Primary and Secondary Bonds
Elements interact with each other constantly in the natural world. There are only a few elite ones who are noble enough to remain to themselves. But in general every element interacts with at least another, giving rise to a variety of structures, phenomena and compounds we see every day. These interactions take place in the most basic form as bond formation.
There are various kinds of bonds but they are all grouped under two main categories, primary and secondary bonds. Primary bonds are those which are strong in nature. They have electronic attractions and repulsions just like secondary bonds but in equilibrium they are stronger than the later. They are broadly classified into three types: Ionic bonds, Covalent bonds and Metallic bonds.
These are bonds formed from the donation and acceptance of electrons between elements, giving rise to strong compounds. These bonds are electrically neutral when the compound is in the solid state but on dissociation in solutions or in the molten state they give positively and negatively charged ions. For instance, NaCl or sodium chloride is a compound formed from ionic bonds between positively charged Na+ ions and negatively charged Cl- ions. This compound is hard but brittle and does not conduct electricity when it is solid but does so when in mixed in a solution or in liquid state. Furthermore, it has a very high melting point, in other words, strong heat is required to break the bonds between the constituent ions. All these strong characteristics of this compound are attributed to it by the presence of strong ionic bonds between its constituent elements.
Covalent bondsare those bonds formed when electrons are shared between elements giving rise to compounds. These bonds enable the constituent elements to complete their incomplete noble gas configuration. Thus these bonds are strong since no element wishes to lose their invite into the elite society of the nobles. For example, the dioxygen molecule is formed from covalent bonds between two oxygen atoms. Each oxygen atom is two electrons short of the next noble gas configuration, which is of the neon atom. Therefore when these atoms come closer and share two electrons each, they give rise to a double covalent bond between the two shared electron pairs of the atoms. Covalent bonds are also possible for single and triple bonds where bonds are formed between one and three pairs of electrons respectively. These bonds are directional and generally insoluble in water. Diamond, the hardest known naturally occurring substance on Earth, is formed from covalent bonds between carbon atoms arranged in a 3D structure.
Metallic bonds, as the name suggests, are bonds found only in metals. Metals are elements of the electropositive nature, thus it is very easy for the constituent atoms to lose their outer shell electrons and form ions. In metals, these positively charged ions are held together in a sea of negatively charged free electrons. These free electrons are responsible for the high electric and thermal conductivities of metals.
Van der Waal's Forces
Secondary bonds are bonds of a different kind to the primary ones. They are weaker in nature and are broadly classified as Van der Waal's forces and hydrogen bonds. These bonds are due to atomic or molecular dipoles, both permanent and temporary.
Van der Waal's forces are of two types. The first type is as a result of electrostatic attraction between two permanent dipoles. Permanent dipoles are formed in asymmetric molecules where there are permanent positive and negative regions due to difference in electronegativities of the constituent elements. For example, water molecule is made of one oxygen and two hydrogen atoms. Since each hydrogen requires one electron and oxygen requires two electrons to complete their respective noble gas configurations, thus when these atoms approach each other they share a pair of electrons between each hydrogen and the oxygen atom. This way all three achieve stability through the bonds formed. But since oxygen is a highly electronegative atom, therefore the shared electron cloud is attracted more towards it than the hydrogen atoms, giving rise to a permanent dipole. When this water molecule approaches another water molecule, a partial bond is formed between the partially positive hydrogen atom of one molecule and the partially negative oxygen of another. This partial bond is due to an electric dipole and thus is called a Van der Waal's bond.
The second type of Van der Waal's bond is formed due to temporary dipoles. A temporary dipole is formed in a symmetric molecule but which has fluctuations of charges giving rise to partial dipole moments for only a few moments. This can also be seen in atoms of inert gases. For instance, a molecule of methane has one carbon atom and four hydrogen atoms joined together by single covalent bonds between the carbon and the hydrogen atoms. Methane is a symmetric molecule but when it is solidified, the bonds between the molecules are of weak Van der Waal's forces and thus such a solid cannot exist for a long time without tremendously cared for laboratory conditions.
Hydrogen bonds are relatively stronger than Van der Waal's forces but compared to primary bonds they are weak. Bonds between hydrogen atom and atoms of the most electronegative elements (N, O, F) are called hydrogen bonds. It is based on the fact that hydrogen being the smallest atom provides very little repulsion when interacting with highly electronegative atoms in other molecules and thus succeeds in forming partial bonds with them. This makes hydrogen bonds strong but weaker compared to primary bonds since the interactions here are permanent dipole interactions. Hydrogen bonds are of two types- intermolecular and intramolecular. In intermolecular hydrogen bonds, the bonds are between hydrogen atom of one molecule and electronegative atom of another. For example, o-nitrophenol. In intramolecular hydrogen bonds, the bonds are between hydrogen atom and electronegative atom of the same molecule but such that they do not have any covalent interactions. For example, p-nitrophenol.