Group 1A elements (alkali metals) and Group 2A elements (alkaline earth metals) differ significantly in their reactions with water.
Group 1A (Alkali Metals)
- Reactivity: Alkali metals (such as lithium, sodium, potassium, etc.) are highly reactive with water. The reactivity increases down the group.
- Reaction with Water: When alkali metals react with water, they form hydroxides and hydrogen gas. The general reaction is: \[ 2 \text{M} + 2 \text{H}_2\text{O} \rightarrow 2 \text{MOH} + \text{H}_2 \] (where M represents an alkali metal).
- Nature of Reaction: The reaction is vigorous, especially for the heavier alkali metals, such as potassium and sodium, often producing enough heat to ignite the hydrogen gas released.
Group 2A (Alkaline Earth Metals)
- Reactivity: Alkaline earth metals (such as magnesium, calcium, strontium, and barium) are less reactive with water compared to alkali metals. Their reactivity increases down the group.
- Reaction with Water: The trend is that beryllium reacts very little with water, while magnesium reacts slowly with hot water or steam. Calcium and heavier alkaline earth metals react more readily with water, producing hydroxides and hydrogen gas. The general reaction for calcium is: \[ \text{Ca} + 2 \text{H}_2\text{O} \rightarrow \text{Ca(OH)}_2 + \text{H}_2 \]
- Nature of Reaction: The reactivity is less vigorous than that of alkali metals. Calcium's reaction with cold water is noticeable, while magnesium's reaction is more subdued unless heated.
Summary
- Alkali Metals (Group 1A): Very reactive, vigorous reactions with water, releasing hydrogen gas and forming strong hydroxides.
- Alkaline Earth Metals (Group 2A): Less reactive, with varying rates of reaction depending on the specific metal, generally forming weaker hydroxides.
This distinction is important for understanding the chemical behavior of these elements and their compounds in various environments.