Intermolecular forces are the forces that exist between molecules. These forces determine many properties of substances, including boiling point, melting point, and solubility. The strength of intermolecular forces varies depending on the type of force and the molecules involved.
The strongest type of intermolecular force is the ionic bond, formed between atoms with opposite charges. Ionic bonds are very strong, but they can be easily broken by adding energy, such as heat. The covalent bond is the next strongest intermolecular force, formed when atoms share electrons. Covalent bonds are not as strong as ionic bonds, but they are much harder to break. The weakest type of intermolecular force is the van der Waals force, which is a temporary attraction between molecules that occurs when they come close to each other.
The Three Types of Intermolecular Forces
In order for a solution to form, the solvent molecules must be attracted to the solute molecules more than they are attracted to each other. This is because the solute molecules are going to be more spread out in the solution than they were in the pure solvent. The forces that cause this attraction are called intermolecular forces.
London Dispersion Forces
London dispersion forces are the weakest type of intermolecular force. They are present between all molecules, even those that do not have permanent dipoles. London dispersion forces occur because the electrons in the outermost orbital of an atom are not evenly distributed. This creates a temporary dipole, which can induce a dipole in another nearby atom. The strength of the London dispersion force increases as the number of electrons in the outer orbital increases.
Dipole-dipole forces are electrostatic interactions between molecules that have permanent dipoles (unevenly distributed electrons). A dipole is formed when the electron pushing elements (fluorine, chlorine, bromine, and iodine) bond with an electron pulling element (oxygen, nitrogen, and carbon). This creates a molecules with a center of positive charge and a center of negative charge. The positive side of one molecule is attracted to the negative side of another molecule. Dipole-dipole forces are generally weaker than ionic or covalent bonds, but stronger than Van der Waals forces.
Hydrogen bonding is a special type of dipole-dipole attraction between molecules, not a covalent bond to a hydrogen atom. For hydrogen bonding to occur, you need:
- A hydrogen atom covalently bonded to a very electronegative atom, such as N, O, or F. Other pairs of atoms can form hydrogen bonds (I-Br, for example), but N-H, O-H, and F-H pairs are by far the most common.
- Another very electronegative atom nearby (within about 4 Angstroms) to which the first atom can bond. Nitrogen can hydrogen bond to itself, as can oxygen; fluorine rarely serves as the “hydrogen bond acceptor.”
Keep in mind that hydrogen bonds are much weaker than covalent bonds: a single water molecule can be held together by two or three other water molecules via hydrogen bonds, but it takes about 10^5 similar molecules to make up a drop of water!
The Strength of Intermolecular Forces
Intermolecular forces are the forces that hold molecules together. They are much weaker than the ionic or covalent bonds that hold atoms together in molecules, but they are important nevertheless. The strength of intermolecular forces varies depending on the type of force. The strongest intermolecular force is the ionic bond, followed by the covalent bond, followed by dipole-dipole forces, dipole-induced dipole forces, and London dispersion forces.
London Dispersion Forces
London Dispersion forces are the weakest of the intermolecular forces. They are present between all molecules, even those that do not have a dipole. London Dispersion forces result from the fact that the electrons in atoms are not always evenly distributed. This can cause an atom to have a temporary dipole. When this happens, the atom is attracted to other atoms that have a temporary dipole in the opposite direction. The size of the London Dispersion force depends on two things: the number of electrons in the atom and how easily the electrons can move around. The more electrons an atom has, the greater its chances of having a temporary dipole. The more easily the electrons can move around, the more likely it is that a dipole will form.
Dipole-dipole forces are attractive forces between molecules that have permanent dipoles. A dipole exists when the electron pushing elements, found on the left side of the periodic table, exchanges electrons with the electron pulling elements, on the right side of the table. This creates a separation of charge and a dipole. Oxygen (O) and fluorine (F) form a dipole because oxygen exchanges electrons more readily than fluorine. The result is that the fluorine end, F-, becomes slightly negative and the oxygen end, O-, becomes slightly positive.
Polar molecules, like water (H2O), have a stronger dipole-dipole force than nonpolar molecules, like methane (CH4). This is because polar molecules have a larger separation of charge than nonpolar molecules. The stronger the dipole, the stronger the dipole-dipole force.
Hydrogen bonding is a special type of dipole-dipole attraction between molecules, not a covalent bond to a hydrogen atom. Hydrogen bonds can occur between molecules (H₂O) or within different parts of a single molecule (DNA).
Intermolecular hydrogen bonds are much weaker than ionic or covalent bonds, but they are still strong enough to make water a liquid at room temperature and to make DNA a double helix.
From the data, it appears that the strongest type of intermolecular force between cus and ags is ionic bonds.