Gallium (Ga) Study Guide: Properties, Reactions & Uses

Introduction

Gallium (Ga) is a fascinating metallic element from Group 13 of the periodic table, known for its remarkably low melting point, allowing it to melt in the palm of one’s hand. Beyond this unique physical property, gallium holds significant real-world importance, primarily in advanced technological applications such as semiconductors for high-speed electronics, LEDs, and high-efficiency solar cells. Its alloys also offer non-toxic alternatives in various instruments.

CBSE/JEE Quick Revision Notes

Atomic Number: 31
Symbol: Ga
Atomic Mass: 69.723 u (or g/mol)
Group: 13 (Boron family, p-block element)
Period: 4
Block: p-block
Electronic Configuration: [Ar] 3d¹⁰ 4s² 4p¹
Valency (Common): +3
Oxidation States: +3 (most common and stable), +1 (less stable due to inert pair effect)
Nature: Silvery-white, soft metal, unusually low melting point (29.76 °C), high boiling point (2204 °C).
Density: 5.91 g/cm³ (at 25 °C).
Amphoteric Nature: Gallium and its compounds (e.g., Ga₂O₃, Ga(OH)₃) react with both acids and strong bases.

Electron Configuration & Bonding Behavior

Electronic Configuration

The ground state electronic configuration of Gallium is [Ar] 3d¹⁰ 4s² 4p¹. The presence of filled 3d¹⁰ orbitals significantly influences its properties, particularly its ionization energies and the stability of its oxidation states.

Bonding Behavior and Oxidation States

Dominant Oxidation State (+3): Gallium predominantly exhibits a +3 oxidation state, formed by the loss of its three valence electrons (two 4s and one 4p electrons).
Inert Pair Effect (+1): Due to the “inert pair effect,” the ns² electrons (here, 4s²) become less involved in bonding down Group 13. While more prominent for heavier elements like Thallium (Tl), Gallium can form some compounds in the +1 oxidation state, though these are generally less stable than their +3 counterparts. For example, GaCl is less stable than GaCl₃.
Covalent Character: Gallium compounds tend to exhibit significant covalent character, particularly with electronegative elements. This is due to the high ionization energy sum for forming the Ga³⁺ ion and the high charge-to-radius ratio of the Ga³⁺ ion, which causes strong polarization of anions.
Coordination Chemistry: Ga³⁺ forms stable complex ions, often exhibiting coordination numbers of 4 (tetrahedral, e.g., [GaCl₄]⁻) or 6 (octahedral, e.g., [Ga(OH)₆]³⁻).

Crucial Chemical Reactions

1. Reaction with Air/Oxygen

Gallium reacts with oxygen upon heating to form gallium(III) oxide. At room temperature, a protective oxide layer forms, preventing further oxidation. 4Ga(s) + 3O₂(g) → 2Ga₂O₃(s)

2. Reaction with Acids

Gallium reacts with non-oxidizing acids, displacing hydrogen due to its position above hydrogen in the electrochemical series. It passivates with strong oxidizing acids like concentrated nitric acid.

With Hydrochloric Acid: 2Ga(s) + 6HCl(aq) → 2GaCl₃(aq) + 3H₂(g)

3. Reaction with Strong Bases (Amphoteric Nature)

Gallium’s amphoteric nature is evident in its reaction with strong bases to form gallates.

With Sodium Hydroxide: 2Ga(s) + 2NaOH(aq) + 6H₂O(l) → 2Na[Ga(OH)₄](aq) + 3H₂(g) (Sodium tetrahydroxogallate(III), sometimes written as NaGaO₂.2H₂O) Or the oxide reacts: Ga₂O₃(s) + 2NaOH(aq) + 3H₂O(l) → 2Na[Ga(OH)₄](aq)

4. Reaction with Halogens

Gallium reacts vigorously with halogens to form trihalides.

With Chlorine: 2Ga(s) + 3Cl₂(g) → 2GaCl₃(s)

5. Reaction with Water

Gallium does not react with water or steam at ordinary temperatures, forming a passive oxide layer that protects the metal. At high temperatures, some reaction may occur.

Industrial and Biological Importance

Industrial Importance

Semiconductors: Gallium arsenide (GaAs) is a crucial semiconductor material. It is used in high-frequency integrated circuits, microwave circuits, infrared LEDs, laser diodes, and high-efficiency multi-junction solar cells due to its superior electron mobility and direct band gap compared to silicon. Gallium nitride (GaN) is also used for blue and white LEDs and high-power electronic devices.
Alloys: Gallium forms low-melting point alloys. For example, Galinstan (an eutectic alloy of Gallium, Indium, and Tin) is a non-toxic liquid metal used as a replacement for mercury in thermometers, heat transfer fluids, and various electronic applications.
High-Temperature Thermometers: Due to its very high boiling point and low melting point, pure gallium is used in some high-temperature thermometers.
Medical Imaging: Certain radioactive isotopes of gallium (e.g., Gallium-67, Gallium-68) are used in nuclear medicine for diagnostic imaging (PET scans) to detect tumors and inflammation. Gallium citrate is absorbed by cells with high metabolic activity, including cancer cells and inflammatory cells.

Biological Importance

No Known Biological Role: Gallium has no known essential biological role in humans or other organisms.
Biomedical Applications: Despite no natural biological role, Ga(III) ions mimic Fe(III) ions due to similar ionic radii and charge. This allows gallium compounds to bind to iron-binding proteins like transferrin in the body. This property is exploited in:
- Anti-cancer agents: Gallium compounds are being investigated as potential anti-cancer drugs, as cancer cells often have increased iron uptake requirements.
- Anti-inflammatory agents: Gallium can inhibit the activity of certain enzymes involved in inflammation.