • However, this method of classification could not explain the position of certain elements, rare earth metals, and isotopes. So, it was rejected.
  • So, Henry Mosley, in the year 1923, gave a new property of elements “Atomic number”. He considered that the atomic number of an element a more fundamental property than the atomic mass.

MODERN PERIODIC LAW

  • According to modern periodic law: The properties of elements are a periodic function of their atomic numbers.
  • When the elements are arranged according to increasing atomic numbers, then the elements having same valence electrons occur at regular intervals.

MODERN PERIODIC TABLE OR LONG FORM OF PERIODIC TABLE

  • The arrangement of elements in the modern (long form) periodic table is based on their electronic configurations.
ModernPeriodicTable

FEATURES OF THE MODERN PERIODIC TABLE

  • The modern periodic table consists of 18 groups and 7 periods.
  • Periods: The horizontal rows of elements in a periodic table are called periods. There are seven periods in the long form of periodic table.
    • First Period – 2 elements- Very Short Period
    • Second Period- 8 elements- Short Period
    • Third period-8 elements- Short period
    • Fourth period- 18 elements- long period
    • Fifth period- 18 elements- long period
    • Sixth period- 32 elements- Very long period
    • Seventh period- 32 elements- Very long period
  • The number of elements in a period is fixed by the maximum number of electrons which can be accommodated in the various shells of an atom.
  • Groups: The vertical columns in a periodic table are called groups. There are 18     groups in the long form of periodic table.
  • All the elements in a group have similar electronic configuration and show similar properties.
Group number Group name Property
Group 1 or IA Alkali metals They form strong alkalis with water
Group 2 or IIA Alkaline earth metals They also form alkalis but weaker than group 1 elements
Group 13 or IIIA Boron family Boron is the first member of this family
Group 14 or IVA Carbon family Carbon is the first member of this property
Group 15 or VA Nitrogen family This group has non-metals and metalloids
Group 16 or VIA Oxygen family They are also known as chalcogens
Group 17 or VIIA Halogen family The elements of this group form salts.
Group 18 Zero group They are noble gases and under normal conditions they are inert.

ARRANGEMENT OF METALS, NON-METALS AND METALLOIDS:

  • The diagonal line at the right side of the table separates the elements into two groups: the metals and the nonmetals.
  • The elements that are on the left of this line tend to be metals, while those to the right tend to be nonmetals.
arrMetalNonmetalMetaloids
  • Here, the metals have been separated from non-metals by some elements called ‘metalloids’ which are placed diagonally in the periodic table.
  • These metalloids are: Boron (B); Silicon (S); Germanium (Ge); Arsenic (As); Antimony (Sb).

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