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 higher reactivity of cesium with water compared to sodium is option (ii), the first ionization energy of cesium being less than that of sodium. This is because a lower ionization energy means cesium can lose its valence electron more easily than sodium, which results in a more violent reaction with water.

Step 1: Understanding metallic character and reactivity

Metallic character refers to the properties of elements that are typical of metals, such as high electrical and thermal conductivity, malleability, and ductility. In general, metallic character increases as we move down and to the left in the periodic table. Therefore, cesium, being further down and to the left of sodium in the periodic table, has a greater metallic character than sodium.

Step 2: Understanding ionization energy

Ionization energy is the energy required to remove an electron from an atom. In general, ionization energy decreases as we move down a group in the periodic table. This is due to the increased atomic size and a weaker force between the nucleus and the outer electrons. Since cesium is further down the periodic table than sodium, its first ionization energy is lower than that of sodium.

Step 3: Understanding electron affinity

Electron affinity is the energy change that occurs when an electron is added to a neutral atom. Electron affinity generally becomes more negative as we move across a period in the periodic table. This trend is not directly related to reactivity with water, so it doesn't help explain the difference in reactivity between sodium and cesium.

Step 4: Understanding effective nuclear charge

Effective nuclear charge is the net positive charge experienced by an electron in an atom, taking into account the shielding effect of inner electrons. The effective nuclear charge generally increases as we move across a period in the periodic table. This trend is not directly related to reactivity with water, so it doesn't help explain the difference in reactivity between sodium and cesium.

Step 5: Understanding atomic radius

Atomic radius is the measure of the size of an atom. The atomic radius generally decreases as we move across a period and increases as we move down a group in the periodic table. Since cesium is further down the periodic table than sodium, its atomic radius is larger than that of sodium.

Step 6: Conclusion

Based on our understanding of these properties, the best explanation for the higher reactivity of cesium with water compared to sodium is option (ii), the first ionization energy of cesium being less than that of sodium. This is because a lower ionization energy means cesium can lose its valence electron more easily than sodium, which results in a more violent reaction with water.

Key Concepts

Metallic Character

When we talk about metallic character, we're describing how strongly an element behaves like a metal. This includes traits such as shiny appearance, good conductivity of heat and electricity, and the ability to be shaped or drawn into wires.
In the periodic table, as we move down a group (like going from sodium to cesium), elements tend to display increased metallic character. This is because their atoms become larger and their outer electrons are weaker bound to the nucleus.
Cesium sits further down and to the left than sodium, meaning it's more metallic. A greater metallic character in cesium contributes to a higher reactivity as it can more readily lose electrons.

Ionization Energy

Ionization energy is a key concept in understanding element reactivity. It is the energy required to remove an electron from a neutral atom.
The lower the ionization energy, the easier it is for an element to lose an electron and react. Moving down the periodic table, like from sodium to cesium, the ionization energy generally decreases.
This decrease is due to the increased atomic radius and less effective nuclear attraction on the outermost electrons. As cesium has a lower ionization energy than sodium, it more easily loses electrons, making it more reactive with substances like water.

Effective Nuclear Charge

Effective nuclear charge (\(Z_{eff}\)) is the net positive charge that an electron in an atom experiences. It considers the actual nuclear charge minus the shielding effect of inner-shell electrons.
As we move across a period in the periodic table, the effective nuclear charge felt by electrons generally increases because the number of protons in the nucleus rises without a big increase in electron shielding.
This increased attraction generally means that electrons are held more tightly. However, when moving down a group, such as from sodium to cesium, the effective nuclear charge doesn't increase as significantly, which means cesium's outer electrons are less tightly held, contributing to its greater reactivity.

Atomic Radius

The atomic radius represents the size of an atom, typically measured from the nucleus to the outer boundary of its electron cloud.
In the periodic table, the atomic radius increases as you move down a group. This happens because additional electron shells are added, making the atom larger, as seen in cesium compared to sodium.
The larger atomic radius of cesium results in a weaker hold on its outermost electron, facilitating electron loss. This plays into cesium's increased reactivity compared to sodium, because atoms that can easily lose their outer electrons tend to be more reactive.