Problem 103 - Chemistry: The Central Science

Question

Elemental cesium reacts more violently with water than does elemental sodium. Which of the following best explains this difference in reactivity? (i) Sodium has greater metallic character than does cesium. (ii) The first ionization energy of cesium is less than that of sodium. (iii) The electron affinity of sodium is smaller than that of cesium. (iv) The effective nuclear charge for cesium is less than that of sodium. (v) The atomic radius of cesium is smaller than that of sodium.

Step-by-Step Solution

Answer

The best explanation for the difference in reactivity between elemental cesium and sodium when reacting with water is statement (ii): The first ionization energy of cesium is less than that of sodium. This lower ionization energy allows cesium to lose its valence electron more easily, making it more reactive with water than sodium.

Step 1: Understand the periodic trends

To find out which statement is correct, we need to know the periodic trends and how they affect reactivity. The periodic trends are the variations in elemental properties with increasing atomic number. The important trends to consider are atomic radius, ionization energy, electron affinity, and metallic character.

Step 2: Compare metallic character

Metallic character decreases from left to right, and increases from top to bottom in the periodic table. Since cesium is below sodium in Group 1, cesium has a greater metallic character than sodium. Hence, statement (i) is false.

Step 3: Compare first ionization energies

First ionization energy is the energy required to remove the most loosely bound electron from an atom in its ground state. It increases from left to right and decreases from top to bottom in the periodic table. Since cesium is below sodium, cesium's ionization energy is less than that of sodium. This means cesium's valence electron is more easily removed, making it more reactive. Therefore, statement (ii) is correct.

Step 4: Compare electron affinities

Electron affinity is the energy change when an electron is added to a neutral atom, forming a negative ion. It increases from left to right and decreases from top to bottom in the periodic table. The electron affinity of sodium would be larger than cesium's, due to its position in the periodic table. However, electron affinity is not the main factor affecting the reactivity between elements and water in this case, since both elements belong to the same group and lose electrons while reacting with water. Therefore, statement (iii) is not the best explanation.

Step 5: Compare effective nuclear charges

Effective nuclear charge is the net charge experienced by the valence electrons in an atom. It generally increases from left to right and slightly increases from top to bottom in the periodic table. The effective nuclear charge for cesium would be less than that of sodium due to the increased number of electron shielding layers. However, this trend is not as important with regards to reactivity in this case. Hence, statement (iv) is not the best explanation.

Step 6: Compare atomic radii

Atomic radius is the distance from the nucleus to the outermost electron shell in an atom. It decreases from left to right and increases from top to bottom in the periodic table. The atomic radius of cesium is larger than that of sodium due to its position in the periodic table. A larger atomic radius means electrons are farther from the nucleus and more easily removed, increasing reactivity. Despite this fact, it is not the main factor affecting the reactivity difference between cesium and sodium. Therefore, statement (v) is not the best explanation.

Step 7: Conclusion

The best explanation for the difference in reactivity between elemental cesium and sodium when reacting with water is statement (ii): The first ionization energy of cesium is less than that of sodium. This lower ionization energy allows cesium to lose its valence electron more easily, making it more reactive with water than sodium.