Question

For the four kinds of crystal binding - - covalent, ionic. metallic, and molecular - how would the density of valence electrons vary throughout the solid? Would it be constant, centered on the atoms, or largest between the atoms? Or would it alternate, with a net charge density positive at one atom and negative at the next?

Short answer

For a covalent bond, the electron density is concentrated around the atoms. In ionic bonds, electron density centers on individual atoms and alternates, with a net positive charge density at one atom and negative at the next. For metallic bonds, the electron density is relatively constant throughout the solid due to the 'electron sea'. In the case of molecular bonds, electron density is high around each molecule but low between molecules.

Step-by-step solution

Analyze Covalent Bonding

In covalent bonding, electrons are shared between adjacent atoms, forming a localized bond. The electron density tends to be concentrated around the atoms themselves. The electron density is therefore not constant and is not centered between the atoms.

Analyze Ionic Bonding

In ionic bonding, one atom gives up an electron to another, resulting in charged ions. These ions are then attracted to each other. The electron density is thus centered on the individual atoms and alternate, with a net charge density which is positive at one atom and negative at the next.

Analyze Metallic Bonding

In metallic bonding, electrons are delocalized and free to move through the solid, forming an 'electron sea'. This leads to a relatively constant electron density throughout the solid.

Analyze Molecular Bonding

Molecular bonds happen in molecules and in some crystals like ice. The bonds can be covalent within the molecule causing high electron density around each molecule. But between the molecules, electron density can be extremely low, making the electron distribution not constant.

Key concepts

Covalent Bonding

Covalent bonding is a fundamental type of chemical bonding where atoms share pairs of electrons. This sharing allows atoms to achieve more stable electron configurations. In solids with covalent bonds, electron density tends to be concentrated around the atoms themselves rather than between them. This is because the shared electrons are still relatively localized near the nuclei of their respective atoms.
This localized electron density is why covalent crystals, such as diamond, are known for their hardness. The concentrated electron density around the atoms provides strong directional bonds that resist external forces. However, the electron density is not uniform throughout the solid, it varies, being denser around the bonding regions of the atoms. In covalent networks, these bonds form a lattice, contributing to the unique properties of the material.

Ionic Bonding

Ionic bonding occurs when one atom donates an electron to another, creating positive and negative ions. This electron transfer leads to the formation of a lattice of ions where the overall electron density centers on the ions themselves. In ionic compounds, the pattern alternates between positive and negative ions, with electron density concentrating around the negatively charged ions.
The electrostatic attraction between oppositely charged ions results in the formation of a stable structure. However, the distribution of electron density in an ionic crystal is quite different from covalent materials. Here, due to the nature of ionic bonds, electron density is not shared between atoms but sits prominently around individual ions. This often leads to compounds that are brittle and hard but have high melting and boiling points due to the strong attractions between ions.

Metallic Bonding

Metallic bonding is characterized by a 'sea of electrons' that are free to move around the lattice of positive metal ions. This delocalization of electrons leads to a relatively uniform electron density across the entire metallic structure. Unlike ionic or covalent bonds, electrons in metal do not belong to any specific atom.
This unique property gives metals their characteristic features—high electrical conductivity, malleability, and ductility. The free-moving electrons allow metals to conduct electricity efficiently, as they are not bound to any one part of the lattice. Furthermore, the uniform distribution of electron density acts as a cushion that allows metal atoms to slide past each other without breaking the metallic bond.

Molecular Bonding

Molecular bonding occurs within molecules or in crystals formed by molecules, such as ice. Often containing covalent bonds within the molecule itself, these structures exhibit high electron density around each molecule. However, the bonds that hold the molecules together in the solid phase can be weak, such as van der Waals forces, leading to lower electron density between the molecules.
This method of bonding results in solids that often have lower melting and boiling points. While the electron density is high in areas within the molecules, it is significantly lower between them, giving these materials their characteristic softness and lower density. Molecular bonded substances usually show poor conductivity because the electron density is not sufficient to support the flow of current throughout the crystal.