The basic components of matter, from the cells of the human body to the air, water, and minerals, are formed by different types of atoms and molecules (Castillero Mimenza, 2017). These tiny particles, considered the fundamental unit of matter, allow us to understand various biological processes, such as depolarization, which are relevant to neuroscience. However, the formation of living organisms or the various materials observed daily requires atoms to group and interact in specific ways. In line with Castillero Mimenza (2017), chemistry, through the study of the composition of matter, has successfully identified the elements that enable the bonding between atoms: the so-called chemical bonds.
Chemical Bonds: The Foundation of Matter
A chemical bond is the result of the union between atoms, molecules, or ions that give rise to more complex and stable compounds, modifying their physical and chemical properties (Ondarse Álvarez, 2021). To achieve greater stability, atoms complement their electrical charges by sharing, donating, or accepting electrons from their outermost layer, thus forming ions (positive or negative) that are attracted to each other by electrostatic force. The reason for this behavior, according to Ondarse Álvarez (2021), is that the protons in the atomic nucleus have a positive charge, and the electrons surrounding it have a negative charge, so atoms seek to balance their electrical charges.
The attraction between the electron and the nucleus depends on the position, electronegativity, and electronic stability of each atom and can be so strong that it prevents repulsion between atoms (Castillero Mimenza, 2017). This forms a chemical bond in which one atom loses electrons, and another gains them, reaching a final state in which the two atoms together have a stable electrical charge (Castillero Mimenza, 2017). In this way, in line with Zita Fernandes (2020), all the compounds that exist in nature originate.
Characteristics
Polarity
Polarity is a property of molecules formed by covalent bonds, depending on the nature of the atoms involved (Ondarse Álvarez, 2021). When two atoms of the same element or elements with a very small electronegativity difference (less than 0.4) bond, the pair of electrons they share is equally attracted by both atoms, resulting in a uniform distribution of electric charges in the molecule. These molecules are called "nonpolar" or "apolares," and this type of bond is termed "nonpolar covalent bond”. Conversely, when two atoms of elements with an electronegativity difference greater than 0.4 bond, the more electronegative atom attracts the electrons of the bond more strongly, leading to a non-uniform distribution of charge in the molecule. According to Ondarse Álvarez (2021), these molecules are called “polar", and this type of bond is termed "polar covalent bond”.
Electrovalency
The main characteristic of ionic bonds is that the atoms forming them have a significant difference in electronegativity (greater than 2), causing one to donate and the other to receive electrons upon bonding (Ondarse Álvarez, 2021). According to Ondarse Álvarez (2021), this electrical capacity of atoms is referred to as electrovalency, and it depends on certain elements being naturally more inclined to be electron "donors" (Groups IA, IIA, IIIA of the periodic table), while others are, on the contrary, electron "acceptors" (Groups VA, VIA, and VIIA).
Sea of Electrons
A phenomenon that occurs among metallic atoms of the same type, bonding through metallic bonds, is known as the "sea of electrons" (Ondarse Álvarez, 2021). This means that atomic nuclei are surrounded by a sea of their electrons. According to Ondarse Álvarez (2021), metals exhibit low electronegativity, brightness, and malleability, properties attributed to the nature of the bond that joins them in their solid state: a rigid order that mobilizes valence electrons from their orbitals, allowing for excellent conductivity of electricity and heat, and the ability to reflect almost all the light that illuminates them.
Chemical Bond Breakage
Under certain conditions, the breaking of chemical bonds that hold the atoms of a substance together can occur (Ondarse Álvarez, 2021). This happens when heat, electricity, or other substances that interfere with the bonding are applied, causing the release of the atoms. An example of this is "electrolysis," a process in which hydrogen and oxygen, which form water, are separated by subjecting it to an electric current. According to Ondarse Álvarez (2021), another example is the denaturation of a protein, which involves the destruction of its chemical bonds by exposing it to very high temperatures.
