Group 1 (alkali metals), Group 2 (alkaline earth metals), and Group 13 (represented by Al) are all typical metals that readily form cations. Let us study the periodicity of chemical reactivity and ionic radii.
Basic Knowledge
Alkali metals (Li, Na, K, Rb, Cs): All form monovalent cations; they are silvery white, soft, and react vigorously with water: 2Na + 2H2O → 2NaOH + H2. Reactivity increases in the order Li<Na<K<Rb<Cs. Flame-test colors are Li (red), Na (yellow), K (purple), Rb (red-purple), and Cs (blue).
Alkaline earth metals (Ca, Sr, Ba) (in high school, Be and Mg may sometimes be excluded): All form divalent cations. Flame-test colors are Ca (orange-red), Sr (crimson), and Ba (yellow-green). Difference between Mg and Ca: Mg reacts with hot water, whereas Ca reacts with water at room temperature: Ca + 2H2O → Ca(OH)2 + H2.
Carbonates: Alkali-metal carbonates are soluble in water, whereas alkaline-earth-metal carbonates (including Mg) are insoluble. Limewater, Ca(OH)2 + CO2 → CaCO3↓ + H2O, is a reaction for detecting carbon dioxide.
Aluminum Al: Group 13, a trivalent cation, and an amphoteric metal (dissolves in both acids and strong bases). 2Al + 6HCl → 2AlCl3 + 3H2, 2Al + 2NaOH + 6H2O → 2Na[Al(OH)4] + 3H2. It is refined by electrolysis of molten salts (the Hall–Héroult process).
Alkali metals (Group 1, monovalent cations, react vigorously with water)
Flame test (Li red · Na yellow · K purple · Ca orange · Sr crimson · Ba yellow-green · Cu blue-green)
Ammonia-soda process (Solvay process) (NaCl+CaCO3→Na2CO3, NH3 recycled as a catalyst)
Amphoteric metals (Al, Zn, Sn, Pb = “Ah, smoothly” mnemonic)
Tetrahydroxoaluminate ion ([Al(OH)4]-, the form dissolved in NaOH)
Alumite (the hard Al2O3 anodic oxide film on the Al surface)
Further Study (Principles and Applications)
The industrial production of sodium carbonate Na2CO3, the ammonia-soda process (Solvay process): CO2 and NH3 are bubbled into saturated brine to precipitate NaHCO3, which is heated to obtain Na2CO3. NH3 and CO2 are recycled, making this a classic green process with high atom efficiency. It is essential in the glass, detergent, and pulp industries.
Aluminum is the most abundant metallic element in Earth’s crust (8%), but because Al3+ is stable, the pure metal was treated as a precious metal until the end of the nineteenth century. In 1886, Hall and Héroult established a method of refining bauxite into Al2O3 (the Bayer process), dissolving it in cryolite Na3AlF6, and electrolyzing it. Electrolysis requires a large amount of electricity, so Al is called “canned electricity.” Aluminum recycling requires only about 3% of the energy needed to produce new metal, making it a representative example of a circular economy.
- Alkali metals are monovalent; alkaline earth metals are divalent
- Reactivity with water: Li<Na<K<Rb<Cs
- Remember the flame-test colors (Li red, Na yellow, K purple, Ca orange, Sr crimson, Ba yellow-green, and so on)
- Na2CO3 is produced by the Solvay process
- Al is amphoteric and dissolves in both HCl and NaOH
- Al refining uses electrolysis of molten cryolite (Hall–Héroult process)
- Mg reacts with hot water; Ca reacts with water at room temperature
Points to Note (Common Confusions and Frequent Errors)
① Mg is sometimes not included among the alkaline earth metals (it does not show a flame test and has low reactivity with water). ② Alkali metals are stored under kerosene because they react with air and water. ③ Amphoteric metals are “Ah, smoothly” = Al, Zn, Sn, Pb. ④ NaHCO3 (baking soda) decomposes upon heating: 2NaHCO3 → Na2CO3 + H2O + CO2. ⑤ Do not confuse passivation (Al, Fe, and Ni in concentrated nitric acid) with amphoterism (Al, Zn, Sn, and Pb).
Practice
- Write the equation for the reaction between sodium and water.
- Write two major reactions by which Na2CO3 is obtained in the Solvay process.
- Write the equations for dissolving Al in HCl and NaOH, respectively.