Intermolecular forces, phase changes and crystals. Discover the vocabulary of how matter holds together in liquids and solids.
These terms were extracted from Chemistry by OpenStax, eCampusOntario (© 2016 Rice University; adapted by BCcampus) which is licensed under a Creative Commons Attribution 4.0 International License.
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intermolecular attraction between two permanent dipoles
The effect of a dipole-dipole attraction is apparent when we compare the properties of HCl molecules to nonpolar F₂ molecules.
(also, London dispersion force) attraction between two rapidly fluctuating, temporary dipoles; significant only when particles are very close together
This attractive force is called the London dispersion force in honor of German-born American physicist Fritz London who, in 1928, first explained it.
occurs when exceptionally strong dipoles attract; bonding that exists when hydrogen is bonded to one of the three most electronegative elements: F, O, or N
Molecules with F-H, O-H, or N-H moieties are very strongly attracted to similar moieties in nearby molecules, a particularly strong type of dipole-dipole attraction called hydrogen bonding.
temporary dipole formed when the electrons of an atom or molecule are distorted by the instantaneous dipole of a neighboring atom or molecule
The presence of this dipole can, in turn, distort the electrons of a neighboring atom or molecule, producing an induced dipole.
temporary dipole that occurs for a brief moment in time when the electrons of an atom or molecule are distributed asymmetrically
At any moment in time, an atom or molecule can develop a temporary, instantaneous dipole if its electrons are distributed asymmetrically.
noncovalent attractive force between atoms, molecules, and/or ions
The phase in which a substance exists depends on the relative extents of its intermolecular forces (IMFs) and the kinetic energies (KE) of its molecules.
measure of the ability of a charge to distort a molecule’s charge distribution (electron cloud)
The measure of how easy or difficult it is for another electrostatic charge (for example, a nearby ion or polar molecule) to distort a molecule’s charge distribution (its electron cloud) is known as polarizability.
attractive or repulsive force between molecules, including dipole-dipole, dipole-induced dipole, and London dispersion forces; does not include forces due to covalent or ionic bonding, or the attraction between ions and molecules
All of the attractive forces between neutral atoms and molecules are known as van der Waals forces, although they are usually referred to more informally as intermolecular attraction.
force of attraction between molecules of different chemical identities
The IMFs of attraction between two different molecules are called adhesive forces.
flow of liquid within a porous material due to the attraction of the liquid molecules to the surface of the material and to other liquid molecules
These are examples of capillary action—when a liquid flows within a porous material due to the attraction of the liquid molecules to the surface of the material and to other liquid molecules.
force of attraction between identical molecules
The various IMFs between identical molecules of a substance are examples of cohesive forces.
energy required to increase the area, or length, of a liquid surface by a given amount
Surface tension is defined as the energy required to increase the surface area of a liquid, or the force required to increase the length of a liquid surface by a given amount.
measure of a liquid’s resistance to flow
The viscosity of a liquid is a measure of its resistance to flow.
temperature at which the vapor pressure of a liquid equals the pressure of the gas above it
Trends in observed melting and boiling points for the halogens clearly demonstrate this effect, as seen in Table 1.
mathematical relationship between the temperature, vapor pressure, and enthalpy of vaporization for a substance
The quantitative relation between a substance’s vapor pressure and its temperature is described by the Clausius-Clapeyron equation:
change from a gaseous to a liquid state
Inside the lighter’s fuel compartment, the butane is compressed to a pressure that results in its condensation to the liquid state, as shown in Figure 3.
change from a gaseous state directly to a solid state
The reverse of sublimation is called deposition, a process in which gaseous substances condense directly into the solid state, bypassing the liquid state.
state of a system in which reciprocal processes are occurring at equal rates
Such is an example of a dynamic equilibrium, the status of a system in which reciprocal processes (for example, vaporization and condensation) occur at equal rates.
change from a liquid state to a solid state
We will often use values such as boiling or freezing points, or enthalpies of vaporization or fusion, as indicators of the relative strengths of IMFs of attraction present within different substances.
temperature at which the solid and liquid phases of a substance are in equilibrium; see also melting point
We will often use values such as boiling or freezing points, or enthalpies of vaporization or fusion, as indicators of the relative strengths of IMFs of attraction present within different substances.
change from a solid state to a liquid state
Trends in observed melting and boiling points for the halogens clearly demonstrate this effect, as seen in Table 1.
temperature at which the solid and liquid phases of a substance are in equilibrium; see also freezing point
Ethane (CH₃CH₃) has a melting point of −183 °C and a boiling point of −89 °C. Predict the melting and boiling points for methylamine (CH₃NH₂).
temperature at which a liquid’s vapor pressure equals 1 atm (760 torr)
The normal boiling point of a liquid is defined as its boiling point when surrounding pressure is equal to 1 atm (101.3 kPa).
change from solid state directly to gaseous state
Some solids can transition directly into the gaseous state, bypassing the liquid state, via a process known as sublimation.
