Chemical periodicity

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Chemical periodicity

Atomic radii
Atomic radius means the distance from the centre of the nucleus to the outermost shell of electrons. But absolute size of an atom is difficult to be defined because of the following reasons:
(1) atom does not have well defined boundary. So probability of finding electron is never zero even at large distances from the nucleus.

(2)it is not possible to isolate an atom  and measure it's radius. the probability distribution is also affected by presence of other atoms in its neighbourhood.

We can only arbitrarily define Atomic radius as effective size  which means the distance of closest approach of one atom to another atom in a given bonding situation. 
 Atomic radii may be called as , covalent radius, Van der Waal's radius , ionic or crystal radius, etc depends upon the types of bonding.

Covalent radius
One half of the distance between the centres of nucleii of two similar atoms bonded together in a molecule.
For example :- the inter - nuclear distance between two atoms in H2 molecule is 0.74A° .
    Atomic radius of H2 =0.74/2
In case of hetronuclear diatomic molecules
In case of HCl
              r(HCl)=r(H)+r(Cl)

Multiple bond radii

Single bond>double bond>triple bond

Metallic or crystal radius
One half of the distance between nuclei of two adjacent atoms of a metal in metallic Lattice.

               
         r(metallic) = internuclear distance between two adjacent atoms in metallic Lattice/2


Van der waal's radius
One half of the internuclear distance between the adjacent atoms belonging to two nearest neighbouring molecules of the same substance in the solid state.
    



r(van der waal's radius)= internuclear distance between two non-bonding nearest neighbouring atoms/2

Vandar waal's radius>Metallic radius>covalent radius

Trends of atomic radii

Left to right :-decrease in atomic radii
Top to bottom:- increase in atomic radii

MCQ

Q(1) p-block belonging to 2nd period in increasing order of their radii
(a)F<O<N<C<B
(b)B<C<N<O<F
(c)C<B<O<N<F
(d)O<C<N<F<B

Q2.order of increasing radiii
(a)Cl-<S2-<Ca2+<K+
(b)Ca2+<K+<Cl-<S2-
(c)K+<Ca2+<Cl-<S2-
(d)S2-<Ca2+<Cl-<K+

Q3. Arrange the in order of increasing radii
(a)K+<Na+<Rb+
(b)Rb+<Na+<K+
(c)Na+<K+<Rb+
(d)Rb+<K+<Na+

Q4.(a)P<Si<Al
(b)Si<P<Al
(c)P<Al<Si
(d)Al<Si<P

Q5.which of the following has large size
(a)K+>Cs+
(b)Cs+>K+
(c)Cs+~K+
(d)none of these

Size of cation and anion

  • Size of cation is smaller than a neutral atom
  • With the removal of electron from the atom , magnitude of nuclear charge remains same .
  • Now same charge acts on lesser number of electrons.
  • So effective nuclear charge per electron increase, and electons are strongly attracted and pulled towards Nucleus
Example.   Atomic radius of Na = 1.54A°
                    Ionic radius of Na+=0.95

Size of anion
  • Size of anion is always larger than size of neutral atom.
  • With addition of electrons to the atom effective nuclear charge remains same .
  • Now the same charge acts on more number of electrons.
  • So electrons are less attracted by nucleus by which size of anion increase.
Example:-
        Atomic radius of Cl=0.99A°
     Atomic radius of anion Cl-=1.81A°

                    


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