Electronic structure and electron deficiency
The aluminum atom is characterized by electron deficiency: its four outer-level valence orbitals contain only three electrons. This determines its key chemical properties.
In the excited state, one $s$-electron transitions to a $p$-orbital. This transition accounts for its acceptor properties—the ability to accept electron pairs. Due to the presence of a vacant $3d$-subshell, the metal can exhibit coordination numbers 4 and 6, forming complexes (e.g., $Na[AlCl_4]$ and $Na_3[AlF_6]$).
Due to the shortage of electrons, aluminum compounds tend to dimerize. For instance, gaseous aluminum chloride at ~180 °C associates into the $Al_2Cl_6$ dimer. This association proceeds via a donor-acceptor mechanism utilizing lone pairs of chlorine, with the metal atom transitioning to $sp^3$-hybridization.
Interaction with simple substances
In air, aluminum becomes coated with a microscopic yet extremely durable oxide film, which grants it high corrosion resistance.
Upon heating or under harsh conditions, the metal exhibits high reactivity:
- With oxygen: burns with the release of a large amount of heat (exothermic reaction).
- With sulfur: upon heating, the powder mixture glows red-hot and reacts violently to form a sulfide.
- With non-metals: at 1000 °C, it reacts with nitrogen to yield a refractory nitride (AlN), and at 2000 °C, it reacts with carbon to form an orange carbide (Al_4C_3).
Aluminum powder is a potent reducing agent. In the process of aluminothermy, it reduces iron, chromium, and manganese from their oxides. The reaction is initiated with a burning magnesium strip, after which it proceeds with such massive heat release that the reduced metals are formed as molten beads.
Behavior towards water, alkalis, and acids
Aluminum is a typical amphoteric metal.
- Water: due to the oxide film, the metal does not react with pure water.
- Alkalis: alkaline solutions dissolve the protective film. Afterward, the exposed metal reacts with water in the presence of the alkali, releasing hydrogen and forming a complex salt (sodium tetrahydroxoaluminate).
- Acids: in dilute acids (hydrochloric, sulfuric), the metal dissolves rapidly with the evolution of hydrogen. With nitric acid, the reaction is specific: strongly dilute HNO_3 reacts without gas evolution (forming an ammonium ion), whereas concentrated nitric acid passivates the surface, completely halting the process.
Aluminum oxide, hydroxide, and salts
The Al–O bond is very strong, but the Al–F bond is even stronger, which makes aluminum fluoride insoluble and resistant to hydrolysis. Binary compounds with chlorine, sulfur, and carbon undergo complete hydrolysis.
Aluminum oxide ($Al_2O_3$) occurs in nature as the inert mineral corundum. It does not react with solutions, but upon fusion with solid alkalis or carbonates, it yields aluminates. To obtain the reactive gamma form ($\gamma$-oxide), one must gently dehydrate the hydroxide at a temperature no higher than 300 °C.
Aluminum hydroxide ($Al(OH)_3$) is amphoteric. Weak bases (ammonia, sodium bicarbonate) are used for its precipitation in the laboratory. If a strong alkali is used, the precipitate dissolves instantly with the formation of hydroxo complexes.
A distinct group of salts is formed by alums—double sulfates. Potassium aluminum alum crystallizes with 12 water molecules and is used in medicine due to its astringent action. Its anhydrous form (burnt alum) is used as a drying powder.
Briefly: answers to topical questions
Which non-metals does aluminum react with upon heating, and what products are formed?
Upon heating, aluminum reacts with oxygen, sulfur, nitrogen, and carbon.
- Oxygen — aluminum oxide ($Al_2O_3$) is formed.
- Sulfur — aluminum sulfide ($Al_2S_3$) is formed.
- Nitrogen — at 1000 °C, aluminum nitride ($AlN$) is formed.
- Carbon — at 2000 °C, aluminum carbide ($Al_4C_3$) is formed.
Problems may also mention the interaction of aluminum with iodine ($I_2$).
How does the reaction of aluminum with nitric acid proceed depending on its concentration?
The nature of the interaction between aluminum and nitric acid is determined by its concentration.
| Concentration | Reaction features | Food products |
|---|---|---|
| Strongly dilute | Reacts without gas evolution | Aluminum nitrate and ammonium nitrate ($NH_4NO_3$) |
| Concentrated | No reaction | Surface passivation occurs (a durable film is created) |
Hydrogen is never evolved in the reaction with nitric acid.
How does aluminum interact with aqueous solutions of salts of other metals?
Aluminum is capable of displacing less active metals from aqueous solutions of their salts. The substitution reaction yields an aluminum salt and releases the free metal.
For instance, during interaction with copper(II) sulfate in an aqueous medium, aluminum reduces copper: $2Al + 3CuSO_4 \xrightarrow{H_2O} Al_2(SO_4)_3 + 3Cu$
Displacement of iron from iron(III) chloride is also possible upon heating.
Which metals can be reduced from their oxides by aluminothermy?
Many metals can be reduced from their oxides via aluminothermy, notably iron, manganese, and chromium. Aluminum powder acts as a strong reducing agent.
The process is accompanied by a large release of heat, so the reduced metals are obtained in a molten state as metallic beads. For example, interaction with magnetite ($Fe_3O_4$) yields pure iron.
Which complex anions are formed when aluminum is dissolved in aqueous alkali solutions?
When aluminum is dissolved in aqueous alkali solutions, hydroxo complexes with coordination numbers of 4 or 6 are formed.
- Diaquatetrahydroxoaluminate ion — $[Al(OH)_4(H_2O)_2]^-$ (formed considering coordination number 6).
- Tetrahydroxoaluminate ion — $[Al(OH)_4]^-$ (abbreviated form).
The process is accompanied by the dissolution of the protective oxide film, oxidation of the metal by water, and the evolution of hydrogen gas.
It is easy to remember aluminum's behavior toward nitric acid by the rule of extremes: "Concentrated protects (passivates), dilute destroys (down to ammonium)"
Class and exam questions
Why does aluminum not react with pure water despite being located before hydrogen in the reactivity series?
The reason lies in the presence of an extremely thin, dense, and very durable aluminum oxide film on the metal's surface. It mechanically isolates the metal from water; if the film is removed with alkali, the reaction proceeds violently.
How to precipitate aluminum hydroxide without it dissolving?
One must treat aluminum salt solutions with weak bases, such as an aqueous solution of ammonia or sodium bicarbonate. A minimal excess of strong alkali will lead to dissolution of the precipitate and complex formation.
What happens upon strong ignition of aluminum chloride crystal hydrate?
An anhydrous salt is not formed; instead, aluminum oxide is produced. Water is split off and gaseous hydrogen chloride is volatilized during the process.
What else to study on this topic
- Mechanism of aluminum compound dimerization and $sp^3$ hybridization
- Stepwise hydrolysis of the aluminum cation and its effect on medium acidity
- Diagonal relationship in the chemical properties of aluminum and beryllium
- Chemistry of industrial electrolysis: role of cryolite and side reactions at the anode
- Formation of unstable univalent aluminum compounds (Al-I)
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