Question

Predict whether each of the following oxides is ionic or molecular: ${\mathrm{SnO}}_2,{\mathrm{Al}}_2{\mathrm{O}}_3,{\mathrm{CO}}_2,{\mathrm{Li}}_2\mathrm{O},{\mathrm{Fe}}_2{\mathrm{O}}_3,{\mathrm{H}}_2\mathrm{O}.$ Explain the reasons for your choices.

Short answer

SnO2, Al2O3, Li2O, and Fe2O3 are ionic compounds because they consist of a metal and a non-metal, where the metal loses electrons to form cations and the non-metal gains electrons to form anions. CO2 and H2O are molecular compounds as they consist of only non-metal elements that share electrons to form covalent bonds.

Step-by-step solution

Understanding Ionic and Molecular Compounds

Ionic compounds consist of ions held together by ionic bonds, which are formed due to the electrostatic attraction between oppositely charged ions. Generally, these compounds involve the reaction between a metal and a non-metal, in which the metal loses its valence electrons to become positively charged cations while the non-metal gains those electrons to become negatively charged anions. Molecular compounds, on the other hand, comprise molecules held together by covalent bonds formed due to the sharing of electrons between atoms. These compounds usually involve the combination of two or more non-metal elements. With this knowledge in mind, we can now analyze each given oxide.

Analyzing SnO2

In the compound SnO2, Sn (Tin) is a metal and O (Oxygen) is a non-metal. Tin is likely to lose electrons to create cations, and Oxygen tends to gain electrons to create anions. Therefore, SnO2 is likely an ionic compound.

Analyzing Al2O3

In the compound Al2O3, Al (Aluminium) is a metal and O (Oxygen) is a non-metal. Aluminium can lose electrons to form cations, while Oxygen gains electrons to form anions. Therefore, Al2O3 is likely an ionic compound.

Analyzing CO2

Since both C (Carbon) and O (Oxygen) are non-metal elements, they tend to share electrons to form covalent bonds. As a result, CO2 (Carbon Dioxide) is a molecular compound.

Analyzing Li2O

In the compound Li2O, Li (Lithium) is a metal and O (Oxygen) is a non-metal. Lithium can lose electrons to form cations, while Oxygen gains electrons to form anions. As a result, Li2O is likely an ionic compound.

Analyzing Fe2O3

In the compound Fe2O3, Fe (Iron) is a metal and O (Oxygen) is a non-metal. Iron tends to lose electrons to become cations, and Oxygen tends to gain electrons to become anions. Therefore, Fe2O3 is likely an ionic compound.

Analyzing H2O

In the compound H2O, H (Hydrogen) is a non-metal element and O (Oxygen) is a non-metal. Both Hydrogen and Oxygen share electrons to form covalent bonds. Thus, H2O (Water) is a molecular compound. In conclusion, SnO2, Al2O3, Li2O, and Fe2O3 are ionic compounds, and CO2 and H2O are molecular compounds.

Key concepts

Ionic Bonds

Ionic bonds are a type of chemical bond that occurs due to the electrostatic attraction between positively charged ions, called cations, and negatively charged ions, called anions. These bonds typically form when a metal reacts with a non-metal.
For example, when a metal like sodium (Na) reacts with chlorine (Cl), sodium loses one electron to become a Na⁺ cation, and chlorine gains an electron to become a Cl⁻ anion. The opposite charges attract, creating a stable ionic bond, resulting in the formation of sodium chloride ( NaCl ext ).
- Ionic bonds form strong attractions between ions. - They usually result in the formation of crystalline solids, such as salt. - These compounds often have high melting and boiling points.
Ionic compounds generally dissolve in water, conducting electricity in solution, as the ions carry the charge through the liquid. This behavior distinguishes them from molecular compounds, which do not dissociate into ions.

Covalent Bonds

Covalent bonds are formed through the sharing of electrons between two atoms, usually non-metals. This type of bond allows each atom to achieve a full outer electron shell, resulting in stable molecules.
For example, in a water molecule (H₂O), each of the two hydrogen atoms shares an electron with oxygen, forming two covalent bonds. Sharing electrons in this manner leads to the creation of covalent compounds, often referred to as molecular compounds. - Covalent bonds generally form between non-metal atoms. - They create discrete molecules, unlike the lattice structures formed by ionic compounds. - Molecular compounds can exist in all three states of matter (solid, liquid, gas) at room temperature.
Covalently bonded compounds, like carbon dioxide (CO₂) or water, may not conduct electricity because there are no free ions to carry a charge. Their properties vary significantly compared to ionic compounds, often featuring lower boiling and melting points.

Oxides

Oxides are compounds formed by the combination of oxygen with another element. They form a broad category and can be either ionic or molecular, depending on the elements involved.
- **Ionic oxides** usually involve metals, such as sodium oxide ( Na_2O ext ) or aluminum oxide ( Al_2O_3 ext ). In these compounds, the metal donates electrons to oxygen, forming ionic bonds. - **Molecular oxides** typically involve non-metals, like carbon dioxide ( CO_2 ext ) or sulfur dioxide ( SO_2 ext ), where the atoms share electrons through covalent bonding.
Oxides can vary widely in properties based on their composition. Ionic oxides are generally solid and have a crystalline formation, while molecular oxides can appear in gaseous, liquid, or solid form, depending on the temperature and pressure.
Understanding the nature of oxides is crucial for predicting their behavior in chemical reactions and their impact on the environment, such as their role in acid rain or as a nutrient in soil.