It is important to understand how atoms are bonded together in order to understand the function of many materials. “A Chemical Bond is the force of attraction that holds the atoms together in a molecule of a compound”. there are three types of Chemical Bond or ways in which atoms can be bonded together: Ionic bond, Covalent bond and Co-ordinate Covalent Bond.
Ionic bonds occur when one atom loses an electron, leaving it with a positive charge, while another atom gains an electron, giving it a negative charge. This leaves the two ions charged and attracted to each other.
Covalent bonds happen when two atoms share electrons equally, creating a neutral molecule between them. This bond is much more stable than ionic bonding because there are no charges attracting or repelling each other.”
however, Atoms tend to form a bond due to following two reasons.
- Atoms attain maximum stability by containing eight electrons in their valence shell like inert gas (octet rule).
- Atoms are much smaller in size and also have high energy, therefore, they combine together and form molecules that are relatively bigger in size and have low energy.
There are three types of the chemical bond:
- Ionic bond or Electrovalent bond.
- Covalent bond.
- Dative or Co-ordinate Covalent Bond.
What is Ionic Bond?
“the bond formed by the complete transfer of electrons from one or more electrons from an electropositive atom to more electronegative atom is called ionic bond”.
“the electrostatic force of attraction which holds positive and the negative ions together in an ionic compound is called ionic bond”.
An ionic bond is usually formed between an element of low electronegativity (metals) and elements of higher electronegativity (non-metals). If the difference of the electronegativity is equal or greater than 1.7 between two atoms usually leads to an ionic bond.
Formation of NaCl (Ionic Compound)
Formation of NaCl consists of the following steps:
Sodium is ground state has electronic configuration, Na ( Z = 11) = 1s2, 2s2, 2p6, 3s1.
It has only one valence electron. The loss of this valence electron requires some energy, the resulting Na+ ions have the complete octet.
Na(g) → Na+(g) + e– ΔH = +495 KJ/mol
Chlorine is ground state has electronic configuration, Cl ( Z = 17) = 1s2 , 2s2, 2p6, 3s2, 3p5.
Chlorine has seven valence electrons. It needs one more electron to complete its octet. The gain of an electron by chlorine releases energy.
Cl(g) + e– → Cl—(g) ΔH = -348 KJ / mol
The oppositely charged ions (Na+ and Cl–) formed are held together by the electrostatic force of attraction, which involves the formation of the crystal lattice in which the energy is released.
Na + + Cl– → NaCl(s) ΔH (U) = -788 KJ / mol
Properties of Ionic Compounds
- They are non-volatile, crystalline solids at room temperature due to the strong attractive forces.
- They possess high melting and boiling points.
- They are soluble in water and insoluble in non-polar solvents.
- They are strong electrolytes because they conduct electricity in the molten or aqueous solution form.
- They are undergoing fast reactions.
- The bond is an ionic compound that is non-directional.
What is a Covalent Bond?
“The force of attraction produces as a result of mutual sharing of electrons between two atoms is called the covalent bond, indicated by a complete line”.
A covalent bond is usually formed between two like or unlike non-metal atoms. Both the atoms contribute an equal share of electrons in the formation of the covalent bond. The shared pair of electrons equally belongs to both the bonded atoms.
Hydrogen has one electron in its valence shell, two electrons, one from each hydrogen atom shared to form hydrogen.
H* + H’ → H : H or H – H
Types of Covalent Bond
Single Covalent Bond
A type of covalent bond which is formed by the mutual sharing of one electron from each atom is called the single covalent bond. It is denoted by a single complete line.
Formation of Cl2 Molecule
Electronic configuration of chlorine (Z = 17) 1s2, 2s2, 2p6, 3s2, 3p5. There are seven electrons in the valence shell of the chlorine atom. In a chlorine molecule, each contributes one valence electron.
Double Covalent Bond
A type of covalent bond which is formed by the mutual sharing of two electrons from each atom is called the double covalent bond. It is denoted by the double complete line.
Formation of O2 Molecule
Electronic configuration of oxygen ( Z = 8) 1s2, 2s2, 2p4. There are six electrons in the valence shell of the oxygen atom. In an oxygen molecule, each contributes two valence electrons.
Triple Covalent Bond
A type of covalent bond which is formed by the mutual sharing of three electrons from each atom is called the triple covalent bond. It is denoted by three complete lines.
Formation of N2 Molecule
Electronic configuration of Nitrogen (Z = 7) 1s2, 2s2, 2p3. There are five electrons in the valence shell of the nitrogen atom. In a nitrogen molecule, each contributes three valence electrons.
Properties of Covalent Compounds
- Covalent compounds exist as separate covalent molecules because the particles are electrically neutral and have little attractive forces, therefore, covalent compounds are volatile liquid or gases or low melting point solids.
- They are non-electrolytes.
- They are generally insoluble in water and similar polar solvents but soluble in non-polar solvents.
- The reactions of covalent compounds are much slower than ionic compounds.
- A covalent bond in covalent compounds is directional in nature.
Polar Covalent Bond
A covalent bond in which the shared pair of electrons is attracted unequally by the two bonded atoms is known as a polar covalent bond.
When a covalent bond is formed between two dissimilar atoms having different values of electronegativity. The shared pair of electrons is slightly shifted towards more electronegativity atoms. As a result of which atoms become partially charged such molecule is referred to as a dipole. A polar covalent bond has a partial ionic character.
Non-Polar Covalent Bond
A covalent bond in which shared pair of electrons is attracted equally by two bonded atoms is known as a non-polar bond.
When a covalent bond is formed between similar atoms or atoms having nearly the same electronegativity value, shared pair of electrons is attracted equally from both sides no separation of charges takes place hence, no poles have appeared. Such molecule is called non-polar molecules.
What is a Dative or Coordinate Covalent Bond?
The type of chemical bond, which is form b7y one-sided sharing of electron pair by one of the bonded atoms, is known as a coordinate covalent bond or dative bond.
The dative bond is represented by the use of an arrow (→) from the donor atom to an acceptor atom.
The atom, which donates the lone pair of electrons, is called the donor, and the atom, which accepts this lone pair to complete its valence shell called the accepter.
Sigma Bond (σ)
A type of covalent bond, which is formed by end-on or head-to-head overlapping of half-filled atomic orbitals, is referred to as a sigma bond.
Only one sigma bond can be formed between two atoms. All single covalent bonds are sigma bonds.
On the basis of overlapping orbitals there are three types of the sigma bond:
- s-s sigma bond (e.g: formation of H2 molecule)
- s-p sigma bond (e.g: formation of HF molecule)
- p-p sigma bond (e.g: formation of F2 molecule)
the relative bonds strengths of sigma bonds are:
s-s > s-p > p-p
Due to spherical charge distribution in s-orbital generally, s-s overlapping is not so effective as s-p and p-p overlapping. Whereas, p-orbital has directional charge distribution and longer lobes which cause more effective overlapping. Thus s-s sigma bond is relatively weak.
The type of covalent bond which is formed between two already sigma bonded atoms by sideway overlaps of two half-filled atomic p-orbitals whose axis are parallel to each other is called pi-bond.
Every double covalent bond consists of one sigma bond and one pi bond while every triple covalent bond contains one sigma and two pi bonds.