Class 10, Science, Chapter-4, Lecture-1, Catenation by Carbon (Notes)
CARBON
Atomic number - 6
Electronic configuration - 2, 4
Valency - 4
Atomic mass - 12
Percentage of carbon in the earth’s crust = 0.02 % (as carbonates, bicarbonates, coal, petroleum)
Percentage of carbon in the earth’s atmosphere = 0.03 % (as carbon dioxide)
COVALENT BOND
Chemical bonds which are formed by sharing of electron pair(s) between two atoms are termed as covalent bonds.
Examples of covalent bonds:
- The bond in H2 molecule
- The bond in O2 molecule
- The bond in N2 molecule
- The bond in NH3 molecule
- The bond in CH4 molecule
Mechanism of bond formation in H2 molecule
Hydrogen atom has one valence electron in K shell.
Two Hydrogen atoms share 1 electron each to complete their duplet.
One pair of shared electrons form a covalent single bond.
Electron-dot structure:
Mechanism of bond formation in N2 molecule
Nitrogen atom has 5 valence electrons in L shell.
Two Nitrogen atoms share 3 electrons each to complete their octet.
Each Nitrogen atom has one lone pair.
Three pairs of shared electrons form a covalent triple bond.
Electron-dot structure:
Mechanism of bond formation in NH3 molecule
H atom has one valence electron in K shell.
Nitrogen atom has 5 valence electrons in L shell.
Three Hydrogen atoms share 1 electron each to complete their duplet and nitrogen atom’s octet.
Nitrogen atom has one lone pair.
Three pairs of shared electrons form three covalent single bonds.
Electron-dot structure:
Carbon always forms covalent bonds.
Reasons
- Carbon atom cannot gain four electrons to complete its octet as its nucleus cannot hold on to 4 extra electrons.
- Carbon cannot lose four electrons due to requirement of large amount of energy in doing so because of its small size.
Covalent Compounds have low melting point or boiling point
Reason:
The intermolecular force of attraction between molecules of covalent compounds is smaller than the force of attraction between ions of an ionic compound.
So, the amount of energy required to separate the molecules of covalent compounds is less.
Covalent Compounds are poor conductors of electricity.
Reason:
Covalent compounds are formed by sharing of electrons. So charged ions are not present in such compounds. The absence of charged particles or free electrons makes these compounds poor conductors of electricity.
CATENATION:
The ability of an element to form long chains by making bonds with atoms of the same element is termed as catenation.
Reasons for large number of carbon compounds:
- CATENATION: The ability of carbon to form different kinds of long chains by making bonds with carbon atoms.
- TETRAVALENCY of carbon makes it capable of bonding with large number of elements such as oxygen, hydrogen, nitrogen, Sulphur, chlorine, etc.