What are Covalent bonds and its types? CBSE Class 11 Chemistry Notes
A covalent bond is a link between two atoms or two ions in which the electron pairs are shared between them. A covalent bond is also known as a molecular bond. Covalent bonds are formed between two non-metal atoms with identical or relatively close electronegativity values. The term “covalent bond” was introduced in 1939. Gilbert N. Lewis, in 1916 described the sharing of electron pairs between atoms. However, Irving Langmuir introduced the term “covalence” in 1919, it means the number of electron pairs shared by neighbouring atoms. The electron pairs that form a covalent bond are called bonding pairs or shared pairs. A covalent bond is made when two or more non-metal atoms bond by sharing valence electrons.
The octet rule is a chemical rule of thumb that suggests that atoms of main-group elements tend to bond in such a way that each atom has eight electrons in its valence shell, and attains the same electronic configuration as a noble gas. The rule is particularly applicable to carbon, nitrogen, oxygen, and the halogens, but also to metals such as sodium or magnesium.
The Octet Rule requires all atoms in a molecule to have eight valence electrons–either by sharing, losing or gaining electrons–to become stable. Valence electrons are the electrons present in the outermost shell of an atom. These electrons are involved in bonding. Valence electrons play a crucial role in the formation of covalent compounds. In the term covalent, “co” means share and ‘valent’ refers to valence electrons. For Covalent bonds, the atoms tend to share their electrons with each other thus the atoms involved in covalent bonding obeys the Octet Rule, and attains the noble gas configuration because each atom wants to become as stable as the noble gases have their outer valence shell filled thus have no charge.
Formation of Covalent Compounds
An atom is made of a positively charged nucleus and the negatively charged electrons that revolve around the nucleus. Suppose, there are two chlorine atoms, each chlorine atom has seven valence electrons. Thus, each of these chlorine atoms needs only one more electron to complete its outer most shell. These atoms bond by sharing two electrons, forming a single bond. These electrons are concentrated in between the two chlorine atoms, thus locked into position by magnetic attraction. The negative electrons are attracted to the positively charged nucleus of each atom, thereby keeping the atoms together.
Some atoms are more electronegative than others. In such a case, the sharing of electrons is not equal, as electronegativity is the ability of an atom to draw electrons to itself. If a covalent bond is made between one atom that is more electronegative than the other, the electrons will not be shared evenly in the bond. Uneven sharing results in the formation of a dipole, which means the separation of charges between two covalently bonded atoms.
For example, Hydrogen fluoride is a simple covalent compound that has a dipole. The fluorine atom is highly electronegative compared to the hydrogen. The shared electrons are, more attracted to the fluorine atom, resulting in an uneven distribution of charge. Thus, fluorine has a partial negative charge and hydrogen has a partial positive charge.
Types of Covalent Bonds
There are two important types of covalent bonds:
- Nonpolar or pure covalent bonds and
- Polar covalent bonds
Nonpolar covalent bonds occur when atoms equally share electron pairs. Only identical atoms (having the same electronegativity) are involved in equal sharing. This mostly occurs in gas molecules, also known as diatomic elements. Some examples of molecules with nonpolar bonds are H2, N2, and CH4.
As there is an increase in electronegativity difference, the electron pair in a bond is more closely associated with one nucleus than the other. If the electronegativity difference is between 0.4 and 1.7, then the bond is polar. If the electronegativity difference is more than 1.7, the bond is ionic.
Polar Covalent Bond is formed when the shared electrons between the bonding atoms are not shared equally. This occurs when one atom has a higher electronegativity than the other atom. The atom with the higher electronegativity will have a stronger pull for electrons (like a Tug-Of-War game, the stronger one usually wins). As a result, the shared electrons will be closer to the atom with the higher electronegativity, thus it is unequally shared. A polar covalent bond will render the molecule having a slightly positive side (the side containing the atom with a lower electronegativity) and a slightly negative side (containing the atom with the higher electronegativity). As a result of polar covalent bonds, the covalent compound will have an electrostatic potential, which makes the resulting molecule slightly polar, allowing it to form weak bonds with other polar molecules. Molecules forming weak bonds with each other as a result of an unbalanced electrostatic potential can be seen in hydrogen bonding, where a hydrogen atom will interact with electronegative hydrogen, fluorine, or oxygen atom from another molecule or chemical group.
A single bond is formed when two electrons–one pair of electrons–are shared between two atoms. It is shown by a single line between the two atoms. This form of bond is weaker and has a smaller density of electrons than a double bond and a triple bond, but it is the most stable because it has a lower level of reactivity. It means that there is less susceptibility of losing electrons to atoms.
A Double bond is formed by sharing of two pairs of electrons between the two atoms. It is presented by two horizontal lines between two atoms in a molecule. This type of bond is stronger than a single bond, but it is less stable, this is due to its greater amount of reactivity compared to a single bond.
A Triple bond is formed when three pairs of electrons are shared between two atoms in a molecule. It is the least stable out of the three general types of covalent bonds as it is very vulnerable to lose an electron.
Covalent bonds between atoms are quite strong, but attractions between molecules/compounds, or intermolecular forces, can be relatively weak. Covalent compounds generally have low boiling and melting points and are found in all three physical states (solids, liquids, and gas) at room temperature.
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