Many existing substances and compounds owe their properties to chemical bonds. Bonds hold atoms of specific elements together to make up a molecule. The nature of the bond that links them determines the strength of such a link.
Chemical bonds
A chemical bond involves the interaction between the electrons of individual atoms of the constituent elements that forms a permanent connection between them. This results in the formation of a chemical compound. Usually, individual elements have different properties from those of the molecule they are part of.
The formation of chemical compounds through bonding results from the fact that elements tend to achieve the lowest possible energy state. This makes them chemically inert. The atoms of the elements are drawn to the electron configuration that is closest in the periodic table to helium (Group 18). This is expressed in the duplet and octet rules. The duplet rule describes the tendency of atoms to have two valence electrons on the outer shell. The octet rule is similar in that the elements tend to have eight valence electrons.
The duplet and octet rules are reflected by chemical elements in that they form one or more chemical bonds. The number of bonds an atom is capable of forming is referred to as valence. Yet, a single element may be characterised by different valencies.
In order to fully characterise a chemical bond, we often also provide other information about it such as its energy, bond length and the difference in electronegativity of the constituent elements.
The importance of electronegativity in bond formation
Electronegativity is a term used to describe the phenomenon of attraction of shared electrons by the atoms of the elements that form a given chemical bond. Electronegativity refers directly to the bond energy between atoms. The way that chemical elements achieve electron configurations of the nearest noble gas in the periodic table, in other words how they form chemical bonds, depends directly on the electronegativity of their individual components.
Particular elements interact with electrons of other atoms with varying strengths, in line with how they are ranked on a scale created by Linus Pauling (Pauling Electronegativity Scale). Metals are characterised by low electronegativity values. As a result, they weakly attract other electrons and give up their own electrons easily. They are also called electropositive elements. Cesium (or artificially obtained francium) have the lowest electronegativity of 0.7. Nonmetals behave differently. Fluorine is the most electronegative element (4.0) of them all. Nonmetals strongly attract valence electrons of the atoms which they want to bond to.
Types of chemical bonds
Ionic bonds
An ionic bond is formed between metal atoms and nonmetal atoms, which significantly differ in their Pauling electronegativity. The more electronegative element attracts electrons that are added to its valence shell. This affinity results in the element having an excess of electrons and therefore becoming a negative ion, or anion. At the same time, the (electropositive) atom that has lost its electrons now has a shortage of electrons, so it becomes a positive ion, or cation. It is assumed that in order to form an ionic bond, the difference in electronegativity should be at least 1.7. The resulting ions (cation and anion) attract each other as a result of electrostatic attraction of oppositely charged ions. It should be noted, however, that in real life, there are no bonds that are 100 percent ionic. The percentage share of this bond depends on the difference in electronegativity between the constituent atoms: the greater the difference, the greater the percentage share of ionic bonding.
Covalent bonds (atomic)
A covalent bond occurs between atoms of nonmetals that have a small difference in their Pauling electronegativity. The elements that form a covalent bond “share” valence electrons in such a way that each of them can reach the lowest possible energy-state. The resulting pair of electrons is known as a shared pair. It is located between the atoms in the form of an electron cloud. If there is a difference in electronegativity between the constituent elements, the covalent bond becomes polarised, and the shared pair of electrons is shifted toward the element with higher electronegativity (which attracts electrons more strongly). The molecule then becomes a dipole, i.e. it has positive and negative poles. If two atoms involved in the covalent bond are the same, they form a non-polar covalent bond. The electron pair is not shifted in any direction, because the difference in electronegativity on the Pauling scale is 0.
Coordinate covalent bonds
This is a type of bond where one of the atoms gives up its electron pair becoming the so-called donor. The atom in a molecule or in an ion with an incomplete valence shell becomes the so-called acceptor. Another name for this bond is dative bond. The coordinate covalent bond is similar to the covalent bond in a sense. In this case, however, electron sharing occurs as a result of only one atom donating its pair of electrons.
Metallic bonds
Metallic bonds are special types of bonds found in metals and their alloys. Cations in metals form a specific crystal lattice and are positively charged. Electrons travel on the valence shells of metal atoms. They form an electron cloud and move freely between metal cations in the crystal lattice. They are called delocalised electrons. Since they are negatively charged, they balance the positively charged cations, making metals electrically neutral.
Intermolecular interactions
There are many compounds in the real world whose atoms are not connected by chemical bonds. They interact with each other as a result of much weaker, short-range forces, called van der Waals forces, and hydrogen bonds.
Van der Waals forces
These are short-range interactions that occur between non-polar molecules. They play an important role in macromolecules such as polymers. Moreover, they influence the physical state of specific elements of matter. The most common example of van der Waals attractions is the graphite lead in pencils. As you press your pencil to paper, layers of graphite (that are weakly bound to each other) slide across one another and deposit themselves on a page.
Hydrogen bonds
Hydrogen bonds are around 10 times weaker compared to covalent bonds. They may occur either within a single molecule or between different molecules. They are formed between hydrogen atoms bound to atoms of electronegative chemical elements, and atoms of highly electronegative elements that have lone electron pairs. This type of chemical attraction is characteristic of the -OH, -SH and -NH2 groups. Hydrogen bonds play an important role in all types of biological systems. It leads to association, i.e. holding together larger clusters of molecules, which changes their properties such as their boiling point, density or solubility.