Main Types of Chemical Bonds
Different atoms come together to form various molecules through three main types of chemical bonds (Castillero Mimenza, 2017). The nature of these bonds depends on the types of atoms involved, which can be metallic or non-metallic. According to Castillero Mimenza (2017), metallic atoms have a limited tendency to attract electrons, while non-metallic atoms have a higher tendency.
Ionic Bond
Ionic bond refers to the force that holds together a metallic element, such as sodium or magnesium, and a non-metallic element, for example, chlorine or sulfur (Zita Fernandes, 2020). When the nucleus of the non-metallic element attracts the outermost electron of the metallic element, the latter transfers it to the former (Castillero Mimenza, 2017). Thus, stable electrochemical compounds are formed (Castillero Mimenza, 2017). By losing electrons, the metal becomes a positive ion called a cation (Zita Fernandes, 2020). On the contrary, by gaining electrons, the non-metal transforms into a negative ion called an anion. These ions aggregate in a three-dimensional lattice held together by electrostatic forces between opposite charges. According to Zita Fernandes (2020), these forces are known as ionic compounds.
The Earth's crust is mainly composed of this type of compounds. Many rocks, minerals, and gemstones are examples of ionic compounds (Zita Fernandes, 2020). For example, sodium chloride NaCl, where sodium, the metal, donates an electron to chlorine, the non-metal. Or magnesium chloride MgCl2, where magnesium Mg donates two electrons to two chlorine atoms (Zita Fernandes, 2020). Materials resulting from this type of bonding are usually hard and require a lot of energy to melt, but they can also be easily compressed and broken (Castillero Mimenza, 2017). Additionally, as mentioned by Castillero Mimenza (2017), they often dissolve easily.
Covalent Bond
Atoms with similar or identical electronegative properties can form a type of bond called covalent, where electrons are shared between them without altering their quantity (Castillero Mimenza, 2017). This bond is common in organic matter, such as that constituting the human body, and is more stable than ionic bonding. Furthermore, compounds with this bond usually have a low melting point and generally do not conduct electricity (Castillero Mimenza, 2017). Zita Fernandes (2020) mentions that, depending on the electron affinity of the atoms and the number of shared electrons, the covalent bond can be classified into various types.
One of these types is the non-polar or pure covalent bond, which occurs when electrons are evenly distributed between the two atoms constituting the molecule (Zita Fernandes, 2020). This happens when the molecule is symmetrical, composed of two atoms of the same element, such as hydrogen, oxygen, or carbon (Castillero Mimenza, 2017; Zita Fernandes, 2020). According to Castillero Mimenza (2017), these molecules do not dissolve in water under any circumstances.
Another type is the polar covalent bond, which occurs when electrons are more concentrated in one atom than in the other, due to its greater attraction to them (Zita Fernandes, 2020). This creates a charge difference between the ends of the molecule, which are called poles (Zita Fernandes, 2020). Electrons are not lost or gained in this type of bond but are shared unevenly (Castillero Mimenza, 2017). According to Zita Fernandes (2020), an example of this type of bond is hydrogen fluoride H-F, where fluorine has a higher electronegativity than hydrogen and therefore attracts the shared electrons more.
Atoms forming covalent bonds share a certain number of electron pairs between them, which can be one, two, or three (Ondarse Álvarez, 2021). This determines whether the bond is single, double, or triple, respectively. The simplest type of covalent bond is the single bond, where two atoms contribute one electron each to form a shared electron pair (Zita Fernandes, 2020). Single covalent bonds are symbolized with a single line A - A (Ondarse Álvarez, 2021). An example of this bond is between two chlorine atoms, each having seven valence electrons and needing one more to complete their outer shell. When they join, according to Zita Fernandes (2020), they form the chlorine molecule Cl2, which is more stable than isolated atoms.
