Types of chemical bond

Types of chemical bond

All the elements differ with each other in their valence shell electronic configuration. So the way in which they combine to form compounds also differs. Hence, there are different types of chemical bonding possible between atoms which make the molecules. Depending on the type of bond they show different characteristics or properties. Such types of bonding that are considered to exist in molecules are categorized as shown below. Among these, let us learn about the Ionic bond, Covalent bond and Coordinate bond in this chapter and other types of bond in the higher classes.

1. Ionic (or) Electrovalent bond

An ionic bond is a chemical bond formed by the electrostatic attraction between positive and negative ions. The bond is formed between two atoms when one or more electrons are transferred from the valence shell of one atom to the valence shell of the other atom. The atom that loses electrons will form a cation (positive ion) and the atom that gains electrons will form an anion (negative ion). These oppositely charged ions come closer to each other due to electrostatic force of attraction and thus form an ionic bond. As the bond is between the ions, it is called Ionic bond and the attractive forces being electrostatic, the bond is also called Electrostatic bond. Since the valence concept has been explained in terms of electrons, it is also called as Electrovalent bond.

Formation of ionic bond

Let us consider two atoms A and B. Let atom A has one electron in excess and atom B has one electron lesser than the stable octet electronic configuration. If atom A transfer one electron to atom B, then both the atoms will acquire stable octet electronic configuration. As the result of this electron transfer, atom A will become positive ion (cation) and atom B will become negative ion (anion). ese oppositely charged ions are held together by electrostatic force of attraction which is called Ionic bond or Electrovalent bond.

In general, ionic bond is formed between a metal and non-metal. The compounds containing ionic bonds are called ionic compounds. Elements of Group 1 and 2 in periodic table, i.e. alkali and alkaline earth metals form ionic compounds when they react with non-metals.

Illustration 1 – Formation of Sodium Chloride (NaCl)

The atomic number of Sodium is 11 and its electronic configuration is 2, 8, 1. It has one electron excess to the nearest stable electronic configuration of a noble gas - Neon. So sodium has a tendency to lose one electron from its outermost shell and acquire a stable electronic configuration forming sodium cation (Na⁺).

e atomic number of chlorine is 17 and its electronic configuration is 2, 8, 7. It has one electron less to the nearest stable electronic configuration of a noble gas - Argon. So chlorine has a tendency to gain one electron to acquire a stable electronic configuration forming chloride anion (Cl⁻).

When an atom of sodium combines with an atom of chlorine, an electron is transferred from sodium atom to chlorine atom forming sodium chloride molecule thus both the atoms achieve stable octet electronic configuration.

Illustration 2 – Formation of Magnesium Chloride (MgCl₂)

The atomic number of Magnesium is 12 and the electronic configuration is 2, 8, 2. It has two electron excess to the nearest stable electronic configuration of a noble gas - Neon. So magnesium has a tendency to lose two electrons from its outermost shell and acquire a stable electronic configuration forming magnesium cation (Mg²⁺).

As explained earlier two chlorine atoms will gain two electrons lost by the magnesium atom forming magnesium chloride molecule (MgCl₂)

Characteristics of Ionic compounds

The nature of bonding between the atoms of a molecule is the primary factor that determine the properties of compounds. By this way, in ionic compounds the atoms are held together by a strong electrostatic force that makes the compounds to have its characteristic features as follows:

a. Physical state – ese compounds are formed because of the strong electrostatic force between cations and anions which are arranged in a well-de ned geometrical pattern. us Ionic compounds are crystalline solids at room temperature.

b. Electrical conductivity – Ionic compounds are crystalline solids and so their ions are tightly held together. The ions, therefore, cannot move freely, so they do not conduct electricity in solid state. However, in molten state and their aqueous solutions conduct electricity.

c. Melting point–the strong electrostatic force between the cations and anions hold the ions tightly together, so very high energy is required to separate them. Hence ionic compounds have high melting and boiling points.

d. Solubility – Ionic compounds are soluble in polar solvents like water. ey are insoluble in non-polar solvents like benzene (C₆H₆), carbon tetra chloride (CCl₄).

e. Density, hardness and brittleness–Ionic compounds have high density and they are quite hard because of the strong electrostatic force between the ions. But they are highly brittle.

f. Reactions – Ionic compounds undergo ionic reactions which are practically rapid and instantaneous.

2. Covalent bond

Atoms can combine with each other by sharing the unpaired electrons in their outermost shell. Each of the two combining atoms contributes one electron to the electron pair which is needed for the bond formation and has equal claim on the shared electron pair. According to Lewis concept when two atoms form a covalent bond between them, each of the atoms attains the stable electronic configuration of the nearest noble gas. Since the covalent bond is formed because of the sharing of electrons which become common to both the atoms, it is also called as Atomic bond.

Formation of Covalent bond

Let us consider two atoms A and B. Let atom A has one valence electron and atom B has seven valence electrons. As these atoms approach nearer to each other, each atom contributes one electron and the resulting electron pair fills the outer shell of both the atoms. Thus both the atoms acquire a completely filled valence shell electronic configuration which leads to stability.

Illustration 1 – Formation of hydrogen molecule (H₂)

Hydrogen molecule is formed by two hydrogen atoms. Each having one valence electron (1s¹), it is contributed to the shared pair and both atoms acquire stable completely filled electronic configuration.

Illustration 2 – Formation of chlorine molecule (Cl₂)

Chlorine molecule is formed by two chlorine atoms. Each chlorine atom has seven valence electrons (2,8,7). ese two atoms achieve a stable completely lled electronic configuration (octet) by sharing a pair of electrons.

