All matter is made up of atoms. Atoms are tiny particles of different types that join together to form molecules and other types of chemical compounds. What holds the different atoms together in a polyatomic substance such as a molecule or an ionic compound is what we call the chemical bond.
A chemical bond can be defined as a force of an electrostatic nature that holds two atoms together thanks to the interactions between the nuclei and the electronic clouds of both . Since there are different types of atoms, including metallic atoms, non-metallic atoms, metalloids, and noble gases, various combinations can occur in which the atoms interact in different ways, giving rise to different types of chemical bonds.
One of the main characteristics of atoms that determines the type of bond that will form between them is their metallic character. It is not the same to unite a metallic atom with another, than to unite a metal with a non-metal, or a non-metal with another non-metal. Even when joining two nonmetals together, the bond can be of different types, depending on the difference between the electronegativities of the two elements.
Types of chemical bonds and electronegativity
Depending on the characteristics of the two linked atoms, different types of bonds can be given. Broadly speaking, we can identify four main types, which are:
- The ionic bond .
- The polar covalent bond .
- The pure or nonpolar covalent bond .
- The metallic bond .
The most important property that determines the type of bond that will form between two atoms is the difference between their electronegativities. Electronegativity is the ability of an atom to attract bonding electrons when a chemical bond is formed. This is a periodic property that increases as you move from bottom to top along a group on the periodic table, and as you move from left to right across a period, fluorine being the most electronegative element of all.
Electronegativity is measured on a scale that goes from 0.7 (corresponding to the francium atom, the least electronegative of all) to 4 (corresponding to fluorine). This scale is known as the Pauling electronegativity scale and is very useful for predicting the type of bonds that will form between two atoms.
Using Electronegativity to Predict Bond Type
When two atoms bond with each other, they seek to complete their octet, that is, they seek to surround themselves with a total of 8 valence electrons. For this reason, when the bond is formed, there is immediately a competition to keep the bonding electrons of the other.
The atom that is more electronegative gets all the electrons. If this happens, this atom becomes negatively charged, while the less electronegative one, the one that lost the electrons, remains positively charged. These two ions are attracted to each other thanks to their opposite charges, thus forming the ionic bond. This is particularly common when we are bonding a metal with a non-metal, such as magnesium chloride shown below.
On the other hand, if both atoms have the same electronegativity (which could happen if both atoms are the same, for example), neither would win the competition for the other’s electrons, so they would have no choice but to share the electrons. in order to simultaneously satisfy their respective octets. In this case, because valence electrons are being shared, the bond is called a covalent bond .
But what happens if we join two atoms that have similar but not the same electronegativities? In that case, the bond will neither be completely ionic nor completely polar. In these cases, the two atoms do not perfectly share electrons, generating opposite partial charges at each end of the bond. These types of bonds are called polar covalent bonds , or simply polar bonds .
Finally, when we join two metals together, neither an ionic nor a covalent bond is formed. In this case, a special type of chemical bond called a metallic bond is established . In this type of bond, the metal atoms are usually packed in a cubic structure like the ones shown in the following figure.
Typical cubic cells of the crystalline structure of metals. From left to right, these cells are: simple cubic cell, face-centered cubic cell, and body-centered cubic cell.
Conventional criteria to define bond types based on electronegativity
The following table summarizes the criteria for deciding whether the bond between two atoms will be ionic, polar covalent, nonpolar, or metallic.
link type electronegativity difference Example ionic bond >1.7 NaCl; LiF polar bond Between 0.4 and 1.7 OH; HF; NH nonpolar covalent bond <0.4 CH; IC pure covalent bond 0 H H; ooh; FF metal bond does not depend on electronegativity Fe, Mg, Na, Ti…
As can be seen in the table, the bond will be ionic when the difference in electronegativity is more than 1.7. It is considered pure covalent if there is no difference, or if the difference is very small. Some authors distinguish the first case from the second, considering as pure covalent bonds only those in which two equal atoms join, while when the difference is very small, they are classified as nonpolar or apolar bonds.
Lastly, if two metals are being bonded, then the bond is classified as a metallic bond.
Characteristics of the different types of links
The ionic bond is so named because it is formed by two ions with opposite charges. It is formed when a metal with very low electronegativity, usually an alkali or alkaline earth metal, is joined with a nonmetal with a very high electronegativity, usually a halogen.
This type of bond is not directional because electrons are not shared along the axis that joins both atoms. It is also not possible to recognize discrete units when ionic compounds are formed, because each cation can be found surrounded by multiple anions and these, in turn, are attached to other cations, without belonging exclusively to any of them.
Compounds with ionic bonds are generally soluble in water and produce solutions that conduct electricity.
polar covalent bond
In this case, a bond is formed in which electrons are shared, but not equally, generating a partial negative charge on the most electronegative atom, and a partially positive charge on the least electronegative. This type of link gives rise to discrete units called molecules in which each atom is always linked to the same other atoms.
Many compounds with polar bonds have polar molecules that can become soluble in water.
The pure or nonpolar covalent bond
This link occurs when two identical atoms join, as occurs in the molecules of Cl 2 , O 2 and N 2 . Because there is no difference in electronegativity, the electrons are shared perfectly equally. Compounds that only contain covalent bonds are necessarily nonpolar, and are compounds that are not soluble in water.
multiple covalent bonds
Both in the pure covalent bond and in the polar one, covalent bonds can occur in which more than one pair of electrons are shared, giving rise to multiple covalent bonds. Depending on whether 2, 4, or 6 electrons are shared, the bond will be classified as a single, double, or triple covalent bond, respectively.
the metallic bond
As already mentioned before, this type of bond is formed between metal atoms. Its most important characteristic is the presence of what is called the “conduction band”, through which the valence electrons of the metal can move freely from one side to the other. This freedom of movement is what makes metals very good conductors of electricity.
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