Another type of covalent bond is the double bond, which occurs when two atoms share two pairs of electrons, totaling four electrons (Zita Fernandes, 2020). Double covalent bonds are symbolized with two parallel lines A = A (Ondarse Álvarez, 2021). An example is the bond between two oxygen atoms, each having six electrons in their outer shell and needing two more for stability. According to Zita Fernandes (2020), by sharing four electrons in total, both atoms end up with eight electrons in their valence shell.
Yet another type is the triple covalent bond, involving the sharing of three pairs of electrons, totaling six electrons between two atoms (Zita Fernandes, 2020). Triple covalent bonds are represented by three parallel lines A ≡ A (Ondarse Álvarez, 2021). An example of this bond is between carbon and nitrogen in the hydrogen cyanide molecule H - C - N, where carbon contributes four electrons, and nitrogen contributes two, as mentioned by Zita Fernandes (2020).
Finally, the coordinated or dative covalent bond is formed when only one of the atoms in the bond provides the two electrons that are shared (Zita Fernandes, 2020). An example is the bond between the nitrogen of ammonia NH3 and the boron of boron trifluoride BF3, where nitrogen donates a pair of electrons to boron, which has none to share. Thus, according to Zita Fernandes (2020), both atoms achieve having eight electrons in their valence shell.
Metallic Bond
Metallic bond is the attractive force that binds metallic elements such as sodium (Na), barium (Ba), calcium (Ca), magnesium (Mg), gold (Au), silver (Ag), and aluminum (Al) (Zita Fernandes, 2020). This force originates from the interaction between positive ions and negatively charged electrons that flow freely between them (Zita Fernandes, 2020). Positive ions form a crystalline network where negative electrons move easily, following regular patterns (Castillero Mimenza, 2017). This structure determines the properties of metals, which are usually solid, strong, and malleable, meaning they can be molded without breaking (Castillero Mimenza, 2017). Additionally, metal atoms are closely packed, facilitating the movement of electrons within the atom network (Zita Fernandes, 2020). According to Castillero Mimenza (2017), this also explains the electrical conductivity of metals because their electrons are free.
Chemical Bonds Between Molecules
The main chemical bonds are not the only ones that can form between molecules; there are also other molecular-level modalities (Castillero Mimenza, 2017). One of them is the Van der Waals or Dipole-Dipole forces, which occur between symmetrical molecules that attract or repel each other based on the interaction of their molecules or ions (Castillero Mimenza, 2017). This type of bond can be of three classes: between two permanent dipoles, between two induced dipoles, or between a permanent dipole and an induced one (Castillero Mimenza, 2017). An example of this bond is found in formaldehyde molecules H2C = O, which are polar and have a partial negative charge on oxygen and a partial positive charge on hydrogen (Zita Fernandes, 2020). Thus, according to Zita Fernandes (2020), the positive side of one formaldehyde molecule is attracted to the negative side of another formaldehyde molecule.
Another modality of chemical bond between molecules is the hydrogen bond or hydrogen bridge, which forms between hydrogen and another element with high polarity (Castillero Mimenza, 2017). In this bond, hydrogen has a positive charge and binds to polar atoms with high electronegativity, creating an interaction or bridge between them (Castillero Mimenza, 2017). Although this bond is weak, it allows two atoms that normally would not come together to connect, creating complex and stable molecules, both organic and inorganic (Castillero Mimenza, 2017; Ondarse Álvarez, 2021). Zita Fernandes (2020) mentions an example of this bond between water H2O and ammonia NH3.
References
Castillero Mimenza, O. (2017, septiembre 7). Los 5 Tipos de Enlaces Químicos: Así se Compone la Materia. Psicología y Mente. https://psicologiaymente.com/miscelanea/tipos-enlaces-quimicosOndarse Álvarez, D. (2021). Enlace Químico: Tipos, Ejemplos y Características. Enciclopedia Humanidades. https://humanidades.com/enlace-quimico/
Zita Fernandes, A. (2020, noviembre 3). Los 10 Tipos de Enlaces Químicos. Diferenciador. https://www.diferenciador.com/tipos-de-enlaces-quimicos/
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