Intermolecular force
Intermolecular forces are the attractive forces that exist between molecules, as opposed to the forces that hold atoms together within a molecule. They play a significant role in determining the physical properties of substances, influencing characteristics such as boiling and melting points, solubility, and viscosity. There are several types of intermolecular forces, including dipole-dipole forces, hydrogen bonding, ionic interactions, and weaker forces like induced dipole or London dispersion forces. Dipole-dipole forces arise from the attractions between the positive and negative poles of polar molecules. Hydrogen bonding, a stronger subset of dipole forces, occurs when hydrogen atoms are covalently bonded to highly electronegative atoms like nitrogen, oxygen, or fluorine. Ionic interactions are the strongest intermolecular forces, formed through the transfer of electrons between atoms, resulting in charged ions. The behavior of molecules changes with temperature, transitioning between solid, liquid, and gas states as thermal energy influences the strength and presence of these intermolecular bonds. Understanding these forces is crucial for comprehending how various substances behave under different conditions.
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 much weaker than molecular bonds, which are created by intramolecular forces, or forces within molecules.
Dipole-dipole forces, hydrogen bonding, and ionic interactions 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 force found in 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. They are attracted to these oppositely charged atoms. When they align because of these attractions, they form temporary hydrogen bonds. These bonds are stronger than dipole forces.
Ionic forces are the strongest. 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 molecules generally lack one or two 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 dipoles.
These attractive forces between molecules determine the physical properties of matter, including boiling point, evaporation, melting point, solubility, surface tension, vapor pressure, and viscosity.
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, but they are not very sticky; instead, they move around one another, forming and breaking intermolecular bonds. When enough thermal energy is present, it overcomes the intermolecular bonds completely. No intermolecular bonds form, 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 hydrogen bond of two hydrogen atoms and an oxygen atom. A water molecule is somewhat triangular. The single electrons of the hydrogen atoms are continuously moving but generally stay in the area closest to the oxygen atom, or inside the water molecule. This is referred to as concentration. For this reason, the outer area of the hydrogen molecule is positively charged. Most of the eight electrons of the oxygen atom will be away from the hydrogen atoms, making the outer side of the oxygen atom in the water molecule negatively charged. 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), the hydrogen bonds break. The water changes to steam, or water vapor, its gaseous state.
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