To determine which statement is true about the elements of Group IA (alkali metals) and Group IIA (alkaline earth metals), we need to analyze the properties of these groups based on their electronic configurations, ionization energies, atomic sizes, and reactivities.
### Step-by-Step Solution:
1. **Identify the Groups**:
- Group IA (Alkali Metals): Lithium (Li), Sodium (Na), Potassium (K), Rubidium (Rb), Cesium (Cs), Francium (Fr).
- Group IIA (Alkaline Earth Metals): Beryllium (Be), Magnesium (Mg), Calcium (Ca), Strontium (Sr), Barium (Ba), Radium (Ra).
2. **Analyze Ionization Energies**:
- Ionization energy is the energy required to remove an electron from an atom.
- Alkali metals have a single electron in their outermost shell (ns¹), while alkaline earth metals have two electrons (ns²).
- As we move from left to right across a period, ionization energy increases due to the increasing effective nuclear charge.
- Therefore, the ionization energy of alkali metals is **lower** than that of alkaline earth metals.
3. **Compare Atomic Sizes**:
- Atomic size generally decreases across a period due to increased nuclear charge pulling electrons closer to the nucleus.
- In the same period, alkali metals are larger than alkaline earth metals because they have one less proton attracting the same number of electrons.
- Thus, the statement that the size of alkaline earth metals is larger than that of alkali metals is **false**.
4. **Evaluate Reactivity**:
- Reactivity in metals is often determined by how easily they can lose their outermost electrons.
- Alkali metals (with ns¹ configuration) can achieve a stable noble gas configuration by losing just one electron, making them highly reactive.
- Alkaline earth metals need to lose two electrons (ns²) to achieve stability, making them less reactive compared to alkali metals.
- Therefore, the statement that the reactivity of alkaline earth metals is less than that of alkali metals is **true**.
5. **Conclusion**:
- The true statement among the options provided is: "The reactivity of alkaline earth metals is less than that of alkali metals."
### Final Answer:
The correct statement is: **The reactivity of alkaline earth metals is less than that of alkali metals.**
