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There are basically three types of chemical bonds in nature that hold atoms, molecules and ions together. These are the ionic bond, the covalent bond, and the metallic bond. Of the three, ionic and covalent bonds are the most common, and are responsible for the existence of virtually all organic and inorganic substances that we know of.
These two bonds are very different and give rise to ionic compounds or substances and to covalent compounds or substances that have a series of markedly different characteristics and properties.
Later, we will make a comparison of the ionic bond with the covalent bond, highlighting the most important differences between these two types of bonds and the chemical substances that possess them. However, before getting to that point and in order to understand it better, it is necessary to understand why atoms bond with each other and what determines the type of bond that occurs between two atoms.
Why do atoms bond with each other?
The existence of the chemical bond has to do with the stability of the atoms and, in particular, with their electronic configuration. This refers to the particular way how electrons are distributed around the nucleus of an atom.
It turns out that, as far as electron configurations are concerned, some are better than others, and only the elements in the group of noble gases (group 18 of the periodic table) have what we can call a stable electron configuration. This electronic configuration is characterized by having the valence shell s and p orbitals completely filled with 8 electrons.
No other element in the periodic table has this stable electronic configuration, so the other atoms seek to unite with each other in order to satisfy their need to surround themselves with 8 and only 8 valence electrons, just like noble gases, giving rise to the Chemical bond.
The need to surround yourself with 8 valence electrons is called the octet rule, and there are basically two ways to achieve it: give up (when you have too many) or remove (when you lack) valence electrons from another atom, or share the electrons of valence to mutually satisfy the same need. Depending on the case in question, an ionic bond or a covalent bond will be formed.
ionic bond
An ionic bond is the type of chemical bond found in ionic compounds. It is a link that occurs due to the force of electrostatic attraction existing between particles with opposite charges called ions, and hence its name. Positively charged ions are called cations, while negative ones are called anions.
An ionic bond is formed when a highly electronegative, nonmetallic atom removes one or more electrons from a highly electropositive atom (usually a metal). When this happens, the nonmetal is left with a negative charge, thus becoming an anion, while the metal is left with a positive charge, becoming a cation. By having opposite charges, these ions attract each other, forming the ionic bond.
the covalent bond
The covalent bond is a type of bond that occurs mainly between atoms of similar elements, almost always non-metals. Unlike ionic bonding, in covalent bonding there is no net transfer of electrons from one atom to another, as this would only help one atom to complete the octet, but not the other. Instead, the atoms share their valence electrons, through which they manage to complete the octet of both atoms at the same time.
Differences between ionic bond and covalent bond
It has already been clarified what a chemical bond is and the ionic and covalent bonds have been defined. Now we will analyze the main differences between these two types of bonds and between the compounds that contain them.
Types of elements that unite
| ionic bond | Covalent bond |
| Always between different elements and also of different types. In general, it occurs between metals and non-metals. Example: | It occurs between atoms of the same element or of very similar elements with similar electronegativities. It almost always occurs between nonmetals and nonmetals. |
Ionic bonds occur mainly between metals and non-mentals. The reason is that the former always have some electrons left over compared to noble gases, while non-metals generally lack some electrons. For this reason, when a metal is joined with a non-metal, the transfer between both elements occurs so that both satisfy the octet rule.
In the case of the covalent bond, as two identical or very similar atoms will have the same need to acquire electrons to complete their octet, the only way to achieve this is by sharing the electrons.
electronegativity differences
| ionic bond | Covalent bond |
| Electronegativity difference > 1.7 | Pure or non-polar covalent: < 0.4 Polar covalent: Between 0.4 and 1.7 |
One way to tell if two atoms will form an ionic or covalent bond is based on the difference in their electronegativities. When the difference is very large, the bond will be ionic, while when it is small or non-existent, it will be covalent.
Among covalent bonds, one can distinguish pure or nonpolar covalent bonds that occur between identical atoms (as in the H 2 molecule ) or between atoms with very similar electronegativities (as between C and H). If there is a difference in electronegativity, but it is not very large, a covalent bond occurs in which the electrons spend more time around one of the atoms, giving rise to a polar bond.
binding energies
| ionic bond | Covalent bond |
| They are between 400 and 4000 kJ/mol | They are between 100 and 1100 kJ/mol |
In general, the ionic bond is stronger than the covalent bond, although that depends on the atoms that are bonded. As a consequence, the binding energies in ionic compounds are almost always higher than those of covalent compounds.
Types of compounds that form
| ionic bond | Covalent bond |
| Ionic compounds such as lithium fluoride (LiF) or potassium chloride (KCl). | Molecular compounds such as methane (CH 4 ) and covalent network solids (or simply covalent solids) such as diamond (allotrope of carbon). |
Ionic bonds give rise to ionic compounds, while covalent bonds can give rise to either molecular compounds such as water or carbon dioxide, or covalent network compounds such as diamond, graphite and zeolites, in which that millions of atoms are linked to each other forming a two-dimensional or three-dimensional network that is very stable and resistant.
Differences in physical and chemical properties of the compounds that form
The fact of having ionic bonds or covalent bonds gives the different compounds very different properties. The following table summarizes the most important differences between ionic compounds and the two main classes of substances with covalent bonds, namely molecular substances and covalent solids.
| Property | ionic compounds | molecular compounds | covalent solids |
| melting and boiling point | Very high melting and boiling points. | Low melting and boiling points | Very high melting and boiling points. |
| physical state at room temperature | They are solid at room temperature. | They can be both solid and liquid or gas at room temperature. | They are solid at room temperature. |
| Solubility | They are usually soluble in water and other polar solvents. | Polar molecular compounds are soluble in polar solvents. Nonpolars are insoluble in water and other polar solvents but soluble in many nonpolar organic solvents. | They are not usually soluble in any solvent. |
| Electric conductivity | They do not conduct electricity in the solid state, but they do in solution or in the liquid state (molten salts). | They do not conduct electricity. They are insulating materials. | Some are conductors (like graphite), while others are not (like diamond). |
| structure type | crystalline solids. | Some are crystalline, others amorphous. | crystalline solids. |
| Mechanical properties | hard brittle solids | They are generally soft | hard brittle solids |
Summary of the differences between ionic bond and covalent bond
| ionic bond | Covalent bond | |
| Definition | The force that holds oppositely charged ions together in ionic compounds. | The force that holds two atoms together that share valence electrons. |
| Types of elements that unite | Always between different elements and also of different types. In general, it occurs between metals and non-metals. Example: | It occurs between atoms of the same element or of very similar elements with similar electronegativities. It almost always occurs between nonmetals and nonmetals. |
| electronegativity differences | Electronegativity difference > 1.7 | Pure or non-polar covalent: < 0.4 Polar covalent: Between 0.4 and 1.7 |
| binding energies | They are between 400 and 4000 kJ/mol | They are between 100 and 1100 kJ/mol |
| Types of compounds that form | Ionic compounds such as lithium fluoride (LiF) or potassium chloride (KCl). | – Nonpolar molecular compounds such as methane (CH4). – Polar molecular compounds such as water (H 2 O) – Covalent network solids (or simply covalent solids) such as diamond (allotrope of carbon). |
References
Brown, T. (2021). Chemistry: The Central Science (11th ed.). London, England: Pearson Education.
Chang, R., Manzo, Á. R., Lopez, PS, & Herranz, ZR (2020). Chemistry (10th ed.). New York City, NY: MCGRAW-HILL.
The chemical bond and molecular geometry. (2020, October 29). Retrieved from https://espanol.libretexts.org/@go/page/1851
