Intermolecular and Intramolecular Forces (Episode 3)
Molecule, groups of two or more atoms, will form through intramolecular bonds, which are bonds between atoms. Covalent bonds are the strongest of the intramolecular bonds. This type of bond involves the sharing of electrons between atoms and is generally between nonmetals. Many atoms have room to gain electrons to fill their outer shell. The number of other atoms a particular atom can bond to is determined by how much room this particular atom has.
For example, examine Carbon, which has a total of six valence electrons but only four electrons in its outer shell. Carbon needs four more electrons to fill the outer shell. It can can obtain these four electrons by sharing with other atoms. Let’s shift our focus to carbon dioxide to better understand sharing of electrons. Carbon Dioxide consists of one Carbon and two Oxygen atoms as its name would suggest. The carbon must have a double bond with each Oxygen atom. The molecule has a total of sixteen valence electrons, and as shown by the Lewis structure of Carbon Dioxide, Carbon needs to be double bonded to each Oxygen to complete its octet.
Now, to mention the four major elements of the human body, Hydrogen can form one bond, Oxygen can form two, Nitrogen can form three, and Carbon can form four. It is also important to note that double bonds and triple bonds can form between certain atoms, and these types of bonds are stronger and shorter than single bonds. In addition to considering the different types of covalent bonds, we can also determine if a particular covalent bond is polar or nonpolar. A nonpolar covalent bond means equal electron sharing, and a polar covalent bond means unequal sharing of electrons.
An atom’s electronegativity difference determines its polarity. A more electronegative atom would pull electrons more towards itself and obtain a partial negative charge, while the other atom obtains a partial positive charge. If you are looking at a particular bond, calculating the electronegativity difference can help you determine if the bond is polar or nonpolar. The bond is nonpolar covalent if the electronegativity difference is less than or equal to 0.5. The bond polar above that but becomes ionic when the electronegativity is greater than 2. So atoms of the same element will have a nonpolar covalent bond, but atoms of different elements may or may not. A notable property of molecules with many polar bonds (polar molecules) is that they are usually hydrophilic. In contrast, molecules with many nonpolar bonds tend to be nonpolar molecules and are hydrophobic. Lipids have many nonpolar bonds, which make them insoluble in water.
Ionic bonds are the second strongest type of bond. Unlike covalent bonds, ionic bonds is not the sharing electrons. They actually are the stealing of electrons, and they are between a metal and a nonmetal. To understand an ionic bond, we must understand what ions are. Ions form when atoms lose or gain electrons. Ions that lose electrons become positively charged and are known as cations. Ions that gain electrons become negatively charged and are known as anions. For ionic bonding, there is a transfer of electrons. In Sodium Bromide, Sodium is the cation, and Bromide is the anion because Sodium transfers an electron to Bromide. Since these newly formed ions have opposite charges, they are actually attracted to each other. The larger the charge and the smaller the distance, the stronger the interaction. That means highly charged ions that are small will have the strongest ionic bonds. If you want an equation to remember this, refer to Coulomb’s law, which shows that the force of attraction is proportional to the magnitude of the charge and inversely proportional to the distance between charges.
We discussed two intramolecular forces, but we will ignore metallic bonding. Almost all questions you will see on this topic on intramolecular will be related to ionic or covalent bonding. Intermolecular forces are not about attraction between atoms but rather attraction between different molecules. Ion-ion interactions involve structures formed due to networks of ionic bonds. These are the strongest because they are attractions due formal charges rather than partial charges. The other intermolecular forces involve partial charges. The strongest after ion-ion involves interaction between ions and a dipole, and they are called ion-dipole interactions. The dipole of a molecule refers to its separation of charges. In this intermolecular force, cations attract to the electronegative portion of a molecule, and anions attract to the less electronegative portion of a molecule. Another interaction is the dipole-dipole interaction, and the strongest of the dipole-dipole interactions are hydrogen bonds. Hydrogen bonds can occur between Hydrogen and Nitrogen, Oxygen, or Fluorine. I remember it as NOF, but most people use FON to remember which elements are involved in hydrogen bonding. Hydrogen bonding is vital to biology because it gives water tremendous properties. It gives water a high boiling point, adhesion and cohesion, surface tension, and excellent solvent capabilities. Because of its high melting point and boiling point, water is less volatile. The last, the weakest, and the intermolecular force common to all types of molecules is the Van Der Waals force. This force is caused by momentary dipoles making one part of a molecule more negative than another. This momentary dipole can induce dipoles in nearby molecules, which leads to some attraction.
