Covalent Bond: Definition, Examples, Types, Properties

– In this subject, we will discuss the Covalent Bond (Definition, Examples, Types, Properties)

Covalent Bond: Definition, Examples, Types, Properties

Covalent Bond 

– The electron transfer theory could not explain the bonding in molecules such as H2, O2, Cl2, etc., and in organic molecules, that had no ions.

– It was G.N. Lewis who suggested that two atoms could achieve stable 2 or 8 electrons in the outer shell by sharing electrons between them.

– Let us consider a general case where atom A has one valence electron and another atom B has seven valence electrons.

– As they approach each other, each atom contributes one electron and the resulting electron pair fills the outer shell of both the atoms.

– Thus A acquires stable 2 electrons and B, 8 electrons in the outer shell.

Covalent Bond: Definition, Examples, Types, Properties

– The shared pair is indicated by a dash (–) between the two bonded atoms.

– A shared pair of electrons constitutes a Covalent bond or Electron-pair bond.

– In fact, the positive nuclei of atoms A and B are pulled towards each other by the attraction of the shared electron pair.

– At the same time, the nuclei of two atoms also repel each other as do the two electrons.

– It is the net attractive force between the shared electrons and the nuclei that holds the atoms together.

– Thus an alternative definition of a covalent bond would be: The attractive force between atoms is created by the sharing of an electron pair.

– The compounds containing a covalent bond are called covalent compounds.

Conditions for the formation of Covalent Bond

– The conditions favorable for the formation of an ionic bond are:

(1) Number of valence electrons

Each of the atoms A and B should have 5, 6, or 7 valence electrons so that both achieve the stable octet by sharing a 3, 2, or 1 electron pair.

– H has one electron in the valence shell and attains a duplet.

– The non-metals of groups VA, VIA, and VIIA respectively satisfy this condition.

(2) Equal electronegativity

– The atom A will not transfer electrons to B if both have equal electronegativity and hence electron sharing will take place.

– This can be strictly possible only if both atoms are of the same element.

(3) Equal sharing of electrons

– The atoms A and B should have equal (or nearly equal) electron affinity so that they attract the bonding electron pair equally.

– Thus equal sharing of electrons will form a nonpolar covalent bond.

– Of course, precisely equal sharing of electrons will not ordinarily occur except when atoms A and B are atoms of the same element, for no two elements have exactly the same electron affinity.

Some examples of covalent compounds

– The construction of Lewis structures of simple covalent compounds will be discussed. 

Covalent bond in Hydrogen, H2

– A hydrogen molecule is made of two H atoms, each having one valence electron.

– Each contributes an electron to the shared pair and both atoms acquire stable helium configuration.

– Thus stable H2 molecule results.

Covalent Bond: Definition, Examples, Types, Properties

Covalent bond in Chlorine, Cl2

– Each Cl atom (2, 8, 7) has seven valence electrons.

– The two Cl atoms achieve a stable electron octet by sharing a pair of electrons.

Covalent bond in Water, H2O

– The oxygen atom (2, 6) has six valence electrons and can achieve a stable octet by sharing two electrons, one with each H atom.

– Thus Lewis’s structure of water can be written as:

Covalent bond in Ammonia, NH3

– Nitrogen atom (2, 5) has five valence electrons and can achieve the octet by sharing three electrons, one each with three H atoms.

– This gives the following Lewis structure for ammonia:

Covalent bond in Methane, CH4 

– Carbon atoms (2, 4) have four electrons in the valence shell.

– It can achieve the stable octet by sharing these electrons with four H atoms, one with each H atom.

– Thus the Lewis structure of methane can be written as:

Covalent Bond: Definition, Examples, Types, Properties

Examples of multiple covalent compounds

– In many molecules, we find that to satisfy the octet, it becomes necessary for two atoms to share two or three pairs of electrons between the same two atoms.

– The sharing of two pairs of electrons is known as a Double bond and the sharing of three pairs of electrons is a Triple bond.

– Let us consider some examples of compounds containing these multiple covalent bonds in their molecules.

Covalent bond in Oxygen, O2

– The conventional Lewis structure of oxygen is written by the sharing of two pairs of electrons between two O atoms (2, 6).

– In this way, both the O atoms achieve the octet

Covalent Bond: Definition, Examples, Types, Properties

– The above structure of oxygen implies that all the electrons in oxygen, O2, are paired whereby the molecule should be diamagnetic.

– However, the experiment shows that O2 is paramagnetic with two unpaired electrons.

– This could be explained by the structure.

– Although writing Lewis structures work very well in explaining the bonding in most simple molecules, it should be kept in mind that it is simply the representation of a theory.

– In this case, the theory just doesn’t work. 

Covalent bond in Nitrogen, N2

– The two atoms of nitrogen (2, 5), each having five electrons in the valence shell, achieve the octet by sharing three electron pairs between them.

Covalent Bond: Definition, Examples, Types, Properties

Covalent bond in Carbon Dioxide, CO2

– Carbon (2, 4) has four valence electrons. It shares two electrons with each O atom (having six valence electrons).

– Thus the C atom and both the O atoms achieve their octet.

Properties of covalent compounds

– While the atoms in a covalent molecule are firmly held by the shared electron pair, the individual molecules are attracted to each other by weak van der Waals forces.

– Thus the molecules can be separated easily as not much energy is required to overcome the intermolecular attractions.

– This explains the general properties of covalent compounds.

(1) Gases, liquids, or solids at room temperature

– The covalent compounds are often gases, liquids, or relatively soft solids under ordinary conditions.

– This is so because of the weak intermolecular forces between the molecules.

(2) Low melting points and boiling points

– Covalent compounds have generally low melting points (or boiling points).

– The molecules are held together in the solid crystal lattice by weak forces.

– On application of heat, the molecules are readily pulled out and these then acquire kinetic energy for free movement as in a liquid.

– For the same reason, the liquid molecules are easily obtained in the gaseous form which explains the low boiling points of covalent liquids.

(3) Neither hard nor brittle

– While the ionic compounds are hard and brittle, covalent compounds are neither hard nor brittle.

– There are weak forces holding the molecules in the solid crystal lattice.

– A molecular layer in the crystal easily slips relative to other adjacent layers and there are no ‘forces of repulsion’ like those in ionic compounds.

– Thus the crystals are easily broken and there is no sharp cleavage between the layers on application of external force.

(4) Soluble in organic solvents

– In general, covalent compounds dissolve readily in nonpolar organic solvents (benzene, ether).

– The kinetic energy of the solvent molecules easily overcomes the weak intermolecular forces.

– Covalent compounds are insoluble in water. Some of them (alcohols, and amines) dissolve in water due to hydrogen bonding.

(5) Non-conductors of electricity

– Since there are no (+) or (–) ions in covalent molecules, the covalent compounds in the molten or solution form are incapable of conducting electricity.

(6) Exhibit Isomerism

– Covalent bonds are rigid and directional, the atoms being held together by a shared electron pair and not by electrical lines of force.

– This affords the opportunity for various spatial arrangements and covalent compounds to exhibit stereoisomerism.

(7) Molecular reactions

– The covalent compounds give reactions where the molecule as a whole undergoes a change.

– Since there are no strong electrical forces to speed up the reaction between molecules, these reactions are slow.

Difference between a covalent and ionic bond

– The following table shows the Comparison of Ionic bond and covalent bond  

Covalent Bond: Definition, Examples, Types, Properties

Reference: Essentials of Physical Chemistry /Arun Bahl, B.S Bahl and G.D. Tuli / multicolor edition. 

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