Which of the following best helps to explain why Na(s) is more reactive with water than Mg(s) is? — Atomic Structure Chemistry Question

STEPS:

1. Analyze the chemical reaction occurring with water: When solid sodium ($\text{Na}$) and magnesium ($\text{Mg}$) metals react with water, they undergo an oxidation-reduction reaction where the neutral metal atoms lose their valence electrons to become positive ions ($\text{Na}^+$ and $\text{Mg}^{2+}$) in solution.
2. Identify the thermodynamic property governing electron loss: The ease with which a metal atom loses its outermost electron is directly dictated by its first ionization energy. A lower first ionization energy means less energy is required to remove the first valence electron, making oxidation thermodynamically and kinetically easier.
3. Compare the periodic trends of Na and Mg: Sodium (Group 1) and magnesium (Group 2) are adjacent elements in Period 3. Magnesium has a higher nuclear charge (12 protons vs. 11 protons) attracting valence electrons in the same $3s$ shell, which holds those electrons more tightly. Therefore, the first ionization energy of sodium is significantly less than that of magnesium.
4. Relate ionization energy directly to reactivity: Because sodium's single valence electron is held less tightly, sodium loses its electron much more easily than magnesium does. This lower energy barrier allows sodium to react violently with water at room temperature, while magnesium reacts incredibly slowly under the same conditions. This matches Option D.

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WHY_OTHERS_WRONG:

• Option A is incorrect: While sodium is physically softer than magnesium, physical hardness is a macroscopic mechanical property related to metallic lattice strength, not a chemical property that dictates the rate of electron transfer to water.
• Option B is incorrect: Although the atomic mass of sodium is slightly less than that of magnesium, mass itself has no bearing on the energy required to remove valence electrons or initiate an oxidation reaction.
• Option C is incorrect: While it is true that a $\text{Na}^+$ ion experiences weaker Coulombic attraction to a neighboring anion than a $\text{Mg}^{2+}$ ion (due to its smaller $+1$ charge), this describes the stability of the reaction products rather than the ease of removing electrons from the neutral metal reactants to start the reaction.