4 Types of Bonds to Know About

A bond is a debt instrument that promises to pay investors principal and interest at maturity. Unlike stocks, bonds can be bought and sold in the open market.

Most bonds are unsecured, meaning that they do not have a collateral backing them. However, there are exceptions. Bonds can be backed by the full faith and credit of the issuer, or by revenue from a specific project or source.

1. Covalent

Covalent bonds result from the sharing of electron pairs between atoms. The shared electrons have lower total energy than the electrons of atoms that are widely separated. A bond is formed when an atom gives up one of its own electrons to another atom that needs them to complete its octet.

A covalent bond forms when atoms with similar electronegativities (electron attracting abilities) share electrons in their outer orbitals. Carbon for example has six electrons in its octet but only four electrons are in its outer orbitals. It can therefore form a covalent bond with hydrogen (which has four electrons in its octet) to produce the methane molecule.

Other examples of simple covalent compounds include the diatomic molecules hydrogen, nitrogen, oxygen and halogens, and elemental gases such as helium and neon. They are also found in a number of multi-atomic molecular compounds such as phosphorus and sulfur.

These covalent bonds are often described using the Lewis dot symbol and given a name such as HN3 or CH4. In some cases, where atoms have unequal electronegativities, there is an imbalance in the bonding pairs. This leads to polar covalent bonds that exhibit a negative charge at one end of the molecule and a positive charge at the other.

2. Ionic

Ionic bonds, ionic compounds and ionization involve charged ions. They are different from covalent and polar bonds in that they transfer valence electrons to form oppositely charged ions.

Ionic bonding usually occurs between elements with a large difference in their electronegativity values, but can also occur between metals and non-metals. The transfer of electrons creates positively charged ions called cations and negatively charged ions called anions. These ions are attracted to each other and form the solid substance we know as a compound.

Ionic bonds can form by a redox reaction when an element with low electron affinity gives away its electrons to achieve its stable electron configuration, or it can accept them from another element.

For example, sodium loses its outer electron to fluorine to form the ionic compound, sodium fluoride. It can also form spontaneously when two metals combine together, such as in sodium chloride (table salt). Ionic bonding is based on electrostatic attraction between oppositely charged ions, not on the VSEPR rules of molecular geometry.

3. Polar

Polar bonds form when atoms with different electronegativities share a bonding pair of electrons. The electronegativities of the two atoms determine whether the bonding pairs are distributed evenly or unequally. The higher the difference in the electronegativities, the more polar the bond.

For example, the carbon-oxygen double bond in methane (CH2CO2) is a polar bond because of the large differences in the electronegativities of carbon and oxygen. This means that each atom attracts the shared electrons more strongly. As a result, the electron density is concentrated on one side of the molecule.

In polar molecules, the electrons are unevenly distributed between the two ends of the molecule. Thus, each end of the molecule has a partial negative electrostatic charge (d) and a partial positive charge (+).

4. Metallic

The metallic bond is a type of chemical bond found in metals. It is distinct from the ionic and covalent bonds in that the electrons are not shared between two individual atoms. Instead, the valence electrons in a metallic bond are delocalized across a larger group of atoms, forming a sea of electrons that can be interacted with by all other atomic nuclei in a particular material. This characteristic of the bond gives metals many of their unique properties, including conductivity and malleability.

When light is incident on a metallic surface, it is absorbed by the sea of electrons and excited. These electrons quickly return to their ground states, releasing the energy in the form of light. The resulting emission of light is what attributes the shiny metallic luster to metals.

Metallic bonding is often seen in minerals containing transition metals, such as gold and silver. These minerals are excellent conductors of heat and electricity and have a high melting point.