18. 3. The successive electrons added into the d shell are shielded from the core nuclear charge (because of teh shape of teh d-orbitals).

18.5

V(+5): [Ar}

Cr(+3): [Ar}3d3

Mn(+2): [Ar}3d5

Fe(+3): [Ar}3d5

Cu(+2): [Ar}3d9

Sc(+3): [Ar}

Ti(+4): [Ar}

18.6 Because they have intermediate values for their ionization potentials, neither very small nor very large, between stable oxidation states.