Illustration 3 – Formation of methane molecule (CH₄)

Methane molecule is formed by the combination of one carbon and four hydrogen atoms. The carbon atom has four valence electrons (2, 4). ese four electrons are shared with four atoms of hydrogen to achieve a stable electronic configuration (octet) by sharing a pair of electrons.

Illustration 4 – Formation of oxygen molecule (O₂)

Oxygen molecule is formed by two oxygen atoms. Each oxygen atom has six valence electrons (2, 6). These two atoms achieve a stable electronic configuration (octet) by sharing two pair of electrons. Hence a double bond is formed in between the two atoms.

Illustration 5 – Formation of nitrogen molecule (N₂)

Nitrogen molecule is formed by two nitrogen atoms. Each nitrogen atom has five valence electrons (2, 5). These two atoms achieve a stable completely filled electronic configuration (octet) by sharing three pair of electrons. Hence a triple bond is formed in between the two atoms.

Characteristics of Covalent compounds

As said earlier, the properties of compounds depend on the nature of bonding between their constituent atoms. So the compounds containing covalent bonds possess different characteristics when compared to ionic compounds.

a. Physical state – Depending on force of attraction between covalent molecule the bond may be weaker or stronger. Thus covalent compounds exists in gaseous, liquid and solid form. Eg. Oxygen-gas; Water-liquid: Diamond-solid.

b. Electrical conductivity – Covalent compounds do not contain charged particles (ions), so they are bad conductors of electricity.

c. Melting point – Except few covalent compounds (Diamond, Silicon carbide), they have relatively low melting points compared to Ionic compounds.

d. Solubility – Covalent compounds are readily soluble in non-polar solvents like benzene (C₆H₆), carbon tetra chloride (CCl₄). ey are insoluble in polar solvents like water.

e. Hardness and brittleness – Covalent compounds are neither hard nor brittle. But they are soft and waxy.

f. Reactions – Covalent compounds undergo molecular reactions in solutions and these reactions are slow.

Fajan’s Rule:

As we know, a metal combine with a non-metal through ionic bond. The compounds so formed are called ionic compounds. A compound is said to be ionic when the charge of the cation and anion are completely separated. But in 1923, Kazimierz Fajans found, through his X-Ray Crystallographic studies, that some of the ionic compounds show covalent character. Based on this, he formulated a set rules to predict whether a chemical bond is ionic or covalent. Fajan’s rules are formulated by considering the charge of the cation and the relative size of the cation and anion.

·           When the size of the cation is small and that of anion is large, the bond is of more covalent character

·           Greater the charge of the cation, greater will be the covalent character

This can be summarized as follows:

For example, in sodium chloride, low positive charge (+1), a fairly large cation and relatively small anion make the charges to separate completely. So it is ionic. In aluminium triiodide, higher is the positive charge (+3), larger is the anion and thus no complete charge separation. So is covalent. The following picture depicts the relative charge separation of ionic compounds:

3. Coordinate covalent bond

In the formation of normal covalent bond each of the two bonded atoms contribute one electron to form the bond. However, in some compounds the formation of a covalent bond between two atoms takes place by the sharing of two electrons, both of which comes from only one of the combining atoms. is bond is called Coordinate covalent bond or Dative bond.

Mostly the lone pair of electrons from an atom in a molecule may be involved in the dative bonding. The atom which provides the electron pair is called donor atom while the other atom which accepts the electron pair is called acceptor atom. The Coordinate covalent bond is represented by an arrow (→ ) which points from the donor to the acceptor atom.

Formation of Coordinate covalent bond

Let us consider two atoms A and B. Let atom A has an unshared lone pair of electrons and atom B is in short of two electrons than the octet in its valence shell. Now atom A donates its lone pair while atom B accepts it. Thus the lone pair of electrons originally belonged to atom A are now shared by both the atoms and the bond formed by this mutual sharing is called Coordinate covalent bond. (A→B)

Illustration 1 – Formation of coordinate covalent bond in ammonium ion (NH4⁺)

Ammonium ion is formed by one ammonia (NH₃) molecule and one hydrogen (H⁺) ion. In ammonia molecule the central nitrogen atom has five valence electrons (2,5) among which three electrons are shared with three hydrogen atoms and still it has an unshared lone pair of electrons. This lone pair electrons are donated to a Hydrogen ion and thus a N→H coordinate covalent bond is formed in ammonium ion molecule (NH₄⁺)

Illustration 2 – Formation of coordinate covalent bond between NH₃ → BF₃ molecules

In some cases, the donated pair of electrons comes from a molecule as a whole which is already formed to another acceptor molecule. Here the molecule ammonia (NH₃) gives a lone pair of electrons to Boron tri fluoride (BF₃) molecule which is electron deficient. Thus a Coordinate covalent bond is formed between NH₃ (donor molecule) and BF3 (acceptor molecule) and is represented by NH₃ → BF₃.

Characteristics of coordinate covalent compounds

The compounds containing coordinate covalent bonds are called coordinate compounds.

a. Physical state – These compounds exist as gases, liquids or solids.

b. Electrical conductivity – Like covalent compounds, coordinate compounds also do not contain charged particles (ions), so they are bad conductors of electricity.

c. Melting point – These compounds have melting and boiling points higher than those of purely covalent compounds but lower than those of purely Ionic compounds.

d. Solubility – Insoluble in polar solvents like water but are soluble in non-polar solvents like benzene, CCl₄, and toluene.

e. Reactions – Coordinate covalent compounds undergo molecular reactions which are slow.