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Classification Of Elements And Periodicity In Properties

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Classification of Elements and Periodicity in Properties PDF Notes, Important Questions and Formulas

Classification of Elements and Periodicity in Properties

 Periodicity of Valence or Oxidation State

  • The chemical behaviour of an element is mainly decided by the electrons present in its outermost shell called valence shell. 
  • These electrons participate in the formation of bonds and decide the combining capacity of the element. The combining capacity of an element is called its valence.
  • The valence of a representative element is equal to the number of electrons in the outermost shell or equal to eight minus the number of electrons present in the outermost shell.





    Example: Consider the two oxygen- containing compounds: OF2 and Na2O
    OF2

    • Order of electro negativity of the three elements is F>O>Na.
    • Each of the atoms of fluorine with outer electronic configuration 2S22P5 shares one electron oxygen in the OF2 molecule.
    • Fluorine is the highest electronegative element and is given the oxidation state 1.
    • Because there are two fluorine atoms in this molecule, oxygen with outer electronic configuration 2s22p4 shares two electron with fluorine atoms and exhibits oxidation state +2.


    Na2O

    • In Na2O, oxygen being more electronegative accepts two electrons, one from each of the two sodium atoms and shows the oxidation state 2.
    • Sodium with electronic configuration 3s1 loses one electron to oxygen and is given the oxidation state +1.

     

    The oxidation state of an element in a particular compound is the charge acquired by its atom on the basis of electronegative consideration from other atoms in the molecule.

     Anomalous Properties of Second Period Elements

  • The first element of each of the Group 1 (lithium), Group 2 (beryllium) and Groups 13–17 (boron to fluorine) differs from the other members of their respective groups. Example: Lithium unlike other alkali metals and beryllium unlike other alkaline earth metals form compounds with pronounced covalent character.

  • The behaviour of lithium and beryllium is similar with the second element of the following group, i.e. magnesium and aluminium.

  • This type of similarity is known as diagonal relationship in periodic properties.

  • Different chemical behaviour of the first member of s-block and p-block: o The first member of a group of elements in the s-block and p-block shows different chemical behaviour to that of the subsequent members in the same group. 

    • The anomalous behaviour is attributed to their small size, large charge/radius ratio and high electronegativity of the elements.
    • The first member of a group has only four valence orbitals (2s and 2p), whereas the second member of the group has nine valence orbitals (3s, 3p, 3d).
    • The maximum covalence of the first member of each group is 4
    • The first member of p-block elements displays greater ability to form pp–pp multiple bonds, i.e. C=C, C≡C, N=N and N≡N, and to second period elements, i.e. C=O, C=N, C≡N and N=O, compared to subsequent members of the group.

Periodic Trends and Chemical Reactivity

 All chemical and physical properties are a manifestation of the electronic configuration of elements.

Variation in a period 

  • The atomic and ionic radii decrease in a period from left to right.
  • The ionisation enthalpies increase and the electron gain enthalpies become more negative across a period.
  • The ionisation enthalpy of the extreme left element in a period is the least negative and the electron gain enthalpy of the element on the extreme right is the most negative.
  • Noble gases with filled shells have positive electron gain enthalpy values.
  • This results in high chemical reactivity at the two extremes and the lowest in the centre.
  • The maximum chemical reactivity at the extreme left (among alkali metals) is exhibited by the loss of an electron, leading to the formation of a cation and at the extreme right (among halogens) shown by the gain of an electron forming an anion.
  • This property can be related with the reducing and oxidising behaviour of elements. It can also be related to the metallic and non-metallic character of elements.
  • The metallic character of an element which is highest at the extremely left decreases and the non-metallic character increases while moving from left to right across the period.
  • Chemical reactivity of an element can be best represented by its reactions with oxygen and halogens.

Reaction with Oxygen

  • Elements on the extremes of a period easily combine with oxygen to form oxides.
  • The normal oxide formed by the element on the extreme left is the most basic. Example: Na2O
  • The oxide formed by the element on the extreme right is the most acidic. Example: Cl2O7
  • Oxides of elements in the centre are amphoteric, i.e. Al2O3 and As2O3 or neutral, i.e. CO, NO, N2O.
  • Oxides which behave acidic with bases and basic with acids are known as amphoteric oxides.
  • Neutral oxides have no acidic or basic properties.

Variation in a Group

  • In a group, the increase in atomic radii and ionic radii with an increase in atomic number results in a gradual decrease in ionisation enthalpies.
  • There is regular decrease in electron gain enthalpies for the main group elements.
  • Thus, the metallic character increases and the non-metallic character decreases.
  • Change in atomic radii is much smaller among the transition elements (3d series) as compared to that of representative elements across the period.
  • Change in atomic radii is still smaller among inner transition elements (4f series).
  • Ionisation enthalpies are intermediate between the s-block and the p-block. Thus, they are less electropositive than Group 1 and 2 metals.
  • The elements present in a group possess the same number of valence electrons. Therefore, on moving down the group, there is no variation in the valence of the elements.
  • The elements present in a group show the same valence. Example: Elements in Group 2 show valence equal to two.

Periodic Properties

General Properties of Periodic Table

  1. There are Seventeen non-metals (including hydrogen) in periodic table.
  2. Five non - metals are solid C, P, S, Se, I.
  3. One non - metal is liquid i.e. Br.
  4. Eleven non-metals are gaseous.
  5. Six gases are monatomic (noble gases) i.e. He, Ne, Ar, Kr, Xe, Rn.
  6. Five gases are diatomic, they are H, F, N, O, Cl
  7. There are eight metalloids in periodic table like B, Si, Ge, As, Sb, Te, Po, At.
  8. Five elements are liquid at room temperature namely Cs, Fr, Ga, Hg, and Br,
  9. S-Block and p-Block together are called Representative elements.
  10. Five elements are radioactive amongst representative elements. They are Po, At,Rn, Fr and Ra
  11. There are seven periods in long-form of periodic table.
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