Melting and Boiling Points of Group 14 (Carbon Group) Elements
Melting Points and Boiling Points
As we move down the group from carbon (C) to lead (Pb), both melting points and boiling points generally decrease. It’s important to note that the melting point of metallic white tin (Sn) is an exception to this trend, being lower than that of lead.
| Element | Melting Point (oC) | Boiling Point (oC) | Electronegativity | Ionization Energy |
|---|---|---|---|---|
| Carbon (C) | 3700 | 3825 | 2.6 | 1087 |
| Silicon (Si) | 1414 | 3265 | 1.9 | 787 |
| Germanium (Ge) | 939 | 2833 | 2.0 | 762 |
| Stannum* (Tin) (Sn) | 232 | 2602 | 2.0 | 709 |
| Plumbum* (Lead) (Pb) | 328 | 1749 | 1.8 | 716 |
| *From Latin |
The high melting and boiling points of diamond (a giant covalent form of carbon) and silicon (Si) are due to their strong covalent bonds.
The decrease in melting and boiling points down the group suggests a corresponding decrease in inter-atomic forces.
The unusually low melting point of metallic white tin (Sn) compared to lead is attributed to its distorted 12-coordinated structure. Note that the melting point mentioned here refers to metallic white tin, as there’s also a brittle grey form of tin with a different melting point.
Brittleness
Carbon in the form of diamond is extremely hard due to the strength of covalent bonds, but it shatters when hit with a hammer due to the rigid structure.
Silicon (Si), germanium (Ge), and grey tin (all sharing a diamond-like structure) are also brittle.
White tin and lead (Pb) have metallic structures, making them malleable and ductile. Lead, in particular, is a relatively soft metal.
Electrical Conductivity
Diamond (a form of carbon) does not conduct electricity because its electrons are tightly bound in covalent bonds and not free to move.
Silicon, germanium, and grey tin are semiconductors, exhibiting limited electrical conductivity.
White tin and lead are good metallic conductors of electricity due to their free moving electrons.
There is a clear trend from non-metallic conductivity (carbon as diamond) to metallic conductivity (white tin and lead).
Electronegativity
Carbon is the most electronegative element in Group 4.
Electronegativity decreases as we move down the group, though not in a regular manner. This irregularity is likely due to the filling of d-orbitals in germanium and tin, and f-orbitals in lead, which do not effectively shield the valence electrons.
Ionization Energies
Ionization energy values generally decrease as we move down Group 4 from carbon (C) to lead (Pb). This decrease is due to the increasing distance of the valence electrons from the nucleus, leading to a weaker attractive force. However, the trend is not perfectly regular. The presence of d-orbitals in germanium (Ge) and tin (Sn) and f-orbitals in lead (Pb) can partially shield the valence electrons from the increasing nuclear charge. This shielding effect reduces the decrease in ionization energy compared to elements without these inner orbitals.
Anomalous Case of Tin: Interestingly, tin (Sn) has a slightly lower ionization energy than lead (Pb) even though it is positioned higher in the group. This anomaly occurs because the greater distance of the valence electrons from the nucleus in tin outweighs the weak shielding effect of its d-orbitals compared to the f-orbitals in lead.
Summary of Physical Properties of Group 4 Elements
- Melting and Boiling Points: Generally decrease from carbon to lead, with high values for diamond (giant covalent structure) and silicon.
- Brittleness: Carbon (diamond), silicon, germanium, and grey tin are brittle; white tin and lead are malleable and ductile.
- Electrical Conductivity: Varies from non-conductive (diamond) to semiconductive (Si, Ge, grey tin) to conductive (white tin and lead).
- Electronegativity: Decreases irregularly down the group, with carbon being the most electronegative.
- Ionization Energies: Decrease irregularly from carbon to lead, influenced by d- and f-orbital fillings.
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