Physical Properties
Atomic Radii, Electronegativity, and Melting Point Anomalies.
Group 13 (Boron, Aluminum, Gallium, Indium, Thallium) is notorious for breaking the smooth trends typically seen in the s-block. The sudden introduction of completely filled $d$ and $f$ orbitals causes drastic changes in effective nuclear charge ($Z_{eff}$), resulting in a series of highly tested physical anomalies.
1. Atomic and Ionic Radii
Usually, atomic radius increases steadily down a group as new electron shells are added. However, Group 13 presents a classic JEE trap:
Aluminum ($143\text{ pm}$) is noticeably larger than Gallium ($135\text{ pm}$).
Reason: Gallium is preceded by the 3d transition series. It possesses a completely filled $3d^{10}$ subshell. The $d$-electrons are highly diffuse and provide extremely poor shielding for the outer valence electrons against the nucleus. As a result, the effective nuclear charge ($Z_{eff}$) in Gallium is unusually high, pulling the valence electrons tightly inward and shrinking the atomic radius. This is known as the d-block contraction.
2. Electronegativity (The "U-Shape" Curve)
In typical groups (like Group 1 or 2), electronegativity decreases smoothly down the group. In Group 13, the trend dips and then actually increases!
| Element | Boron (B) | Aluminum (Al) | Gallium (Ga) | Indium (In) | Thallium (Tl) |
|---|---|---|---|---|---|
| Electronegativity | 2.0 | 1.5 | 1.6 | 1.7 | 1.8 |
Why does it increase after Aluminum?
Electronegativity is closely tied to Effective Nuclear Charge ($Z_{eff}$). Because of the poor shielding provided by the filled $d$-orbitals (in Ga, In) and $f$-orbitals (in Tl), the $Z_{eff}$ acting on the valence shell increases down the group. This causes the nucleus to attract shared pairs of electrons more strongly, leading to a rise in electronegativity from Al to Tl.
3. Melting and Boiling Points
The melting points of Group 13 elements do not follow a regular trend, while the boiling points decrease smoothly down the group.
Melting Point Trend: The Gallium Trough
- Boron ($2453\text{ K}$): Extremely high melting point because it forms a very rigid, 3-dimensional covalent network lattice.
- Gallium ($303\text{ K}$): Has an exceptionally low melting point. In the solid state, Gallium does not form a typical metallic lattice; it exists as discrete diatomic molecules ($Ga_2$). Very little thermal energy is needed to break the weak forces between these molecules. It will literally melt if held in a warm hand ($30^\circ\text{C}$).
Boiling Point Trend: A Smooth Decrease
Unlike its melting point, Gallium's boiling point is very high ($2676\text{ K}$). This gives Gallium a massive liquid range of over $2300^\circ\text{C}$, making it highly valuable for use in high-temperature thermometers.
4. Physical State, Structure, and Density
Physical State & Structure
Boron is a non-metallic, extremely hard, and black solid. It exists in several allotropic forms, all of which are built from the highly complex $B_{12}$ icosahedron (a 20-faced geometric structure).
In contrast, Aluminum, Gallium, Indium, and Thallium are all silvery-white, relatively soft, and highly electrically conductive metals.
Density Trend
Despite the anomalies in atomic radius, the density of Group 13 elements increases smoothly down the group.
This occurs because the sharp increase in atomic mass easily outweighs any irregular fluctuations in atomic volume.
Knowledge Check
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