Covalent Bond Definition

A covalent bond is a type of chemical bond in which two atoms of the same or different elements share one or more pairs of valence electrons in order to complete their respective octets. This type of bond is the one that occurs most frequently among nonmetallic elements, but in some cases it also involves some transition metals and metalloids.

Covalent bonds are the type of bond or chemical bond that holds together all the atoms that make up molecules such as water, carbon dioxide, and glucose, or molecular solids such as graphite and diamond, to name a few. On the other hand, covalent bonds are the type of bond par excellence present in organic compounds that make life possible, particularly in proteins, amino acids, fats and triglycerides, carbohydrates, etc.

The concept of covalent bonding is easy to remember if we consider the word covalent as formed by the words “share” and “valence,” indicating that this type of bond involves almost exclusively the electrons located in the valence shell orbitals of the elements . linked.

The covalent bond is the opposite type of bond to the ionic bond, in which instead of sharing electrons, one of the atoms removes the electrons from the other, the first acquiring a negative electrical charge while the second remains positively charged. These species are called ions (anions the former and cations the latter) and are held together by the electrostatic attraction that occurs between ions of opposite charges.

Characteristics of covalent bonds

Covalent bonds have several characteristics that clearly distinguish them from ionic and metallic bonds. Some of these are:

• They form mainly between non-metallic elements or between elements that have relatively similar electronegativities. An electronegativity difference equal to or less than 1.7 has been arbitrarily chosen to define a bond as covalent.
• Covalent bonds are, on average, weaker than ionic bonds . The energy required to break one mole of a typical covalent bond is usually in the range of 150 to 400 kJ/mol, whereas for ionic bonding, it typically takes between 600 and 4,000 kJ/mol. even more.
• They give rise to molecular compounds , which generally have much lower melting and boiling points than ionic compounds (with the exception of molecular solids such as graphite and diamond whose melting points are very high).
• They are directional , which means that, in the atoms that form several covalent bonds, these are oriented preferentially in certain directions, giving rise to characteristic molecular geometries for each molecular substance. For example, in the case of ammonia (NH ₃ ), the three covalent bonds with hydrogen are oriented along the edges of a trigonal-based pyramid, while in borane (BH 3 ₎ , the three bonds form a equilateral triangle, giving rise to a trigonal plane geometry.
• Covalent bonds are shorter than ionic bonds . Whereas in most ionic compounds the nuclei are between 160 and 370 pm apart, in the case of covalent compounds this distance is between about 80 and 200 pm for the vast majority of single covalent bonds, with only a few exceptions that come close to 260 pm.
• Bond length decreases with bond order , which means that, for the same pair of atoms, the bond gets shorter as more electrons are shared.

Types of covalent bonds

Covalent bonds are very common and they are also very varied, being able to be classified according to different criteria. The most important criteria for the classification of covalent bonds and the types of bonds included in each of them are presented below.

Types of covalent bonds according to the difference in electronegativity

The electronegativity difference determines how equally electrons are shared when a covalent bond is formed. Based on this criterion, we can distinguish two types of covalent bonds:

polar covalent bonds

They are formed when two elements whose electronegativities difference is between 0.4 and 1.7 are joined (these ranges are somewhat arbitrary). In this type of bonds, the electrons are not shared equally since the more electronegative atom retains the electron cloud longer around it than the less electronegative one, the former acquiring a partial negative charge while the less electronegative one acquires a positive partial charge. .

This separation of charges is called an electric dipole and is the reason why this type of bond is called a polar bond. The charge separation is measured through the dipole moment of the bond. Compounds that possess polar bonds may or may not be polar molecules, depending on whether the vector summation of all dipole moments gives a resulting net dipole moment.

nonpolar covalent bonds

They are the covalent bonds that are formed between atoms that have a difference in electronegativities less than 0.4. In this type of bond it is assumed that a dipole is not formed, so the bond is said to be non-polar.

Some people recognize a subclass of nonpolar covalent bonding that they call a pure covalent bond, which occurs when two exactly the same atoms of the same element are covalently bonded (in addition to being the same element, both atoms must also possess the same hybridization). This is the perfect covalent bond in which the electrons are completely equally shared and we can say with certainty that the dipole moment is zero.

Types of covalent bonds according to the overlapping of atomic orbitals (Valence Bond Theory)

The Valence Bond Theory establishes that, for the covalent bond to form, the valence atomic orbitals of the two bonded atoms must overlap, otherwise they will not be able to share electrons. According to this theory, there are two ways in which these orbitals can overlap, giving rise to two types of covalent bonds:

σ (sigma) bonds

The sigma bond is formed by the frontal overlap of the atomic orbital lobes, which is why this bond is formed along the line joining the two nuclei. Two bonded atoms can only form a σ bond between them due to restrictions related to the orientations of the atomic orbitals, since if one orbital points in one direction, the other orbitals in the valence shell must necessarily point in a different direction.

π (pi) bonds

They are those formed by lateral overlapping of atomic orbitals, generally pure atomic orbitals of the po d type. These bonds are only formed when two atoms share more than one pair of electrons, being able to form more than one pi bond.

The electrons that are shared in the pi bonds are located above and below or to the sides of the line that joins the two nuclei, but they never pass through that line.

Types of covalent bonds according to bond order or number of shared electron pairs

As mentioned before, in a covalent bond, two atoms can share one or more pairs of electrons. This number of shared electron pairs is known as the bond order. Based on this bond order, covalent bonds can be classified as:

single covalent bond

It occurs when two atoms share only one pair of electrons. Single covalent bonds are always σ bonds.

double covalent bond

It is the covalent bond in which two pairs of electrons are shared. One of the pairs of electrons forms a σ bond between the two nuclei, while the second pair forms a π bond. It is important to understand that although it is called a double bond and is thought of as consisting of a σ and a π bond, the double bond is actually a single bond.

triple covalent bond

It forms when two atoms share three pairs of electrons. In this case, the bond is made up of one σ bond and two π bonds. However, these two π bonds form a hollow cylinder where the four π electrons meet while the two σ electrons meet in the middle.

Other special types of covalent bonds

Dative or coordinate covalent bonds

In most covalent bonds, both bonded atoms contribute one electron to form each bonding electron pair. However, there is a particular type of covalent bond that is quite common and is formed as a consequence of a Lewis acid-base reaction.

In these cases, only one of the two atoms contributes the pair of electrons to form the covalent bond. This special type of bond is called a dative bond (for obvious reasons, since only one of the atoms gives or contributes the necessary electrons for the bond) or coordinate. This is the type of covalent bond that characterizes coordination compounds.

Covalent bonds of three cores or three centers

In some special molecules, covalent bonds can be formed in which the same pair of electrons is shared between more than two atoms. Such is the case of allyl cations in which a double covalent bond is conjugated with a vicinal carbocation, forming a π bond that encompasses all three atoms allowing the two π electrons to move freely from one end of the bond to the other. This is called relocation.

Examples of Common Covalent Bonds

Some examples of covalent bonds are:

• C–H
• C–C
• C–N
• N–N
• N=N
• C=N
• C–O
• C=O
• OR = OR
• O–H
• Br–Br
• C–F
• C ≡ C
• N ≡ N
• C ≡ N

References

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