Why some elements 2 orbit are uncomplet
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Using electronic configuration as the criterion, we ordinarily recognize four general type of elements; the inert gas elements, the representative elements, the transition elements, and the inner transition elements. The classification of the elements into these groups is dependent on the extent to which the s, p, d and f orbitals are filled.
(i) 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.
(ii) 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.
(iii) 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)d1-10 ns1-2.
(d) According to 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.
(e) Because of the extra stability which is associated with empty, half-filled, and filled subshells, there are some apparent anomalies in electronic arrangements in the transition series. This empirical rule is illustrated by the chromium and copper configuration in the first d series of elements:

(iv) Inner Transition Elements
(a) In these elements last three shells i.e. last, penultimate and prepenultimate 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 up to 32 electrons.
(d) Their outermost electronic configuration is similar to f-block element i.e. (n-2)f1-14(n-1)s2 (n-1)p6(n-1)d0-1ns2
(v) Elements of the seventh period after atomic number 93 (i.e. actinides) are synthetic elements and are called transuranium elements.
(i) 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.
(ii) 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.
(iii) 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)d1-10 ns1-2.
(d) According to 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.
(e) Because of the extra stability which is associated with empty, half-filled, and filled subshells, there are some apparent anomalies in electronic arrangements in the transition series. This empirical rule is illustrated by the chromium and copper configuration in the first d series of elements:

(iv) Inner Transition Elements
(a) In these elements last three shells i.e. last, penultimate and prepenultimate 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 up to 32 electrons.
(d) Their outermost electronic configuration is similar to f-block element i.e. (n-2)f1-14(n-1)s2 (n-1)p6(n-1)d0-1ns2
(v) Elements of the seventh period after atomic number 93 (i.e. actinides) are synthetic elements and are called transuranium elements.
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