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In a covalent bond, the equal sharing of electrons from both the participating atoms takes place. The pairs of electrons participating in this covalent bonding are called shared pairs or bonding pairs.
Key Terms: Bond Energy, Atoms, ions, Covalent bonds, Polarization, Solid State
What are Covalent Bonds?
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A covalent bond is a type of chemical bond which is formed by the sharing of electrons between two or more atoms. Usually, covalent bonds occur between nonmetals or between two similar elements. Two atoms with less difference in their electronegativity do not exchange an electron from their outermost shell. These atoms share electrons so that their valence electron shell is filled which results in the formation of covalent bonds.
For example:
The video below explains this:
Covalent Bond Detailed Video Explanation:
Let’s take the Chlorine molecule ‘Cl2’.
The electronic configuration of the Cl atom is [Ne]3s2 3p5 and it has one electron short of the argon configuration. In the formation of Cl2 molecules, a pair of electrons between the two chlorine atoms are shared, where each chlorine atom contributes one electron to the shared pair. In this process, both chlorine atoms attain the outer shell octet of the nearest noble gas ( Argon ).
Octet Rule
When an atom is surrounded by eight electrons, a stable state is attained. This can be achieved by own electrons and some electrons which are shared. Thus, an atom reacts and continues to form bonds until an octet of electrons is made. This tendency of an atom to prefer 8 electrons in its valence shell is called the octet rule.
In other words:
“The principle of attaining the maximum of eight electrons in the valence shell of atoms is called octet rule.”
In the octet rule, we consider only those atoms where the ending electronic configuration is of the type s2p6, i.e. s and p electrons as d and f electrons are not involved in this rule. Atoms having less than 8 electrons in their valence shell tend to react until they achieve the most stable state possible.
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|---|---|---|
| Electrical properties of solids | Conductors | Insulators |
| Classification Of Crystalline Solids | Density of Unit Cell | Metallic Bonds |
Types of Covalent Bonds
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There are three types of covalent bonds on the basis of the number of electrons shared.
Single Bond
In single covalent bonds, one pair of electrons ( 2 electrons ) take part in bond formation to complete their valence shell and attain a stable electronic configuration. The formation of a single bond takes place between nonmetals or nonmetals and metalloids.
Single bond results in high bond length and weak bond strength. Due to their low reactivity, they are highly stable. A single dash ( - ) is used to represent a single covalent bond.
H2 , F2 , Cl2 , Br2 , I2 , NH3 , C2H6 , etc. are compounds having single covalent bonds.
Some Examples of Single Covalent Bond:
H2O: Here, hydrogen has one electron in its valence shell and it needs one electron to complete its 2s shell, while oxygen has 6 electrons in its outermost shell and it needs 2 electrons to complete its octet.
So, oxygen shares its 2 electrons with 2 hydrogen atoms to form H2O and the other 4 electrons remain as 2 lone pairs of electrons.
Chlorine ( Cl2 ) : Chlorine has 7 electrons in its outermost shell and it requires one electron to complete its octet. Hence, it shares one electron with another chlorine atom to form Cl2 .
Ammonia ( NH3 ) : Nitrogen has 5 electrons in its outermost shell and it requires 3 electrons to become stable. So, it shares its 3 electrons with 3 hydrogen atoms to form ammonia.
Methane ( CH4 ) : Carbon has 4 electrons in its outermost shell and it requires 4 electrons to achieve a stable state. So, it shares its 4 electrons with 4 hydrogen atoms to form methane.
Double Bond
In double covalent bonds, two pairs of electrons ( 4 electrons ) take part in bond formation to complete their valence shell and become stable. Formation of double bond takes place between nonmetals or a nonmetals and metalloid.
Double bonds result in moderate bond length and high bond strength. Due to their moderate reactivity they are unstable. Double dash ( = ) is used to represent a double covalent bond.
O2 , CO2 , C2H4 , etc. are compounds having double bonds.
