Iron occurs 8 valence electron and column 7 elements have 7 valence electrons that's why iron will not be kept in column 7.
The periodic table groupings are seen as vertical columns with numbers ranging from 1 through 18.
The count of electrons occupy in outermost orbital of the particular atom, can be introduced as valence electrons.
Such kind of elements which posses 7 valence electrons , will be placed in column 7. Iron has 8 valence electron but column 7 elements have 8 valence electron that's why iron will not be kept in column 7.
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