CBSE Class 12 Chemistry Notes: Periodic Properties – Bohr’s Classification of Elements

Elements may be classified into four groups on the basis of number of incomplete shells present in them

Inert gases

(a) s- and p-orbitals of the outer most shell of these elements are completely filled. The outermost electronic configuration is ns²np⁶.

(b)  Helium is also inert gas but its electronic configuration is 1s²

Representative or Normal Elements:  

(a)  Outermost shell of these elements is incomplete. The number of electrons in the outermost shell is less than eight.

(b) Inner shells are complete

(c) s-and p-block elements except inert gases are called normal or representative elements.

Transition Elements:

(a)  Last two shells of these elements namely outermost and penultimate shells are incomplete

(b)  The last shell contains one or two electrons and the penultimate shell may contain more than eight up to eighteen electrons.

(c) Their outermost electronic configuration is similar to d-block elements i.e.  (n-1)d^1-10 ns^1-2.

(d)  According to the latest definition of transition elements those elements which have partly filled d-orbitals in neutral state or in any stable oxidation state are called transition elements. According to this definition Zn, Cd and Hg (IIB group) are d-block elements but not transition elements because these elements have d¹⁰ configuration in neutral as well as in stable +2 oxidation state.

Inner Transition Elements:

(a)  In these elements last three shells i.e. last, penultimate and pre-penultimate shells are incomplete.

(b) These are related to IIIB i.e. group 3.

(c)  The last shell contains two electrons. Penultimate shell may contain eight or nine electrons and pre-penultimate shell contains more than 18 upto32 electrons.

(d)  Their outermost electronic configuration is similar to ¦-block elements i.e. (n-2)^1-14  (n-1)s² (n-1)p⁶ (n-1)d^0-2ns²

(e) Elements of the seventh period after atomic number 92 (i.e. actinides) are synthetic elements and are called transuranic elements.

Illustration 1.    Boron is metalloid. Comment

Solution:    It shows the metallic as well as non – metallic nature.

Illustration 2.    Why are inert gases mono – atomic?

Solution:    Inert gas have completely filled octet (ns², np⁶) and so do not form bonds.

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