Chemical Bond

What is a chemical bond | chemical bonding

It is a type of force of attraction that holds together ions or atoms in a molecule of a compound.

Types of chemical bond

There are mainly two types of chemical bonds that are ionic bond and covalent bond, both are described below one by one.

1) Ionic Bond

The chemical bond which is formed by the transfer of electrons between atoms is known as an ionic bond. It is also called an electrovalent bond. When the transfer of electrons takes place, a force of attraction develops between ions (atoms become ions after losing or gaining electrons). So, it a type of force of attraction that keeps atoms held together in a molecule of an ionic compound. Ionic bonds are strongest as compared to all other types of chemical bonds.

An ionic bond generally formed between metal and non-metal atoms or elements. This is because metals tends to lose electrons to complete their octet or gain stability and non-metals tend to gain electrons to complete their octet or to become stable. The metal atom becomes a positively charged ion or cation after losing electrons, whereas, the non-metal atom becomes a negatively charged ion or anion after gaining an electron.

We can say that at least one electron donor and one electron acceptor is required to form an ionic bond. The attraction for electrons is called electronegativity. So, there should be a difference in electronegativity of atoms participating in ionic bond formation, atoms with same the electronegativity do not form an ionic bond.

For example,

• Sodium Chloride (NaCl) is an ionic compound in which Na and Cl atoms has different electronegativity. The electronegativity of sodium is less than Chlorine. So, in the case of NaCl, sodium loses one electron to form sodium ion (Na⁺) and chlorine gains one electron to form chlorine Cl⁺. Due to the opposite charge on these ions force of attraction develops that hold them together and thus a giant ionic lattice is formed.
• Magnesium Chloride is also an ionic compound where magnesium is less electronegative, so electrons are transferred from magnesium to oxygen to form magnesium ion (Mg²⁺) and Oxide ion (O^2-), which are attracted to each other electrostatically to form ionic bond and thus ionic compound.
• In ionic compound calcium chloride (CaCl₂), Ca loses two electrons to form Calcium ion (Ca²⁺). These two electrons are gained by two chlorine atoms to form two chloride ions (Cl^2-). These ions are electrostatically attracted towards each other.

Furthermore, there is no net charge on ionic compounds and the ions neutralize the charge of each other. For example, the positive charge on Na ion is neutralized by the negative charge on Chloride ion.

Furthermore, ionic bonds may not be considered real bonds as a complete transfer of electrons occurs and ions are formed in the ionic bond. It does not involve sharing of electrons and is the electrostatic force that holds the ions.

Properties of ionic bonds:

• They are formed when the complete transfer of electrons occurs between atoms.
• They are formed between ions (cations and anions).
• It is also called an electrovalent bond.
• It is a strong force of attraction that strongly held together ions in ionic compounds, so the ionic compounds generally have high boiling and melting points.
• It is the ionic bond that holds the huge ionic lattice structure of ionic compounds.
• It is formed between atoms with different electronegativity, one should have higher electronegativity and another one should have low.
• They are formed between metal and non-metal as they have different electronegativity.
• They are stronger than covalent bonds as ions strongly attract each other due to huge difference in their electronegativity.

As the name suggests, ionic bonds will form ionic compounds such as;

• Sodium Chloride (NaCl)
• Potassium Bromide (KBr)
• Potassium Chloride (KCl)
• Magnesium Oxide (MgO)
• Potassium iodide (KI)

In the name or chemical formula of ionic compounds, an atom which makes cation (positively charged ion) is written first and it is followed by the atom that makes anion (negatively charged ion).

2) Covalent Bond

The chemical bond in which sharing of electrons occurs between atoms or elements is called a covalent bond. It does not involve the transfer of electrons from one atom to another atom. Covalent bonds are also called molecular bonds. The shared electrons form pairs, if there are two electrons there will be one pair of electrons and there will be 2 pairs of electrons when four electrons are shared and so on. These pairs of electrons that form covalent bonds are called shared pairs or bonding pairs of electrons.

Types of covalent bonds

There are three types of covalent bonds depending on the number of shared pairs of electrons.

i) Single Covalent Bond:

As the name suggests, the covalent bond that has one shared pair of electron or a total of 2 electrons is called a single covalent bond. It is denoted by one dash (-). It is less strong than double and triple covalent bonds.

Example of single covalent bonds

Hydrogen Chloride (HCl): In one molecule of hydrogen chloride (HCl), a single covalent bond exists between hydrogen and chlorine atoms or elements. Let see how these atoms share electrons.

There is one valence electron in hydrogen and seven valence electrons in chlorine. Here, both elements need one electron to complete their octet or to gain stability. Both want electrons, so they tend to share electrons with each other which results in the formation of a single covalent bond or single pair of electrons.

ii) Double Covalent Bond:

It has two shared pair of electrons (a total of four electrons). The atoms participating in bond formation share 2 pairs of electrons. It is denoted by (=). It is stronger than a single covalent bond, however, it is less stable than a single covalent bond as its reactivity is higher than a single covalent bond.