Some Examples of Covalent Bond-
Oxygen ( O2 ) : Oxygen has 6 electrons in its outermost shell and it requires 2 electrons to achieve a stable state. Hence, it shares its 2 electrons with 2 electrons of another oxygen atom to form O2 molecule. Sharing of 2 pairs of electrons implies that there is formation of a double covalent bond.
Carbon Dioxide ( CO2 ) : Carbon has 4 electrons in its outermost shell and it requires 4 electrons to complete its octet, while oxygen has 6 electrons and it requires 2 electrons to complete its octet. So, there is a sharing of 4 electrons ( 2 pairs of electrons) between carbon atom and 2 oxygen atoms to form CO2 molecule.
Triple Bond
In triple covalent bonds, three pairs of electrons ( 6 electrons ) take part in bond formation to complete their valence shell and become stable. Formation of triple bond takes place between nonmetals or a nonmetals and metalloid.
Triple bond results in low bond length and high bond strength. Due to their high reactivity they are highly unstable. Triple dash ( ≡ ) is used to represent a triple covalent bond.
N2 , CN- , C2H2 , etc. are some of the compounds which have triple bonds.
Nitrogen ( N2 ) : Nitrogen has 5 electrons in its outermost shell and it requires 3 electrons to complete its octet. So, it shares its 3 electrons with 3 electrons of another nitrogen molecule to form N2 .
Ethyne ( C2H2 ) : Ethyne has two carbon atoms and two hydrogen atoms. Carbon has 4 electrons in its outer shell and it requires four electrons to complete its octet. On the other hand, hydrogen has one lone electron and it requires one more electron to complete its octet.
So, one hydrogen atom will combine with one carbon atom to form a single bond and another hydrogen atom will combine with another hydrogen atom to form another single bond. Both the carbon atoms have 3 more lone electrons. They will share the 3 electrons resulting in the formation of a triple bond.
Polarization of Covalent Bonds
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In the sigma bonds between two different atoms, the electron cloud is always closer to the more electronegative atom participating in the formation of the sigma bond. Due to this, a permanent dipole arises in the bond and the covalent bond is said to be polarized.
On the basis of Polarity, covalent bonds are of two types:
- Polar Covalent Bond
- Non-polar Covalent Bond
Polar Covalent Bond
The polar covalent bond is formed between two non-metallic atoms that have an electronegativity difference between 0.4 and 1.7. In the bond formation, the shared electrons stay closer to the more electronegative atom.
Example: The water molecule has two polar covalent bonds between oxygen and hydrogens.
In the water molecule H2O, the electrons of the hydrogens stay closer and longer around the oxygen, which is more electronegative.
Nonpolar Covalent Bond
Nonpolar Covalent Bond is formed between atoms with equal electronegativity and the atoms with an electronegativity difference of less than 0.4. In the nonpolar covalent bond, equal sharing of electrons takes place between the atoms.
Example:
Chlorine molecule Cl2 - Two chlorine atoms with the same electronegativity combine to form an electron pair. The bond that is formed between them is a nonpolar covalent bond.
Properties of Covalent Bonds
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If the valence of an atom is not satisfied by sharing a single electron pair between atoms, the atoms share more than one electron pair between them. Some of the covalent bond properties are:
- Covalent bonds are formed between two non-metals like hydrogen, oxygen, etc.
- In covalent bonding, new electrons are not formed. The bond only pairs them.
- Covalent bonds include single, double, or triple bonds where 2, 4, or 6 electrons are shared respectively.
- There exist very powerful chemical bonds between atoms in the covalent bonding.
- A covalent bond normally contains the energy of about ~80 kilocalories per mole (kcal/mol).
- Covalent bonds rarely break once they are formed.
- Most compounds with covalent bonds have very low melting point and boiling points.
- Compounds with covalent bonds usually have lower enthalpies of vaporization and fusion.
