Electronic Configuration

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Electronic Configuration

In 'd' block, electrons are added to an inner 'd' orbital and this shields the outer 's' electrons from the increased nuclear charge. Therefore, the atomic radius decreases only slightly and electronegativity and ionisation energies increase only slightly.

The difference in energy between the second and third shells is less than between the first and second. By the time the fourth shell is reached, there is, in fact an overlap between the third and fourth shells. In other words, from scandium onwards, the orbitals of highest energy in the third shell (the 3d orbital) have higher energy than those of the lowest energy in the fourth shell (the 4s orbital). Hence, when writing the electronic configurations of these 'd' block elements we fill the 4s then the 3d orbitals.

Exceptions do occur:

Cr [Ar] 4s13d5

Cu [Ar] 4s13d10

This can be explained by the extra stability offered by full and half-filled 'd' orbitals.

This property can be illustrated by the examples below:

Fe2+ (3d6) is readily oxidised to Fe3+ (3d5)

Mn2+ (3d5) is not readily oxidised to Mn3+ (3d4)

  Symbol Electronic structure of Atom Common ion Electronic configuration of ion
Potassium K (Ar) 4s1 K+ (Ar)
Calcium Ca (Ar) 4s2 Ca2+ (Ar)
Scandium Sc (Ar) 3d14s2 Sc3+ (Ar)
Titanium Ti (Ar) 3d24s2 Ti4+ (Ar)
Vanadium V (Ar) 3d34s2 V3+ (Ar) 3d2
Chromium Cr (Ar) 3d54s1 Cr3+ (Ar) 3d3
Manganese Mn (Ar) 3d54s2 Mn2+ (Ar) 3d5
Iron Fe (Ar) 3d64s2 Fe2+ (Ar) 3d6
Fe3+ (Ar) 3d5
Cobalt Co (Ar) 3d74s2 Co2+ (Ar) 3d7
Nickel Ni (Ar) 3d84s2 Ni2+ (Ar) 3d8
Copper Cu (Ar) 3d104s1 Cu+ (Ar) 3d10
Cu2+ (Ar) 3d9
Zinc Zn (Ar) 3d104s2 Zn2+ (Ar) 3d10

When transition metals form ions, electrons are lost first from the 4s sub-shell rather than the 3d sub-shell. Thus Fe2+ ions have the electronic structure [Ar]3d6 rather than [Ar]4s23d4.

This occurs due to the presence of electrons in the 3d level, these repel the 4s electrons even further from the nucleus. Therefore, the 4s electrons are pushed to a higher energy level, higher than 3d. Consequently, when transition atoms become ions, the electrons from the 4s level before the 3d.

This means that all transition metals will have similar chemical properties which are dictated by the behaviour of the 4s outer electrons.

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