As we proceed along a transition series, the nuclear charge increases which tends to decrease the size but the addition of electrons in the d-subshell increases the screening effect which tends to counterbalance the effect of the increased nuclear charge

In Cu, all the d-electrons are paired (3d¹⁰ 4s¹). In Cr, all the d-electrons are unpaired (3d⁵ 4s¹). Hence, d-d electron re~ulsions in Cu are much greater than those in Cr. Therefore, Cu atom is larger in size than Cr. In Cu²⁺ (3d⁹), d-d electron repulsions decrease due to presence of one unpaired d-electron. Moreover, the electrons are attracted by 29 protons of the nucleus whereas in Cr²⁺, three unpaired electrons are still present but they are attracted by ony 24 protons of the nucleus. Thus, Cu2+is smaller in size than Cr²⁺.

In hydrated copper sulphate, four water molecules are present as ligands. In the presence of these ligands d-orbitals are no longer degenerate in energy. Hence, d-d transition takes place absorbing red wavelength. The complementary colour, viz., blue is reflected. In anhydrous CuSO₄, d-orbitals remain degenerate.Hence, no d-d transition can occur. The white light is completely reflected back. Hence, it looks white.

Melting points first increase because the number of unpaired electrons increases and hence strength of metallic bond increases. After reaching the maximum, the melting points decrease because the pairing of electrons starts in the d-subshell and number of unpaired electrons decreases and so the strength of metallic bond decreases. The dip in the middle is due to exactly half-filled configuration of d-subshell which has higher stability. Hence, electrons are held tightly by the nucleus. As a result, metallic bond is weaker.