Which of the following alkaline earth metals react more spontaneously with water?

Analysis

The question asks us to identify which of the listed Group 2 elements (alkaline earth metals) reacts most spontaneously with water.

Concept Explanation

In the Periodic Table, reactivity for metals is determined by how easily an atom can lose its valence electrons to form positive ions (cations).

For Group 2 elements (Beryllium, Magnesium, Calcium, Strontium, Barium, Radium), reactivity increases as you move down the group. This happens because:

1. Atomic Size: As you move down the group, the number of electron shells increases.
2. Shielding Effect: The valence electrons are further away from the nucleus and are shielded by more inner shells.
3. Ionization Energy: Because the valence electrons are further from the nucleus and more shielded, the force of attraction between the nucleus and the valence electrons decreases. Consequently, it requires less energy to remove these electrons, making the metal more reactive.

Step-by-Step Solution

1. Locate the elements in Group 2: • Beryllium (Be) - Period 2 • Magnesium (Mg) - Period 3 • Calcium (Ca) - Period 4 • Strontium (Sr) - Period 5

2. Apply the trend: Since reactivity increases down the group, we compare their positions: • Be is the least reactive (it barely reacts with water). • Mg reacts very slowly with cold water. • Ca reacts steadily with cold water. • Sr reacts more vigorously than Calcium.

3. Conclusion: Among the options provided, Strontium is the lowest in the group, meaning it has the lowest ionization energy and the highest tendency to lose electrons, making it the most spontaneous in its reaction with water.

Identification of Answer

Comparing the options, Strontium (Sr) is the most reactive metal listed. Therefore, it reacts more spontaneously with water than Calcium, Magnesium, or Beryllium.

Exam Tip

Remember the mnemonic for Group 2 reactivity: "The further down, the more the boom!" As you go down the group, the metals become "softer" and more reactive. If you ever forget the trend, just remember that Francium (at the bottom of Group 1) is the most reactive metal—the same logic applies to Group 2!