Molecules and Molecular Forces

Questions on this topic? Just ask!

Molecules and Molecular Forces

Matter consists of atoms and molecules. Atoms are the smallest units of matter that retain the physical and chemical properties of individual elements. Molecules, however, consist of two or more atoms chemically bonded together. Molecules generally display characteristics that are distinct from their constituent atoms. For instance, water, a compound whose molecules consist of two hydrogen atoms bonded to one oxygen atom, has drastically different physical and chemical properties than either hydrogen or oxygen alone. A molecule, then, is the smallest unit of a compound that retains the properties of the compound.

Ionic bonds are bonds between ions that attracted to each other because of their opposite electrical charges. The electron imbalance in ions can create negative or positive (e.g., an excess of electrons over protons creates a negative ion). Opposite ions attract one another by electrical force to create an ionic bond. Table salt, or sodium chloride, is a familiar example of ionic bonding. A second and stronger type of intramoleclar force is covalent bonding. Covalent bonds between atoms within a molecule involve the sharing of electrons among nuclei. Covalent bonds are strong intramoleular atomic forces that bond atoms together to form molecules.

Intermolecular forces are those of attraction between separate molecules. Generally, intermolecular forces are weaker than ionic and covalent bonds, but nonetheless are important forces influencing compounds. Hydrogen bonds are intermolecular forces between molecules containing hydrogen, and molecules containing highly electronegative elements such as oxygen, sulfur, or nitrogen. The hydrogen of one molecule, itself bonded to an electronegative element, is attracted to an unshared pair of electrons of another electronegative element of a second molecule. Individually weak, but in numbers quite strong, hydrogen bonds are intermolecular forces that have important influences. For instance, hydrogen bonding between hydrogen and oxygen components of water molecules affects the melting and boiling points of the compound, as well as determining its cohesive and adhesive properties. The hydrogen bonding of water makes life as it is presently known to exist possible by influencing the properties of water. Also, hydrogen bonding is an essential molecular force in the structure of DNA. Hydrogen bonding holds the single strands together to form the characteristic double helix of DNA.

Dipole-dipole interactions are a second type of intermolecular force. Dipole-dipole interactions are less intense versions of hydrogen bonding. Some molecules have regions that are slightly more or slightly less negatively charged than other regions. Such partial charge differences are called dipoles. Opposite charged dipoles are attracted to one another. Thus dipole-dipole interactions are weak electrical forces between molecules. Because they are weak attractions, dipole-dipole intermolecular forces are significant only in closely associated molecules, such as cellular protein receptors and their ligands. One result of polarity in molecules is that found in hydrophobic and hydrophillic interactions. For example, fatty, hydrocarbon molecules that are insoluble in water are said to be hydrophobic. In contrast, charged molecules displaying dipoles are readily soluble in water, and are thus hydrophilic. When hydrophobic molecules, or molecules with hydrophobic regions, are placed in aqueous solution, the attractive forces among charged particles actively exclude the non-charged hydrophobic molecules. As a result, the fatty molecules are pushed together. The resulting intermolecular force pushing like-molecules together is called hydrophobic interaction. When oil and water are mixed, for example, the oil eventually separates out into a layer. The water-insoluble oil molecules are pushed together by hydrophobic interactions. Such intermolecular forces are important in the structure of complex biological molecules such as membranes, proteins, and the configuration of DNA. Combinations of intermolecular forces often determine the overall behavior of molecules.