Question

For each of the following groups, which metal would you expect to have the highest melting point: (a) gold, rhenium, or cesium; (b) rubidium, molybdenum, or indium; (c) ruthenium, strontium, or cadmium?

Short answer

The metals with the highest melting points in each group are: (a) Rhenium, (b) Molybdenum, and (c) Ruthenium. This is due to their stronger metallic bonds compared to the other metals in their respective groups.

Step-by-step solution

Group (a): Gold, Rhenium, or Cesium

Gold (Au) and Rhenium (Re) are transition metals, while Cesium (Cs) is an alkali metal. Transition metals have stronger metallic bonds and higher melting points compared to alkali metals. Therefore, Cesium will have a lower melting point compared to Gold and Rhenium. Comparing Gold and Rhenium, Rhenium has a higher melting point because it has stronger metallic bonds due to its larger number of valence electrons available for bonding. Thus, Rhenium has the highest melting point among Gold, Rhenium, and Cesium.

Group (b): Rubidium, Molybdenum, or Indium

Rubidium (Rb) is an alkali metal, Molybdenum (Mo) is a transition metal, and Indium (In) is a post-transition metal. As mentioned before, transition metals generally have higher melting points than alkali metals, so Molybdenum will have a higher melting point compared to Rubidium. Now, comparing Molybdenum and Indium, Molybdenum is a group 6 transition metal with more valence electrons, resulting in stronger metallic bonds and a higher melting point. Therefore, Molybdenum has the highest melting point among Rubidium, Molybdenum, and Indium.

Group (c): Ruthenium, Strontium, or Cadmium

Ruthenium (Ru) is a transition metal, Strontium (Sr) is an alkaline earth metal, and Cadmium (Cd) is a post-transition metal. Transition metals generally have higher melting points compared to alkali and alkaline earth metals. Thus, Ruthenium will have a higher melting point compared to Strontium. Now we need to compare Ruthenium and Cadmium. Ruthenium is a group 8 transition metal with more valence electrons, which leads to stronger metallic bonds and a higher melting point. As a result, Ruthenium has the highest melting point among Ruthenium, Strontium, and Cadmium.

Key concepts

Transition Metals

Transition metals are elements found in the d-block of the periodic table, characterized by their ability to form multiple oxidation states and complex ions. They are defined as metals that have partially filled d-orbitals. This results in a range of unique properties:

• Multiple Oxidation States: Transition metals can exhibit more than one oxidation state, allowing them to form a variety of compounds.
• Complex Ion Formation: They can form complex ions with molecules or ions surrounding the central atom, which often leads to colorful compounds.
• Strong Metallic Bonds: Due to their ability to delocalize electrons across many atoms, they form strong metallic bonds. This feature contributes to their high melting points.

Transition metals such as Rhenium, Molybdenum, and Ruthenium, exhibit higher melting points than non-transition metals due to these strong metallic bonds.

Metallic Bonds

Metallic bonds are the force of attraction between free-floating valence electrons and the positively charged metal ions. These unique bonds contribute significantly to the physical properties of metals:

• High Melting and Boiling Points: The strength of metallic bonds, resulting from the electrostatic attraction between the electrons and metal ions, contributes to high melting points.
• Electrical Conductivity: The delocalization of electrons allows metals to conduct electricity efficiently.
• Malleability and Ductility: The layers of atoms in a metal can slide over each other, giving metals the ability to bend and stretch without breaking.

The more delocalized valence electrons present, the stronger the metallic bonds. Transition metals, with their extensive d-orbital occupancy, typically experience such bonding.

Valence Electrons

Valence electrons are the outermost electrons of an atom and are essential in determining how an atom interacts with others. They play a critical role in:

• Bond Formation: Valence electrons participate in chemical bonding, including ionic, covalent, and metallic bonds.
• Chemical Reactivity: The number of valence electrons dictates an element's reactivity. For metals, the more valence electrons available, the stronger the bonding.
• Element Position: In the periodic table, elements are grouped based on similar valence electron configurations.

For transition metals, valence electrons include those in the outermost shell and those in the d-subshell. This allows multiple bonding possibilities and contributes to their unique chemical and physical properties, including higher melting points as seen with Rhenium and Ruthenium.