Question

For each of the following groups which metal would you expect to have the highest melting point; (a) gold (Au), rhenium (Re), or cesium (Cs); (b) rubidium (Rb), molybdenum (Mo), or indium (In); (c) ruthenium (Ru), strontium (Sr), or cadmium (Cd)?

Short answer

In summary, the metals expected to have the highest melting points are: (a) Rhenium (Re), (b) Molybdenum (Mo), and (c) Ruthenium (Ru). This is due to their positions in the periodic table, which affect their metallic bond strength, as well as their atomic numbers, which influence the density of their atomic packing.

Step-by-step solution

Group (a): Gold (Au), Rhenium (Re), or Cesium (Cs)

First, let's compare these three elements by their location on the periodic table. Gold (Au) is a transition metal, rhenium (Re) is also a transition metal, and cesium (Cs) is an alkali metal. In general, transition metals exhibit higher melting points than alkali metals due to their greater number of valence electrons, which leads to stronger metallic bonds. Between gold (Au) and rhenium (Re), rhenium has a higher atomic number (75) than gold (79), which implies a denser packing of atoms and stronger bonding. Therefore, rhenium (Re) is expected to have the highest melting point in this group.

Group (b): Rubidium (Rb), Molybdenum (Mo), or Indium (In)

In this group, rubidium (Rb) is an alkali metal, molybdenum (Mo) is a transition metal, and indium (In) is a post-transition metal. As mentioned earlier, transition metals usually have a higher melting point than alkali metals, so molybdenum (Mo) is expected to have a higher melting point than rubidium (Rb). Molybdenum (Mo) also has a higher atomic number (42) than indium (In) (49), which combined with its position as a transition metal, indicates that molybdenum (Mo) is likely to have the strongest metallic bonds and the highest melting point in this group.

Group (c): Ruthenium (Ru), Strontium (Sr), or Cadmium (Cd)

Lastly, in this group, ruthenium (Ru) is a transition metal, strontium (Sr) is an alkaline earth metal, and cadmium (Cd) is a post-transition metal. Transition metals are known to have higher melting points than alkaline earth metals and post-transition metals. Therefore, ruthenium (Ru), as a transition metal, is anticipated to have a higher melting point than strontium (Sr) and cadmium (Cd).

Key concepts

Transition Metals

Transition metals are fascinating elements that live in the d-block section of the periodic table. They are uniquely positioned to exhibit high melting points due to their special characteristics. Transition metals include elements like rhenium (Re), molybdenum (Mo), and ruthenium (Ru), all of which were mentioned in the exercise. What sets transition metals apart is their ability to partially fill their d-orbitals.

This means that they contain unpaired electrons, which leads to their exceptional properties, including high melting points.

• High number of valence electrons: The electrons in the d-orbitals can participate in bonding, resulting in more robust metallic bonds. Think of these bonds as powerful glue that holds the atoms together tightly, requiring more energy (like heat) to break apart.
• Dense atomic packing: Transition metals often have atoms packed together closely, like marbles crammed into a jar. This close packing contributes to their strength and hardiness, and thus higher melting points.

Periodic Table Trends

Understanding periodic table trends is key to predicting how elements behave, including their melting points. The periodic table is organized in a way that highlights the similarities and differences among elements.

• Across a period: As you move from left to right on the periodic table, elements generally increase in melting point. This is due to the increase in the number of valence electrons that strengthen metallic bonds. Transition metals, stationed in the middle, are where you often find these high melting points.
• Within a group: Within groups (vertical columns), as you move down the group, the atomic number increases. Yet, the melting point may decrease because the outer electrons are further from the nucleus and more easily parted. However, for transition metals, despite increasing atomic numbers, melting points can remain high due to strong interatomic forces.

These trends are vital clues for assessing elements like molybdenum (Mo) over rubidium (Rb) or indium (In), as seen in this exercise.

Metallic Bonds

Metallic bonds are central to the structure and melting behavior of metals. These bonds involve a sea of delocalized electrons that are free to move about, allowing metals to conduct electricity and heat efficiently. This type of bond is particularly strong in transition metals due to their configuration.

• Electron sea model: Imagine a sea where electrons freely float around fixed points, representing metal ions. This freedom allows metals to conduct electricity and contribute to their malleability and ductility.
• Strength of metallic bonds: The number of electrons participating in the 'sea' and the charge of the metal cations influence bond strength. Transition metals, with their surplus of electrons, form incredibly strong bonds, thus possessing high melting points. For instance, rhenium (Re) and molybdenum (Mo) exhibit these strong metallic bonds, giving them the highest melting points amongst their groups in the exercise.

Understanding metallic bonds helps explain why some metals, like those aforementioned, endure higher temperatures before melting.