Question

Of the following oxides, the one with the highest melting point is (a) \(\mathrm{Li}_{2} \mathrm{O} ;\) (b) \(\mathrm{BaO} ;\) (c) \(\mathrm{MgO} ;\) (d) \(\mathrm{SiO}_{2}.\)

Short answer

The oxide with the highest melting point is SiO2 due to its strong covalent bonding and network structure.

Step-by-step solution

Identify the Type of Bonding

The first step in solving this exercise is to identify the type of bonding in each of the compounds. Li2O, BaO and MgO are ionic compounds because they consist of metal and non-metal ions that are held together by ionic bonding. SiO2, on the other hand, consists of two non-metals and is held together by covalent bonding.

Consider the Strength of Bonding

Next, we must think about the strength of the bonds. Metal ions with larger charges tend to form stronger ionic bonds, so we might think that MgO, with its +2 metal ion, would have a stronger bond and thus a higher melting point, than the +1 ion in Li2O or BaO. However, we also have to consider the covalent bonding in SiO2. In SiO2 each silicon atom is bonded to four oxygen atoms, forming a large network covalent structure. This makes SiO2 an extremely strong structure.

Analyze the Results

By considering the strength and type of bond in each compound, we can now say that SiO2, with its strong covalent bonding and network structure, will likely have the highest melting point.

Key concepts

Ionic Bonding

Ionic bonding is a type of chemical bond where atoms transfer electrons to each other. This bond occurs between a metal and a non-metal. In ionic compounds, the metal loses electrons to become a positively charged ion, while the non-metal gains those electrons to become negatively charged.
This results in the formation of ionic compounds such as Li\(_2\)O, BaO, and MgO. These ionic compounds are structured in a way that maximizes the attraction between the oppositely charged ions.
### Characteristics of Ionic Bonds- **High Melting Points**: Ionic bonds are typically strong, resulting in compounds that have high melting points.- **Solid State at Room Temperature**: Most ionic compounds are solid at room temperature.- **Conductivity**: When molten or dissolved in water, ionic compounds can conduct electricity because the ions are free to move.
The strength of ionic bonding is influenced by the charge and size of the ions involved. For example, MgO has a higher melting point than Li\(_2\)O because the magnesium ion has a greater charge (+2) compared to the lithium ion (+1), leading to stronger attractions between ions in MgO.

Covalent Bonding

Covalent bonding involves the sharing of electrons between atoms, usually nonmetals, to achieve stability. The shared electrons allow each atom to attain the electronic configuration of a noble gas, which is often the driving factor for bond formation.
One classic example of covalent bonding is found in SiO\(_2\) (silicon dioxide). Here, each silicon atom shares electrons with four oxygen atoms, forming a complex network of Si–O–Si linkages. Due to this extensive network of covalent bonds, the compound exhibits exceptional strength.
### Traits of Covalent Bonds- **Variable Melting Points**: While covalent bonds are strong, they have varied melting points depending on the structure of the molecule.- **Electrical Insulators**: Unlike ionic compounds, covalent compounds usually do not conduct electricity, as there are no free ions.- **Discrete Molecules vs. Networks**: While some covalent compounds exist as small molecules with low melting points, others, like SiO\(_2\), form large, interconnected networks which often have high melting points.

Network Covalent Structure

A network covalent structure is a type of covalent bond arrangement where atoms are bonded in a continuous network. This structure results in materials with exceptional hardness and high melting points.
SiO\(_2\) is a primary example of a compound with a network covalent structure. In this structure, each silicon atom is tetrahedrally coordinated to four oxygen atoms, forming a 3D network. This immense interconnectedness contributes to its unique properties.
### Features of Network Covalent Structures- **High Melting Points**: These structures require a significant amount of energy to break, resulting in high melting points.- **Hardness**: Materials with network covalent structures are typically very hard and durable.- **Non-Conductive**: Similar to most covalent structures, they do not conduct electricity as there are no mobile charge carriers.
Thus, due to the extensive network and strong silicon-oxygen bonds, SiO\(_2\) possesses a much higher melting point compared to ionic compounds such as MgO, highlighting the strength and stability provided by its network covalent structure.