Problem 49 - Chemistry: The Central Science

Question

Which would you expect to be the more ductile element, (a) \(\mathrm{Ag}\) or \(\mathrm{Mo}\), (b) \(\mathrm{Zn}\) or \(\mathrm{Si}\) ? In each case explain your reasoning.

Step-by-Step Solution

Answer

In conclusion: (a) Ag is more ductile than Mo due to its weaker metallic bonds caused by fewer valence electrons. (b) Zn is more ductile than Si, as Zn has more flexible metallic bonds while Si has strong and brittle covalent bonds.

Step 1: (a) Comparing Ag and Mo

To compare the ductility of silver (Ag) and molybdenum (Mo), let's observe the type of bond present in their structures. Silver is an element of Group 11 of the periodic table and has metallic bonds. In contrast, Mo is part of Group 6 and also has metallic bonds. Next, let's analyze their bond strength. Due to the difference in the number of valence electrons, Ag has one valence electron, and Mo has six valence electrons. This makes the metallic bonding in Mo stronger than in Ag, making it harder for Mo to deform without breaking. Therefore, we can conclude that Ag is more ductile than Mo.

Step 2: (b) Comparing Zn and Si

Now let's compare zinc (Zn) and silicon (Si). Zinc belongs to Group 12 of the periodic table and has metallic bonds, while silicon is part of Group 14 and has a covalent bond type configuration. Covalent bonds are generally stronger than metallic bonds, making it more difficult for Si to deform under tensile stress. Furthermore, Si is a brittle material characterized by its inability to deform due to its covalently bonded structure, making it significantly less ductile than metals like Zn, which have a more flexible metallic bond structure. Thus, we can conclude that Zn is more ductile than Si. In conclusion: (a) Ag is more ductile than Mo (b) Zn is more ductile than Si

Key Concepts

Metallic Bonding

Metallic bonding is a type of chemical bond that occurs between metal atoms. These atoms share their valence electrons in a special way that allows for a unique and flexible bond structure. In metals, valence electrons are not bound to any single atom but instead roam freely throughout the metal lattice. This is often described as a "sea of electrons."

Because of this sea of electrons, metallic bonds exhibit certain key characteristics:

• Malleability: Metals can be hammered or pressed into sheets without breaking. This is due to the ability of atoms to slide past each other while remaining bonded.
• Conductivity: The free movement of electrons throughout the metal allows it to conduct electricity effectively.
• Ductility: Metals can be drawn into wires because the atoms can move within the lattice under tension without the bonds breaking.

These features are what make metals like silver (Ag) and zinc (Zn) ductile and easy to shape. The metallic bond provides a great balance of strength and flexibility.

Covalent Bonding

Covalent bonding occurs when two non-metal atoms share pairs of valence electrons. This sharing of electrons allows the bonded atoms to acquire a full valence shell, achieving a more stable configuration. Covalent bonds are typically quite strong and form discrete molecules or complex lattice structures.

One of the key features of materials with covalent bonds is their strength, but this strength comes with increased brittleness. Unlike metals, substances with covalent bonds do not have free-moving electrons, which makes them:

• Rigid: Atoms are held firmly in place, making the material strong but less flexible.
• Brittle: Materials shatter or break upon impact because the rigid structure cannot absorb overload stresses well.

Silicon (Si) is an example of a material with a covalent bond. It forms a crystalline structure that gives it strength but does not allow for much movement of atoms. This is why silicon is less ductile compared to metals like zinc, which rely on metallic bonding.

Valence Electrons

Valence electrons are the outermost electrons of an atom and are crucial in determining how an atom will bond with others. These electrons are involved when atoms form chemical bonds, such as ionic, covalent, or metallic bonds.

Here are some important points about valence electrons:

• Number matters: The number of valence electrons influences the type and strength of the bond. For instance, molybdenum (Mo) with more valence electrons forms stronger metallic bonds compared to silver (Ag) with fewer valence electrons.
• Bond flexibility: In metallic bonding, valence electrons are free to move, which contributes to properties like malleability and ductility. In contrast, covalent bonds fix valence electrons between atoms, providing strength but reducing flexibility.
• Periodic trends: Elements in the same group of the periodic table tend to have similar valence electron configurations, leading to similar bonding properties.

Understanding valence electrons gives insight into why materials behave the way they do, whether they are more ductile, malleable, or brittle.