Question

Cation and anion combines in a crystal to form following type of compound. (a) ionic (b) metallic (c) covalent (d) dipole-dipole

Short answer

The compound is ionic.

Step-by-step solution

Understanding Ionic Compounds

Cations (positively charged ions) and anions (negatively charged ions) can combine to form a compound where the main attraction between ions is due to electrostatic forces, which is characteristic of ionic compounds.

Analyzing Other Options

- Metallic compounds refer to metals held together by a 'sea of electrons.' Anions do not play a role in metallic bonding. - Covalent compounds are formed by the sharing of electron pairs between atoms, not by the combination of cations and anions. - Dipole-dipole interactions are intermolecular forces that occur between polar molecules, not specific forms of a compound.

Identifying the Correct Answer

Since the attraction between cations and anions is purely electrostatic and forms the basis of ionic bonding, the correct type of compound formed when they combine is ionic.

Key concepts

Electrostatic Forces

Electrostatic forces are integral to understanding how cations and anions interact to form ionic compounds. These forces are the result of attractions between oppositely charged ions. When a cation (positive charge) approaches an anion (negative charge), they pull toward each other due to their opposite charges.
This type of interaction is non-directional, meaning the force acts equally in all directions from the ion, contributing to the formation of a balanced and symmetrical structure in the resulting ionic compound.

The strength of electrostatic forces is influenced by the magnitude of the charges on the ions, as well as the distance between them. Mathematically, this relationship is often described by Coulomb's Law:
\[F = k \frac{q_1 q_2}{r^2}\]
where \( F \) is the force, \( k \) is Coulomb's constant, \( q_1 \) and \( q_2 \) are the magnitudes of the charges, and \( r \) is the distance between the centers of the two ions.

Understanding these forces helps explain why ionic compounds have high melting and boiling points; the strong attractions require significant energy input to break apart.

Metallic Bonding

Metallic bonding is a different type of bond that occurs in metal elements and alloys. In this scenario, the positive metal ions are surrounded by a 'sea of electrons'. These delocalized electrons move freely throughout the metal lattice, allowing metals to conduct electricity and heat efficiently.
Unlike ionic compounds, which rely on the electrostatic attraction between cations and anions, metallic bonding does not involve anions. Instead, the metal atoms achieve stability not by gaining or losing electrons to other atoms, but by sharing the electrons in a collective flow.

This electron flow contributes to metals' characteristic properties, such as:

• Electrical conductivity: Easily allows electrons to flow through the material.
• Malleability: Metals can be hammered or rolled into thin sheets.
• Ductility: Metals can be drawn into wires without breaking.
• Luster: The free electrons reflect light, giving metals their shiny appearance.

Understanding metallic bonding provides insight into why metals behave differently from ionic and covalent substances.

Covalent Bonding

Covalent bonding is a type of chemical bond wherein two atoms share one or more pairs of electrons. This predominantly occurs between non-metal atoms, each seeking stability by filling their outer electron shells.

In covalent bonds, the shared electrons contribute to each atom's valence shell, effectively equating to having the noble gas configuration.
For instance, in a water molecule (\( \text{H}_2\text{O} \)), each hydrogen atom shares one electron with oxygen, enabling oxygen to complete its valence shell with eight electrons, following the octet rule.

Covalent bonds are characterized by specific properties:

• Directionality: Bonding occurs in specific directions, creating definite shapes of molecules.
• Low melting and boiling points: Typically, less energy is needed to break covalent bonds compared to ionic ones.
• Varied electrical conductivity: Most are poor conductors, as there are no free electrons or ions to carry charge.

These characteristics differentiate covalent compounds from ionic and metallic bonds, emphasizing their role in organic and biological molecules.

Intermolecular Forces

Intermolecular forces are weak interactions between molecules, playing crucial roles in the properties of molecular substances. These forces are distinct from the intramolecular forces that hold atoms together within a molecule, such as covalent bonds.

Key types of intermolecular forces include:

• Dipole-Dipole Interactions: Occur between polar molecules with permanent dipole moments, such as in hydrogen chloride (HCl).
• Hydrogen Bonds: A strong type of dipole interaction, notable in molecules like water (\( \text{H}_2\text{O} \)), where hydrogen is bonded to electronegative atoms like oxygen or nitrogen.
• Van der Waals Forces: Include London dispersion forces, which are present in all molecules, especially significant in nonpolar substances.

Intermolecular forces are weaker than covalent, ionic, or metallic bonds, but are significant in dictating the physical properties of substances, such as boiling and melting points, solubilities, and phase changes.
Understanding these forces can explain phenomena such as why water has a relatively high boiling point for a molecule of its size, since the hydrogen bonds between water molecules require more energy to break.