Question

Classify each of the following species as metallic, network covalent, ionic, or molecular. (a) W (b) \(\mathrm{NO}_{2}\) (c) C (diamond) (d) \(\mathrm{FeCl}_{2}\) (e) \(\mathrm{C}_{2} \mathrm{H}_{2}\)

Short answer

To summarize: (a) W - Tungsten: Metallic (b) \(\mathrm{NO}_{2}\) - Nitrogen Dioxide: Molecular (c) C (diamond) - Carbon (diamond): Network Covalent (d) \(\mathrm{FeCl}_{2}\) - Iron (II) Chloride: Ionic (e) \(\mathrm{C}_{2} \mathrm{H}_{2}\) - Acetylene: Molecular

Step-by-step solution

(a) W - Tungsten

Tungsten is an element in the periodic table, with the symbol W. It is a metal and has no other atoms bonded to it. Therefore, it is classified as metallic.

(b) \(\mathrm{NO}_{2}\) - Nitrogen Dioxide

Nitrogen dioxide contains non-metal atoms (nitrogen and oxygen) bonded by covalent bonds, creating a molecule. According to its structure, it is classified as molecular.

(c) C (diamond) - Carbon (diamond)

Diamond is a form of carbon where every carbon atom is covalently bonded to four other carbon atoms, creating an extended, highly stable network. Thus, it is classified as network covalent.

(d) \(\mathrm{FeCl}_{2}\) - Iron (II) Chloride

Iron (II) chloride is formed by the metal iron transferring two electrons to two chlorine (a non-metal) atoms, thus creating the ions Fe\(^{2+}\) and Cl\(^-\), held together by electrostatic forces. Therefore, it is classified as ionic.

(e) \(\mathrm{C}_{2} \mathrm{H}_{2}\) - Acetylene

Acetylene consists of two carbon atoms and two hydrogen atoms, all of which are non-metals. They are bonded by covalent bonds, creating a molecule. According to its structure, it is classified as molecular.

Key concepts

Metallic Bonding

Metallic bonding is the force that holds atoms together in metallic substances. It involves the 'sea of electrons' concept, where valence electrons are not bound to individual atoms but float freely around the metal's positively charged nuclei. Imagine a lattice of metal cations immersed in a sea of delocalized electrons. These electrons can move freely, which explains why metals can conduct electricity and heat so well and why they are malleable and ductile.

• Metallic bonds are non-directional, meaning the strength of the bond is the same in all directions around the metal ion.
• The 'sea of electrons' is also responsible for the lustrous appearance of metals.
• An example of metallic bonding can be seen with tungsten (W), as used in light bulb filaments and steel alloys.

Network Covalent Structure

A network covalent structure, also known as a covalent network crystal, consists of atoms connected by covalent bonds in a continuous, extended structure. Unlike individual molecules, these structures do not have a distinct molecular formula and cannot be considered a collection of discrete molecules.
One of the most well-known examples of a network covalent structure is diamond, a form of carbon. In diamond, each carbon atom is tetrahedrally coordinated and bonded to four other carbon atoms. This forms a three-dimensional structure that is extremely hard and has a very high melting point.

Key Characteristics of Network Covalent Structures:

• They are typically very hard and have high melting points.
• They are usually poor conductors of electricity because all electrons are localized in bonds.
• Other examples include quartz (silicon dioxide) and silicon carbide (SiC), both of which are used in high-strength materials.

Ionic Compounds

When a metal and a non-metal react, they can form an ionic compound. In ionic bonding, atoms transfer electrons to achieve a full outer electron shell, resulting in the formation of oppositely charged ions. These ions then attract each other due to electrostatic forces, creating a bond that is distinctly different from covalent bonds found in molecular compounds.
Iron (II) chloride (FeCl₂) is a classic example of an ionic compound. The iron atom donates two electrons to the chlorine atoms, resulting in a positively charged iron ion (Fe²⁺) and two negatively charged chloride ions (Cl⁻). This electrostatic attraction holds the compound together.

Properties of Ionic Compounds:

• They form crystalline structures with high melting points.
• They typically dissolve in water and other polar solvents.
• In solution or when molten, ionic compounds can conduct electricity due to the free movement of ions.

Molecular Compounds

Molecular compounds are composed of molecules formed by atoms sharing electrons through covalent bonds. These compounds can be composed of atoms of the same element, such as oxygen gas (O₂), or different elements such as water (H₂O) and nitrogen dioxide (NO₂).
In the case of nitrogen dioxide and acetylene (C₂H₂), the molecules are formed by covalent bonds between non-metal atoms; this sharing of electrons allows each atom to achieve a full valence shell, resulting in a stable molecule.

Characteristics of Molecular Compounds:

• They often have lower melting and boiling points compared to ionic and network covalent structures due to weaker intermolecular forces known as Van der Waals forces.
• They can be gases, liquids, or solids at room temperature depending on the strength of their intermolecular forces.
• Many molecular compounds are poor conductors of electricity in any state because they do not have ions or a free electron sea.