- Compounds formed by covalent bonding don’t conduct electricity due to the lack of free electrons.
- Covalent compounds are insoluble in water.
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Sample Questions
Ques.1: Explain with the help of suitable example polar covalent bond. (2 Marks)
Answer: When two atoms with different electronegativity are linked with each other by covalent bond, the shared electron pair will not be in the centre because of the difference between their electronegativity.
For example, in hydrogen fluoride molecules- Fluorine has more electronegativity than hydrogen. Thus, the shared electron pair is inclined towards fluorine atom and later will acquire a partial negative charge (∂–).
At the same time hydrogen atom will have a partial positive charge (∂+). This type of covalent bond is known as polar covalent bond.
Ques.2: Give reasons for the following:
(a) Covalent bonds are directional bonds while ionic bonds are non- directional.
(b) Water molecules have a bent structure whereas carbon dioxide molecules are linear.
(c) Ethyne molecule is linear. (3 Marks)
Answer:
(a) Covalent bonds are formed by overlapping of orbitals between different orbitals, the geometry of the molecule is different and the orientation of overlap is also different in the covalent bond. The orientation of overlap is the factor responsible for their directional nature.
(b) Due to the presence of two lone pairs of electrons on oxygen atoms in H2O the repulsion between Ip-lp is more. CO2 undergoes sp hybridization resulting in linear shape (O = C = O) while H2O undergoes sp3 hybridisation resulting in distorted tetrahedral or bent structure.
(c) In ethyne molecule carbon undergoes sp hybridization with two unhybridized orbitals. One carbon atom overlaps axially with another carbon atom to form C – C sigma bond while the other hybridized orbital of each carbon atom overlaps axially with S orbitals of hydrogen atoms forming σ bonds.
Ques. 3: Give reason:
1)Iron displaces copper from a solution of copper salt.The reaction is determined as redox.
2)A non metallic atom [at.no 9] forms a molecule of the same, containing a single covalent bond.
3)A compound has formula H2Y. Y denotes a non metal. State the electronic configuration of Y. (3 Marks)
Answer: 1) Displacement reaction is also a redox reaction because in this reaction, the less reactive compound gets reduced and the more reactive compound gets oxidized.
In the following reaction, Fe displaces Cu from CuSO4.Fe gets oxidized while Cu gets reduced.
Fe(s) + CuSO4(aq) → FeSO4(aq) + Cu(s)
Oxidation state of Fe changes from 0 to +2 and the oxidation state of Cu changes from +2 to 0.
2) A nonmetallic atom [at.no 9] forms a molecule of the same, containing a single covalent bond. The electronic configuration of at no. 9 is (2,7) or 1s2 2s2 2p5. In its outermost shell, there are 7 electrons hence this atom needs only one electron to complete its octet.
So, it will share one electron from another same atom to form a single covalent bond.
3) The non-metal will be Oxygen and the formula is H2O and its electronic configuration is 1s2 2s2 2p4
Ques.4: What are the postulates of orbital overlap concept of covalent bond? (2 Marks)
Answer: According to this concept:
- The covalent bond is formed due to partial overlapping of the two half-filled atomic orbitals of the combining atoms. In partial overlapping, a part of the electron cloud of each of the two half-filled orbitals becomes common. Which results in the reduction of the internuclear repulsion and hence decreases the energy.
- The orbitals undergoing overlap must have electrons with opposite spins.
- Greater the extent of overlapping, stronger is the bond formed.
- Larger the size of the orbitals, less effective is the overlapping and thus weaker is the bond formed.
Ques.5: Explain the formation of covalent bonds in hydrogen molecules. (2 Marks)
Answer: Hydrogen has one valence electron in its outermost shell. So, it requires one more electron to attain a stable electronic configuration. Therefore each hydrogen atom combines with another hydrogen (H) atom to form the H2 molecule. This results in the formation of a covalent bond.
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