Examples of double covalent bonds

Carbon dioxide (CO₂): There is a double covalent bond between carbon and oxygen in a molecule of carbon dioxide. The carbon atom shares its four electrons with four electrons of two oxygen molecules to complete octets of carbon and oxygen as carbon needs four electrons and one oxygen atom needs two electrons to complete their octets.

The carbon atom shares its two electrons with two electrons of the oxygen atom on one side and shares its remaining 2 electrons with 2 electrons of another oxygen atom on another side. So, there are two double bonds on side of the carbon atom as shown in the below image.

Oxygen: A molecule of oxygen contains two oxygen atoms as each oxygen atom needs two more electrons to complete its octet. So, they share two pair of electrons that results in the formation of double covalent bond.

iii) Triple covalent bond:

Here, the atoms participating in bond formation share three pairs of electrons, so it is called a triple covalent bond. It is denoted by (≡). It is least stable than the single and double covalent bonds.

Examples of triple covalent bond:

Nitrogen: The triple covalent bond can be seen in the molecule of nitrogen. One nitrogen atom needs three electrons to complete its octet. So, it shares three pairs of electrons with another nitrogen atom with a similar requirement and thus triple covalent bond is formed.

Types of covalent bond based on the polarity

The covalent bonds also show polar nature in some cases. So, it can be polar or non-polar.

i) Polar covalent bond:

The covalent bond in which a shared pair of electrons is more close to the atom with higher electronegativity (more ability to attract electrons) and thus results in polarity is called a polar covalent bond.

Due to the difference in the electronegativity of the atoms electrons are not shared equally between atoms. So, the polar covalent bond has a slightly positive side towards the atom with low electronegativity and a slightly negative side towards the atom with higher electronegativity. The compounds with polar covalent bonds develop an electrostatic potential. So, it may form weak bonds with other similar polar molecules.

Examples of polar covalent bond:

Water: In the molecule of water (H₂O), hydrogen and oxygen atoms are held together through polar covalent bonds. There is a difference in the electronegativity of hydrogen and oxygen atoms. The oxygen atom has more electronegativity than the hydrogen atom. So, the electron pair is slightly pulled closer to the oxygen atom, which develops a slightly negative charge towards the oxygen atom and a slightly positive charge towards the hydrogen atom.

ii) Non-polar covalent bond

The covalent bond in which electron pairs are equally shared between atoms and not pulled unequally between atoms is called a non-polar covalent bond. It is formed between atoms with equal electronegativity or equal attraction for electrons. It is generally formed between atoms of the same molecule for example diatomic molecules such as gas molecules oxygen (O₂), nitrogen (N₂), etc. Furthermore, due to the equal electronegativity of atoms, there is no net charge on the molecules with a non-polar covalent bond.

Properties of covalent bond:

• They are formed when sharing of electrons takes place between atoms. Their formation does not involve the transfer of electrons.
• Ions are not formed in covalent bonds as the transfer of electrons does not take place.
• They are formed between atoms that have the same electronegativity, so they are generally formed between non-metals.
• Although, electrons are shared equally between atoms, they may be polar in nature due to a slight difference in the electronegativity between the atoms participating in bond formation.
• A covalent bond can be a single, double or triple covalent bond based on the number of shared pairs of electrons.
• A covalent bond is less strong than an ionic bond due to this reason covalent compounds generally have low boiling and melting point and low enthalpy of vaporization and fusion.
• The covalent bonds do not support the flow of current as they do not form ions like ionic bonds.
• Covalent bond generally forms neutral molecules with no net charge as the difference in electronegativity between the atoms participation in a covalent bond is zero.
• A small amount of energy is needed to break the covalent bonds.
• Compounds with covalent bonds are insoluble in water as they are non-polar and water is a polar solvent.

Difference between ionic and covalent bond in tabular form

Although there are lots of differences between ionic and covalent bond, there are also some similarities between them.

Similarities between ionic and covalent bond

• They are strong bonds.
• They are primary bonds.
• They result in the formation of complex structures.
• They form more stable compounds than the original.
• The formation of both ionic and covalent bond is exothermic. When they are formed, the potential energy of the participating elements decreases as energy is released in the form of heat.
• Compounds formed by these bonds have no net charge.
• In both bonds, the valence shell electrons take part in bond formation and both the bonds complete their outermost shell with 8 electrons.
• They have the same bonding method. In both cases, the nuclei of the atoms participating in bond formation attract each other when atoms come closer. The bond will be covalent in nature if the attraction is the same from both atoms. When the attraction is different or polarized the bond will have an ionic character.
• Both ionic and covalent bonds form neutral molecules. In an ionic bond, the opposite charge neutralized the charges of ions and equal sharing of electrons maintains a neutral charge.
• In a solid or crystalline state, both the bonds form compounds with regular pattern and structure. However, both of them may change their shapes physically under the right temperature or pressure.
• The compounds formed by these bonds chemical bonds are not malleable.