Periodic Table of Elements in Chemistry

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Periodic Table of Elements in Chemistry.

Mandeleev’s Periodic Law (1869)

“The physical and chemical properties of elements are a periodic function of then atomic weights, i.e., when the elements are arranged in order of their increasing atomic weights, elements with similar properties are repeated after certain regular intervals”.

Modern Periodic Law

“The physical and chemical properties of the elements are a periodic function of then atomic numbers, i.e., when the elements are arranged in order in order of their increasing atomic weights, elements with similar properties are repeated after certain regular intervals”.

Cause of Periodicity

The cause of periodicity in properties is the repetition of similar outer electronic configurations at certain regular intervals.

Main features of Modern Periodic Table

  • It has 7 horizontal rows called periods and 16 vertical columns called groups or families.
  • The first period is the shortest period consisting of 2 elements.
  • The second and third periods contain 8 elements, a fourth and fifth period contains 18 elements, sixth period contains 32 elements and seventh period is incomplete with 27 known elements.
  • The elements of second period are known as bridge elements.
  • The elements of I, II, XIII, XIV, XV, XVI, XVII and XVIII groups are collective known as normal or representative elements.
  • The elements of XVIII group are known as inert gases or noble gases.
  • The elements of III group to XIII group are known as transition elements. This is because their properties lie between the properties of the reactive metals (alkali and alkaline earth metals) placed on extreme left and the non – metals (halogens and chalcogens = oxygen family) placed on extreme right of the periodic table. Their general electronic configuration is (n – 1)d1-10ns0-2.
  • The series of elements with Z = 58 to Z = 71 which occur in periodic table after lanthanum are called lanthanides or lanthanoids. The series of elements with Z = 90 to Z = 103 which occur in the periodic table after actinum are called actinides or actinoids. These elements (lanthanides and actinides) are also known as inner transition elements. The electronic configuration is,

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(n – 2) f0-14 (n – 1) d0 – 1ns²

s – Block Elements

Elements in which the last electron enters the s – orbital.

General configuration is: ns1 – 2.The elements of I and II groups are s – block elements.

Properties :

  • They are soft metals with low melting and boiling points.
  • They have low ionization energies and high electro positivity.
  • Their metallic character and reactivity increase in a group.
  • They lose the valence electrons readily to form +1 and +2 ions.
  • They are strong reducing agents.
  • They are good conductors of heat and electricity.

p – Block Elements

Elements in which the last electron enters any one of the three p – orbitals.

General configuration is : ns²np1 – 6. The XIII to XVIII groups (excluding He) have p – block elements.

The elements of XVII group are known as halogens (salt producers) and that of XVI group are known as chalcogens (ore – forming).

Properties :

  • The metallic character increases within a group and decreases along a period.
  • Their ionization energies are relatively higher than that of s – block elements.
  • They show more than one oxidation states.
  • Their oxidizing character increase in a period and reducing character increases along a group.

d – Block Elements

Elements in which the last electron enters any one of the five d – orbitals.

General configuration is (n – 1)d1 – 10 ns0 – 2. The elements of groups III to XII belong to this category.

They are called transition elements.

Properties:

  • They are hard, malleable and ductile metals with high m.p. and b.p.
  • They are good conductors of heat and electricity.
  • Their ionization energies are between s and p – block elements.
  • Their compounds are generally coloured and paramagnetic.
  • They show variable oxidation states.
  • They form both ionic and covalent compounds.
  • Most of the transition elements form alloys.

f – Block Elements

Elements in which the last electron enters any one of the seven f – orbitals.

General configuration is (n – 2)f0 – 14 (n – 1)d0 – 1ns². There are 28 f – block elements in all (Lanthanides and Actinides).

They are called inner – transition elements.

Properties :

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  • They are heavy metals with high m.p. and b.p.
  • They show variable oxidation states.
  • Their compounds are generally coloured.
  • They have a high tendency to form complexes.
  • Most of the elements of the actinide series are radioactive.

Prediction of Period, Group and Block

  • For s – block elements group number is equal to the number of valence electrons.
  • For p – block elements group number is equal to 10 + number of valence electrons.
  • For d – block elements group number is equal to the number of electrons in (n – 1)d – subshell + number of electrons in valence shell.

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