Physical properties, atomic and ionic sizes, ionisation enthalpies
Transition metals are hard and high-melting because d electrons join metallic bonding. Size shrinks slowly across a series; 4d and 5d sizes match because of lanthanoid contraction.
Physical properties
Nearly all transition elements are typical metals. They have high tensile strength, are ductile and malleable, conduct heat and electricity well and have a metallic lustre. They are hard and have high melting and boiling points. The reason is that electrons from the (n−1)d orbitals as well as the ns orbitals take part in metallic bonding. The more unpaired electrons, the stronger the bonding. So melting points rise to a maximum near the middle of each series (about d⁵), except for the odd low values of Mn and Tc, then fall. The same pattern shows in the enthalpy of atomisation. Metals of the 4d and 5d series have higher enthalpies of atomisation than 3d metals, so metal-metal bonding is more common in compounds of the heavy transition metals.
Trends in the 3d series, each with its reason
| Property | Trend | Reason |
|---|---|---|
| Melting point and enthalpy of atomisation | Rise to a maximum near the middle, then fall | Number of unpaired d electrons available for metallic bonding first rises, then falls |
| Zn, Cd, Hg | Soft, low melting; Hg is a liquid | d¹⁰: no unpaired d electrons, so metallic bonding is weak |
| Ionic radius (same charge) | Decreases steadily from left to right | Each new electron enters a d orbital; d electrons shield poorly, so the effective nuclear charge rises |
| Density | Increases from Ti to Cu | Metallic radius decreases while atomic mass increases |
| First ionisation enthalpy | Rises from left to right, but less steeply and irregularly | Nuclear charge rises, but extra d electrons shield the outer 4s electrons; removing an electron changes the relative energies of 4s and 3d |
| Second ionisation enthalpy | Unusually high for Cr and Cu | Cr⁺ is 3d⁵ and Cu⁺ is 3d¹⁰: stable configurations are disturbed |
| Third ionisation enthalpy | Very high for Mn and Zn; comparatively low for Fe | Mn²⁺ (d⁵) and Zn²⁺ (d¹⁰) are stable; Fe²⁺ (d⁶) becomes the stable d⁵ Fe³⁺ on losing one electron |
Atomic and ionic sizes, and the 4d and 5d puzzle
Going down a group, we expect sizes to grow. The 4d atoms are indeed larger than the 3d atoms. But the 5d atoms are almost the same size as the 4d atoms: zirconium is 160 pm and hafnium 159 pm. This happens because the 4f orbitals are filled before the 5d series begins. 4f electrons shield the nucleus very poorly, so the atoms shrink steadily across the lanthanoids. This is the lanthanoid contraction. It almost exactly cancels the extra shell, so pairs such as Zr and Hf, or Nb and Ta, have very similar radii and very similar chemistry.
Which ionisation enthalpy is unusually high, and why?
Iron is the opposite case: Fe²⁺ (3d⁶) easily loses one more electron to become the stable 3d⁵ Fe³⁺, so its third IE is comparatively low.
Tungsten, with one of the highest melting points of any metal, was used for filaments in old bulbs. Strong, high-melting transition metals like iron, chromium and nickel make the steel in bridges, rail tracks and pressure cookers.
2 marks: “Why do transition metals have high enthalpies of atomisation? Which 3d metal has the lowest value, and why?” Many unpaired (n−1)d electrons join ns electrons in strong metallic bonding. Zinc has the lowest: it is 3d¹⁰ 4s², with no unpaired d electrons, so its metallic bonding is weak.
2 marks: “The atomic radii of the 4d and 5d series are almost the same. Explain.” 4f orbitals fill before 5d. The poor shielding by 4f electrons causes the lanthanoid contraction, which cancels the size increase expected from the extra shell (Zr 160 pm, Hf 159 pm).
3 marks: “Give reasons: (i) The second ionisation enthalpy of Cu is high. (ii) The third ionisation enthalpy of Mn is high. (iii) Zn, Cd and Hg are soft with low melting points.” (i) Cu⁺ is 3d¹⁰, a stable full set. (ii) Mn²⁺ is 3d⁵, a stable half-filled set. (iii) d¹⁰: no unpaired d electrons, so metallic bonding is weak.
Unpaired d electrons glue the metal. 4f shields badly, so 5d stays as small as 4d.