“Chemical Bond is the force of attraction that holds the atoms together in a molecule of a compound”.
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 which 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.
“the bond formed by the complete transfer of electrons from one or more electrons from an electropositive atom to more electro-negative 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 loose 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 release 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 the 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 in an ionic compound is non-directional.
“The force of attraction produce as a result of mutual sharing of electrons between two atoms is called the covalent bond, indicated by a complete line”.
Covalent bond 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 electron 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 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 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 oxygen molecule, each contributes two valence electrons.
Triple Covalent Bond
A type of covalent bond which is formed by mutual sharing of three electrons from each atom is called the triple covalent bond. It is denoted by three complete line.
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 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 to unequally by the two bonded atoms known as 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 atom. As a result of which atoms become partially charged such molecule is referred to as a dipole. A polar covalent bond has 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 non-polar bond.
When a covalent bond is formed between to similar atoms or atoms having nearly 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.
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 as the donor and atom, which accept this lone pair to complete its valence shell called 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 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 week.
The type of covalent bond which is formed between two already sigma bonded atom 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.