Menu
Home
People
Places
Arts
History
Plants & Animals
Science
Life & Culture
Technology
Reference.org
Molecular solid
open-in-new
<p>A molecular solid is a <a href="/facts/Solid/hj5UsXno">solid</a> consisting of discrete <a href="/facts/Molecule/N9ljSfFq">molecules</a>. The cohesive forces that bind the molecules together are <a href="/facts/Van_der_Waals_force/jyGU8Vh9">van der Waals forces</a>, <a href="/facts/Dipole/G1vRqruD">dipole–dipole interactions</a>, <a href="/facts/Quadrupole/6pCeNmwT">quadrupole interactions</a>, <a href="/facts/Pi_interaction/nzRePWm3">π–π interactions</a>, <a href="/facts/Hydrogen_bond/85V86IIg">hydrogen bonding</a>, <a href="/facts/Halogen_bond/RmK160Em">halogen bonding</a>, <a href="/facts/London_dispersion_force/zU842d26">London dispersion forces</a>, and in some molecular solids, <a href="/facts/Coulomb%27s_law/V0lR5NKv">coulombic interactions</a>. Van der Waals, dipole interactions, quadrupole interactions, π–π interactions, hydrogen bonding, and halogen bonding (2–127 <a href="/facts/Joule/9ndq9jD2">kJ</a> <a href="/facts/Mole_(unit)/kCMFwCc0">mol</a>−1) are typically much weaker than the forces holding together other solids: <a href="/facts/Metal/PT8hNX6R">metallic</a> (<a href="/facts/Metallic_bonding/Ht958du4">metallic bonding</a>, 400–500 kJ mol−1), <a href="/facts/Ionic_compound/jB9nMfrE">ionic</a> (<a href="/facts/Coulomb%27s_law/V0lR5NKv">Coulomb’s forces</a>, 700–900 kJ mol−1), and <a href="/facts/Network_covalent_bonding/7ykpwQqV">network solids</a> (<a href="/facts/Covalent_bond/EzMLf3Pz">covalent bonds</a>, 150–900 kJ mol−1). </p><p>Intermolecular interactions typically do not involve <a href="/facts/Delocalized_electron/jsquhECb">delocalized electrons</a>, unlike metallic and certain covalent bonds. Exceptions are <a href="/facts/Charge-transfer_complex/hbIyNJlo">charge-transfer complexes</a> such as the tetrathiafulvane-tetracyanoquinodimethane (TTF-TCNQ), a <a href="/facts/Radical_ion/N4vFqKrV">radical ion salt</a>. These differences in the strength of force (i.e. covalent vs. van der Waals) and electronic characteristics (i.e. delocalized electrons) from other types of solids give rise to the unique <a href="/facts/Mechanical_properties/Cg2LVoQe">mechanical</a>, <a href="/facts/Electrical_resistivity_and_conductivity/akyCKvMy">electronic</a>, and <a href="/facts/Thermal_conductivity/qWnSG3nQ">thermal properties</a> of molecular solids. </p><p>Molecular solids are poor <a href="/facts/Electrical_conductor/BSuyupLP">electrical conductors</a>, although some, such as <a href="/facts/TTF-TCNQ/hjk4uPhx">TTF-TCNQ</a> are semiconductors (ρ = 5 x 102 Ω−1 cm−1). They are still substantially less than the conductivity of copper (ρ = 6 x 105 Ω−1 cm−1). Molecular solids tend to have lower <a href="/facts/Fracture_toughness/stup0Kd3">fracture toughness</a> (<a href="/facts/Sucrose/7y1ke94L">sucrose</a>, <a href="/facts/Stress_intensity_factor/pLrUo683">KIc</a> = 0.08 <a href="/facts/MPa/Am1To2Qm">MPa</a> <a href="/facts/Metre/hPhzc1CV">m</a>1/2) than metal (<a href="/facts/Iron/QK1JYy8E">iron</a>, KIc = 50 MPa m1/2), ionic (<a href="/facts/Sodium_chloride/MWcXl3Mb">sodium chloride</a>, KIc = 0.5 MPa m1/2), and covalent solids (<a href="/facts/Diamond/IBUdnd1y">diamond</a>, KIc = 5 MPa m1/2). Molecular solids have low <a href="/facts/Melting_point/MY0w0h0k">melting (Tm)</a> and <a href="/facts/Boiling_point/E2BK9QoS">boiling (Tb)</a> points compared to metal (iron), ionic (sodium chloride), and covalent solids (diamond). Examples of molecular solids with low melting and boiling temperatures include <a href="/facts/Argon/QX6NRH6d">argon</a>, <a href="/facts/Water/0lkXnJPK">water</a>, <a href="/facts/Naphthalene/RU2sV8mN">naphthalene</a>, <a href="/facts/Nicotine/VQ6HcIrY">nicotine</a>, and <a href="/facts/Caffeine/Hx6E2QC5">caffeine</a> (see table below). The constituents of molecular solids range in size from condensed <a href="/facts/Monatomic_gas/1cTX2U2g">monatomic gases</a> to small molecules (i.e. naphthalene and water) to large molecules with tens of atoms (i.e. <a href="/facts/Fullerene/LPdnl8ox">fullerene</a> with 60 <a href="/facts/Carbon/ukrgQexo">carbon</a> atoms). </p> Melting and boiling points of metallic, ionic, covalent, and molecular solids<table><tbody><tr><th>Type of Solid</th><th>Material</th><th>Tm (°C)</th><th>Tb (°C)</th></tr><tr><td>Metallic</td><td>Iron</td><td>1,538</td><td>2,861</td></tr><tr><td>Ionic</td><td>Sodium chloride</td><td>801</td><td>1,465</td></tr><tr><td>Covalent</td><td>Diamond</td><td>4,440</td><td>-</td></tr><tr><td>Molecular</td><td>Argon</td><td>-189.3</td><td>-185.9</td></tr><tr><td>Molecular</td><td>Water</td><td>0</td><td>100</td></tr><tr><td>Molecular</td><td>Naphthalene</td><td>80.1</td><td>217.9</td></tr><tr><td>Molecular</td><td>Nicotine</td><td>-79</td><td>491</td></tr><tr><td>Molecular</td><td>Caffeine</td><td>235.6</td><td>519.9</td></tr></tbody></table>