Intermolecular force

Intermolecular forces exist between molecules rather than within them. Some describe intermolecular forces as determining the “stickiness” of molecules. They are attracting forces and therefore are generally weaker than molecular bonds, which are created by intramolecular forces, or forces within molecules.

Dipole-dipole forces, hydrogen bonding, ion-dipole interactions, and London dispersion forces are intermolecular forces. As the polarization and charges of atoms increase, intermolecular attraction increases as well.

Overview

Atoms are made up of subatomic particles called protons, neutrons, and electrons. Protons and neutrons form the center, or nucleus, of an atom, while electrons orbit the nucleus in layers, like a cloud. Electrons closest to the nucleus are most strongly bonded to it, while electrons in the outermost layer, called the valence shell, frequently interact with other atoms. Opposite charges attract, and like charges repel. Electrons are negatively charged, and protons are positively charged, so they attract each other. Neutrons have no charge. Groups of atoms bonded together are molecules. Molecules are affected by the forces between them.

Forces between Molecules

Dipole-dipole force refers to the polar regions of molecules, which are partially positive and partially negative. The slightly positive pole of a molecule is attracted to the slightly negative pole of an adjacent molecule. This attraction is a dipole-dipole force.

Hydrogen bonding is a strong type of dipole-dipole interaction found in many chemical and biological systems. The hydrogen atom is very simple: a single proton and a single electron. Hydrogen atoms bond to atoms such as nitrogen, oxygen, and fluorine. These hydrogen atoms with a partial positive charge are attracted to the lone pair of electrons on the electronegative atoms of the other nearby molecule. When they align because of these attractions, they form hydrogen bonds.

Ion-dipole interactions are among the strongest intermolecular forces. Ions are particles that are charged because they have either more or fewer electrons than protons. Ionic bonds form as the result of the transfer of one or more electrons from a metal to a nonmetal. Metals tend to have few electrons in their outer energy levels and easily lose them. Nonmetal atoms generally lack one to three electrons in their valence shells and readily gain them.

Induced dipole forces, or London dispersion forces, are very weak and brief. When electron clouds in nonpolar molecules shift, they temporarily attract or repel electron clouds in other nonpolar molecules close to them. An atom or molecule with many electrons would more likely be affected by induced dipole forces and temporarily become a dipole.

These attractive forces between molecules determine the physical properties of matter, including boiling point, evaporation, melting point, solubility, surface tension, vapor pressure, and viscosity. Intermolecular forces also influence modern research in materials science, nanotechnology, biomolecular engineering, and climate science.

States of Matter

Matter commonly exists in three states: solid, liquid, and gas. The molecules of a substance behave differently in each state. As a solid, the intermolecular bonds are strong, and the atoms or molecules are close together. Solids have some thermal energy, so the atoms vibrate, but the movement is imperceptible. The introduction of thermal energy increases molecular movement and loosens the bonds, creating a liquid state. The particles of a liquid touch, and are sticky enough to give liquid a definite volume; however, they are free enough to move around one another, forming and breaking intermolecular bonds. When enough thermal energy is present, it largely overcomes the intermolecular bonds, and the liquid becomes a gas with molecules that move about freely and independently. The molecules spread indefinitely.

As an example, water exists as a solid, a liquid, and a gas at different temperatures. Water is a molecule consisting of two hydrogen atoms covalently bonded to one oxygen atom. A water molecule is somewhat triangular. The shared ten electrons in a water molecule are continuously moving but generally stay in the area closer to the oxygen atom than around the hydrogen atom. This is referred to as an uneven distribution of electrons. For this reason, the hydrogen side of the water molecule carries a partial positive charge, and the oxygen side a partial negative charge. These polar charges cause adjacent water molecules to line up; the negative poles are attracted to the positive poles of nearby molecules. In its solid form, the molecules occupy a set space. When heat is added, the kinetic energy of the molecular system increases. The bonds of the water molecule, between the hydrogen and oxygen, bend and stretch. In its liquid state, water molecules have a great deal of energy and form temporary hydrogen bonds. Additional heat increases the kinetic energy. When water reaches its boiling point at a temperature of 100°C (212°F), hydrogen bonds break for water to become a gas. The water changes to steam, or water vapor, its gaseous state.

Bibliography

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