(also, equilibrium vapor pressure) pressure exerted by a vapor in equilibrium with a solid or a liquid at a given temperature
The pressure exerted by the vapor in equilibrium with a liquid in a closed container at a given temperature is called the liquid’s vapor pressure (or equilibrium vapor pressure).
change from liquid state to gaseous state
We will often use values such as boiling or freezing points, or enthalpies of vaporization or fusion, as indicators of the relative strengths of IMFs of attraction present within different substances.
temperature and pressure above which a gas cannot be condensed into a liquid
Finally, notice that the critical point for carbon dioxide is observed at a relatively modest temperature and pressure in comparison to water.
pressure-temperature graph summarizing conditions under which the phases of a substance can exist
Making such measurements over a wide range of pressures yields data that may be presented graphically as a phase diagram.
substance at a temperature and pressure higher than its critical point; exhibits properties intermediate between those of gaseous and liquid states
This unique state of matter is called a supercritical fluid, a topic that will be described in the next section of this module.
temperature and pressure at which the vapor, liquid, and solid phases of a substance are in equilibrium
This temperature-pressure data pair is called the triple point.
(also, noncrystalline solid) solid in which the particles lack an ordered internal structure
The resulting materials are called amorphous solids or noncrystalline solids (or, sometimes, glasses).
solid whose particles are held together by covalent bonds
To break or to melt a covalent network solid, covalent bonds must be broken.
solid in which the particles are arranged in a definite repeating pattern
When we heat a crystalline solid, we increase the average energy of its atoms, molecules, or ions and the solid gets hotter.
spaces between the regular particle positions in any array of atoms or ions
Less commonly, some atoms or ions in a crystal may occupy positions, called interstitial sites, located between the regular positions for atoms.
solid composed of positive and negative ions held together by strong electrostatic attractions
Ionic solids, such as sodium chloride and nickel oxide, are composed of positive and negative ions that are held together by electrostatic attractions, which can be quite strong (Figure 3).
solid composed of metal atoms
Metallic solids such as crystals of copper, aluminum, and iron are formed by metal atoms Figure 4.
solid composed of neutral molecules held together by intermolecular forces of attraction
Molecular solids, such as ice, sucrose (table sugar), and iodine, as shown in Figure 6, are composed of neutral molecules.
defect that occurs when a position that should contain an atom or ion is vacant
Vacancies are defects that occur when positions that should contain atoms or ions are vacant.
crystalline structure that has a cubic unit cell with lattice points at the corners and in the center of the cell
Atoms in the corners of a BCC unit cell do not contact each other but contact the atom in the center.
simplest repeating unit of a body-centered cubic crystal; it is a cube containing lattice points at each corner and in the center of the cube
Simple cubic (which we have already seen), body-centered cubic unit cell, and face-centered cubic unit cell—all of which are illustrated in Figure 5.
equation that relates the angles at which X-rays are diffracted by the atoms within a crystal
This relation is known as the Bragg equation in honor of W. H. Bragg, the English physicist who first explained this phenomenon.
number of atoms closest to any given atom in a crystal or to the central metal atom in a complex
The number of other particles that each particle in a crystalline solid contacts is known as its coordination number.
crystalline structure in which planes of closely packed atoms or ions are stacked as a series of three alternating layers of different relative orientations (ABC)
This structure is also called cubic closest packing (CCP).
redirection of electromagnetic radiation that occurs when it encounters a physical barrier of appropriate dimensions
The size of the unit cell and the arrangement of atoms in a crystal may be determined from measurements of the diffraction of X-rays by the crystal, termed X-ray crystallography.
crystalline structure consisting of a cubic unit cell with lattice points on the corners and in the center of each face
This arrangement is called a face-centered cubic (FCC) solid.
simplest repeating unit of a face-centered cubic crystal; it is a cube containing lattice points at each corner and in the center of each face
Simple cubic (which we have already seen), body-centered cubic unit cell, and face-centered cubic unit cell—all of which are illustrated in Figure 5.
crystalline structure in which close packed layers of atoms or ions are stacked as a series of two alternating layers of different relative orientations (AB)
We find two types of closest packing in simple metallic crystalline structures: CCP, which we have already encountered, and hexagonal closest packing (HCP) shown in Figure 9.
(also, interstice) space between atoms within a crystal
The smaller cations commonly occupy one of two types of holes (or interstices) remaining between the anions.
possessing the same crystalline structure
Isomorphous metals with a BCC structure include K, Ba, Cr, Mo, W, and Fe at room temperature.
open space in a crystal at the center of six particles located at the corners of an octahedron
The larger type of hole is found at the center of six anions (three in one layer and three in an adjacent layer) located at the corners of an octahedron; this is called an octahedral hole.
(also, primitive cubic unit cell) unit cell in the simple cubic structure
This arrangement is called simple cubic structure, and the unit cell is called the simple cubic unit cell or primitive cubic unit cell.
crystalline structure with a cubic unit cell with lattice points only at the corners
This arrangement is called simple cubic structure, and the unit cell is called the simple cubic unit cell or primitive cubic unit cell.
all points within a crystal that have identical environments
The axes are defined as being the lengths between points in the space lattice.
tetrahedral space formed by four atoms or ions in a crystal
The four anions surrounding this hole are arranged at the corners of a tetrahedron, so the hole is called a tetrahedral hole.
smallest portion of a space lattice that is repeated in three dimensions to form the entire lattice
The structure of a crystalline solid, whether a metal or not, is best described by considering its simplest repeating unit, which is referred to as its unit cell.
experimental technique for determining distances between atoms in a crystal by measuring the angles at which X-rays are diffracted when passing through the crystal
The size of the unit cell and the arrangement of atoms in a crystal may be determined from measurements of the diffraction of X-rays by the crystal, termed X-ray crystallography.