Question

Describe the structural units in (a) NaI (b) \(\mathrm{N}_{2}\) (c) \(\mathrm{KO}_{2}\) (d) \(\mathrm{Au}\)

Short answer

Answer: (a) NaI has a cubic crystal lattice formed by alternating Na+ and I- ions. (b) N2 has a diatomic molecule with a triple covalent bond between two nitrogen atoms. (c) KO2 has a layered structure with alternating sheets of oxygen ions and potassium ions. (d) Au consists of gold atoms arranged in a crystalline lattice, with a "sea of electrons" shared between all atoms.

Step-by-step solution

Identify the type of bonds in each compound

The type of bond within a compound can be determined by the bonding elements' electronegativity difference or simply by understanding the nature of the elements involved. Generally, compounds formed between metals and non-metals have ionic bonds, non-metals combine via covalent bonds, and metallic elements form metallic bonds. (a) NaI: Sodium (Na) is a metal and iodine (I) is a non-metal, thus, they form an ionic bond. (b) N2: Nitrogen (N) is a non-metal, and since it is bonded to another nitrogen, it forms a covalent bond. (c) KO2: Potassium (K) is a metal and oxygen (O) is a non-metal, so they form an ionic bond. (d) Au: Gold (Au) is a metallic element, therefore, it has a metallic bond.

Describe the structural units in NaI

NaI is an ionic compound consisting of sodium ions (Na+) and iodide ions (I-). The structural unit of NaI is a cubic crystal lattice formed by alternating Na+ and I- ions, held together by electrostatic attractions between the oppositely charged ions.

Describe the structural units in N2

N2 is a covalent compound, formed by two nitrogen atoms bonded together. The structural unit of N2 is a diatomic molecule consisting of two nitrogen atoms connected by a triple covalent bond (NN).

Describe the structural units in KO2

KO2 is an ionic compound consisting of potassium ions (K+) bonded to superoxide ions (O2-). In KO2, potassium and oxygen ions form a layered structure which has alternating sheets of oxygen ions and potassium ions, held together by electrostatic attractions between the oppositely charged ions.

Describe the structural units in Au

Au is a metallic element and forms a metallic bond. The structural unit of gold consists of gold atoms arranged in a crystalline lattice structure, with electrons shared between all the atoms to form a "sea of electrons" that are free to move, causing metallic bonding and the characteristic properties of metals, such as electrical conductivity and malleability.

Key concepts

Ionic Bonds

Ionic bonds are a type of chemical bond formed between two atoms with a large difference in electronegativity. When a metal atom, like sodium (Na) in NaI, transfers an electron to a non-metal atom, such as iodine (I), each atom becomes an ion. Sodium becomes positively charged (Na⁺), and iodine becomes negatively charged (I). These oppositely charged ions are then strongly attracted to each other, creating an ionic bond.

In everyday substances, ionic compounds arrange themselves in a specific geometric pattern known as a crystal lattice structure, which we'll explore more in the 'Crystal Lattice' section. This rigid arrangement maximizes the attractions between the ions and minimizes repulsion, hence why ionic compounds, like table salt, are typically solid at room temperature with high melting points.

Covalent Bonds

Covalent bonds occur when two non-metal atoms come together by sharing their valence (outermost) electrons, rather than transferring them. This type of bonding is often found in molecules of the same element, like nitrogen (N₂), where two nitrogen atoms share three pairs of electrons, forming what is called a triple bond. Covalent bonding is a strong force that holds the atoms together within a molecule.

The strength and number of shared electron pairs can vary, leading to single, double, or triple bonds, which determine the molecule's properties. Covalent compounds can be gases, liquids, or solids at room temperature and typically have lower melting points compared to ionic compounds.

Metallic Bonds

Metallic bonding is the force that holds together the atoms of a metal, such as gold (Au). Unlike ionic or covalent bonds, metallic bonding involves a 'sea of electrons' that are not associated with any particular atom but float freely around the metal's positively charged atomic nuclei. This unique bonding situation is responsible for many of the characteristic properties of metals, such as their high conductivity of heat and electricity, shining luster, ductility, and malleability.

Within metallic bonding, the structure of a metal can likewise be described as a lattice, albeit different from a crystal lattice. In gold, and other metals, the lattice is typically arranged in compact and efficient patterns that allow the electrons to flow with ease, explaining why metals are such good conductors.

Crystal Lattice

The crystal lattice refers to the organized, three-dimensional arrangement of atoms or ions in a crystalline solid. In an ionic compound like NaI, the lattice is made up of alternating positive and negative ions (Na⁺ and I) in a repeating pattern. This arrangement is not random but highly structured, which maximizes the electrostatic forces of attraction across the entire structure, providing stability and defining the physical properties of the compound such as hardness and melting point.

The strength and geometry of the crystal lattice depend on the size and charge of the ions involved, as well as the conditions under which the crystal formed. For example, certain conditions can cause ionic compounds like KO₂ to form layered lattices. The study of crystal lattices is essential in understanding material properties and is a crucial aspect of fields such as mineralogy, materials science, and solid-state physics.