Problem 54 - Chemistry The Central Science

Question

Which of the following statements does not follow from the fact that the alkali metals have relatively weak metal-metal bonding? $$ \begin{array}{l}{\text { (a) The alkali metals are less dense than other metals. }} \\ {\text { (b) The alkali metals are soft enough to be cut with a knife. }} \\ {\text { (c) The alkali metals are more reactive than other metals. }} \\ {\text { (d) The alkali metals have higher melting points than }} \\ {\text { other metals. }} \\ {\text { (e) The alkali metals have lowization energies. }}\end{array} $$

Step-by-Step Solution

Answer

The statement that does not follow from the fact that alkali metals have relatively weak metal-metal bonding is: \( \text{(d) The alkali metals have higher melting points than other metals.} \)

Step 1: Analyze given properties and statements

Since alkali metals have weak metal-metal bonding, it means that these bonds are easily broken. This will allow alkali metals to possess certain properties. Now, let's analyze each statement to see if they are directly related to weak metal-metal bonding: (a) The alkali metals are less dense than other metals: Alkali metals, particularly those in Group 1 of the periodic table, are known to be less dense than transition metals. This is partially due to having less packed atomic arrangements, which in turn, is affected by their weaker metal-metal bonding. (b) The alkali metals are soft enough to be cut with a knife: The weakness of metal-metal bonding translates to the metal's ability to be deformed and cut easily. Therefore, this statement is related to the weak bonding nature of alkali metals. (c) The alkali metals are more reactive than other metals: The reactivity of alkali metals increases as their metal-metal bonds weakens. This means they have a greater tendency to lose an electron and form a positive ion, also explaining their low ionization energies. (d) The alkali metals have higher melting points than other metals: Melting points depend on the strength of the interaction between atoms. As alkali metals have weak metal-metal bonding, they are expected to have lower melting points compared to other metals. (e) The alkali metals have low ionization energies: Ionization energies depend on how easily an atom can lose an electron. Since alkali metals have weak metal-metal bonding, their atoms can lose an electron more easily, resulting in a low ionization energy.

Step 2: Determine the statement that does not follow the given fact

From the analysis above, the statement that does not follow from the fact that alkali metals have relatively weak metal-metal bonding is: \( \text{(d) The alkali metals have higher melting points than other metals.} \) This statement contradicts the expected properties of alkali metals, which are known to have lower melting points due to their weak metal-metal bonding.

Key Concepts

Metal-Metal Bonding

Metal-metal bonding is the interaction between metal atoms in a metallic substance. The strength of this bonding can significantly influence the physical properties of the metal. For alkali metals, which include lithium, sodium, potassium, rubidium, cesium, and francium, the metal-metal bonds are relatively weaker compared to those in transition metals.

This weak bonding is because alkali metals have a single valence electron that is more loosely held due to its greater distance from the nucleus. As a result, these metals exhibit characteristics such as being less dense and softer; they can be effortlessly cut with a simple knife. Understanding this concept is essential when evaluating why certain properties, like melting points, differ among the metals.

Reactivity of Alkali Metals

Alkali metals are known for their high reactivity, particularly with water and halogens. This reactivity stems from their desire to lose a single valence electron to achieve a stable electron configuration. The ease of electron loss is facilitated by weaker metal-metal bonding, which permits this single valence electron to be readily available for interaction.

The reactivity of these metals increases down the group as the atom sizes grow larger, and the valence electron's orbit is further from the nucleus, making it even easier to remove. Therefore, francium is the most reactive of the alkali metals, although it's rare and highly radioactive. This characteristic reactivity is a major point of interest when studying alkali metals, as it impacts their practical applications and safety precautions during handling.

Ionization Energies

Ionization energy is the amount of energy required to remove the most loosely bound electron from an atom to form a cation. For alkali metals, the ionization energies are considerably low, making them highly reactive. The low ionization energy is directly linked to their weak metal-metal bonding.

As we move down the group in the periodic table, these ionization energies decrease, which means it takes less energy to remove the outer electron. This decrease is because of the increased atomic size, which creates a greater distance between the nucleus and the valence electron, reducing the attraction and thus making it easier for the electron to be ionized. Educating students about ionization energies reinforces the concept of reactivity and the periodic trends observed within the alkali metal group.

Melting Points of Metals

Melting points of metals provide insights into the strength of metal-metal bonding within their structure. Typically, a higher melting point indicates stronger bonding forces holding the metallic lattice together. For alkali metals, their melting points are lower compared to many other metals, indicative of the weaker metal-metal bonds present.

This property becomes particularly noteworthy when comparing across the periodic table: transition metals generally have much higher melting points due to their several shared bonding electrons creating a stronger metallic bond. By contrast, the single valence electron of alkali metals results in a less robust metallic lattice, hence their lower melting points. Understanding this property not only aids in predicting the state of matter under various conditions but also in exploring the suitability of metals for different applications based on their thermal stability.