# 5.3 Comparison of Reactivity: Group 1 (Alkali Metals) vs. Group 2 (Alkaline Earth Metals) The chemical reactivity of an element is largely determined by its tendency to gain, lose, or share electrons. For metals, this tendency is primarily linked to ionization energy. ### Ionization Energy and Reactivity **Definition:** Ionization energy is the minimum energy required to remove one mole of electrons from one mole of gaseous atoms in their ground state. **Relationship to Reactivity:** A smaller ionization energy indicates that an atom can more easily lose its valence electron(s). This ease of electron loss translates directly to greater reactivity for metals. ### Ionization Energy Comparison: Group 1 vs. Group 2 Alkaline earth metals (Group 2) consistently have higher ionization energies compared to alkali metals (Group 1) within the same period. **Reasons:** 1. **Smaller Atomic Size:** Group 2 elements have a slightly smaller atomic radius than Group 1 elements in the same period due to a greater nuclear charge pulling the electron shell inward more effectively. 2. **Greater Nuclear Attraction:** The increased nuclear charge in Group 2 elements (e.g., +2 charge for $\mathrm{Mg}$ vs. +1 for $\mathrm{Na}$) exerts a stronger attractive force on their valence electrons. These factors make it harder to remove an electron from a Group 2 element. ### Reactivity Trend Based on their lower ionization energies, Group 1 elements are significantly more reactive than Group 2 elements in the same period. ## Reactions with Water The difference in reactivity between Group 1 and Group 2 metals is demonstrated by their reactions with water. For more details on Group 2 reactivity with water, refer to . ### Alkali Metals (Group 1) React vigorously and exothermically with water. The reaction produces hydrogen gas ($\mathrm{H_2}$), which often catches fire due to the heat generated. $$2\mathrm{Na}_{(\mathrm{s})} + 2\mathrm{H}_{2}\mathrm{O}_{(\mathrm{l})} \rightarrow 2\mathrm{NaOH}_{(\mathrm{aq})} + \mathrm{H}_{2(\mathrm{g})}$$ ### Alkaline Earth Metals (Group 2) React more slowly with water compared to Group 1 metals. Hydrogen gas is produced, but the reaction is generally less vigorous, and the hydrogen typically does not ignite. Magnesium, for instance, requires heating to react significantly. $$\mathrm{Ca}_{(\mathrm{s})} + 2\mathrm{H}_{2}\mathrm{O}_{(\mathrm{l})} \rightarrow \mathrm{Ca(OH)}_{2(\mathrm{aq})} + \mathrm{H}_{2(\mathrm{g})}$$ ## Reducing Agent Strength **Definition:** A reducing agent is a substance that readily donates electrons to another substance, causing the other substance to be reduced while itself being oxidized. **Comparison:** Alkali metals are generally better (stronger) reducing agents compared to alkaline earth metals. **Reason:** This is directly attributed to their smaller ionization energies, which means they can more easily lose electrons and therefore more effectively reduce other chemical species. ## Physical Properties Comparison While reactivity is the primary focus, physical properties also differ. Group 2 metals generally have higher melting points and densities than Group 1 metals due to stronger metallic bonding (two valence electrons vs. one). ## Basic Nature of Oxides and Hydroxides Both groups form basic oxides and hydroxides, but Group 1 hydroxides are generally more soluble and stronger bases than Group 2 hydroxides.