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What is the Reactivity of the Alkaline Earth Metals?

Alkaline earth metals, comprising beryllium (Be), magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba), and radium (Ra), are reactive metals, though generally less so than the alkali metals. Their reactivity arises from their tendency to lose their two valence electrons to achieve a stable, noble gas electron configuration, forming dipositive ions (M²⁺).

Reactivity Trends and Factors Influencing It

The reactivity of alkaline earth metals generally increases down the group from beryllium to radium. Several factors contribute to this trend:

Ionization Energy: The first and second ionization energies, representing the energy required to remove the first and second electrons, respectively, decrease down the group. This decrease makes it easier for heavier alkaline earth metals to lose their two valence electrons. Beryllium, having the highest ionization energies, is the least reactive, while radium, with the lowest, is the most reactive.
Atomic Size: The atomic radius increases down the group. A larger atomic radius means the valence electrons are further away from the positively charged nucleus. This weaker attraction makes it easier for these electrons to be removed, increasing reactivity.
Electronegativity: Electronegativity, a measure of an atom’s ability to attract electrons in a chemical bond, decreases down the group. This contributes to the increased ease of electron loss.
Hydration Energy: While ionization energy favors reactivity increasing down the group, hydration energy (the energy released when an ion is solvated by water molecules) works in the opposite direction. Smaller ions, like Be²⁺ and Mg²⁺, have higher charge densities and are therefore more strongly hydrated. This significant hydration energy can, in some cases, compensate for the higher ionization energies and influence the overall reaction rate in aqueous solutions.

Reactions with Water

The reaction with water is a key indicator of reactivity.

Beryllium (Be): Under normal conditions, beryllium does not react with water, even at high temperatures. This is due to the formation of a tightly adhering oxide layer on its surface that protects it from further reaction.
Magnesium (Mg): Magnesium reacts very slowly with cold water. However, it reacts readily with steam to form magnesium oxide and hydrogen gas: Mg(s) + H₂O(g) → MgO(s) + H₂(g).
Calcium (Ca), Strontium (Sr), and Barium (Ba): These elements react vigorously with cold water, forming the metal hydroxide and hydrogen gas: M(s) + 2H₂O(l) → M(OH)₂(aq) + H₂(g). The reactivity increases from calcium to barium.
Radium (Ra): Radium reacts most vigorously with water, but its radioactivity limits its use in demonstrating this reactivity.

Reactions with Air

Alkaline earth metals react with oxygen and nitrogen in the air, although the rate of reaction varies.

Beryllium (Be): Beryllium forms a protective oxide layer in air, which passivates the metal and prevents further oxidation at room temperature.
Magnesium (Mg): Magnesium reacts slowly with oxygen at room temperature, forming a thin oxide layer. When heated, it burns brightly in air, forming magnesium oxide and some magnesium nitride.
Calcium (Ca), Strontium (Sr), and Barium (Ba): These metals tarnish rapidly in air, forming oxides, nitrides, and even carbonates due to reaction with carbon dioxide. They are often stored under oil to prevent oxidation.

Reactions with Acids

Alkaline earth metals react with acids, producing hydrogen gas and a salt. The general reaction is: M(s) + 2H⁺(aq) → M²⁺(aq) + H₂(g). The reactivity again follows the trend of increasing reactivity down the group.

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