Carbon is derived from the word “carbo” meaning coal.
1. Atomic number - 6
2. Atomic mass - 12
3. Electronic configuration - 2, 4
4. Valency - 4
The compounds obtained from 'Carbon' are widely used. For example, clothes, medicines, books, food, fertilizer, fuel etc. all living structures are carbon based.
Fig: application of carbon
The amount of carbon present in the earth's crust and in the atmosphere is quite less. The earth's crust has only 0.02% carbon in the form of mineral (like carbonates, hydrogen-carbonates, coal and petroleum) and the atmosphere has 0.03% of carbon dioxide. In spite of this small amount of carbon available in nature, the importance of carbon seems to be immense.
Carbon forms a large number of compounds with hydrogen which are known as hydrocarbons. In addition to hydrogen, carbon compound may also contain some other elements such as oxygen, halogen, nitrogen, phosphorus, sulphur etc. The number of compounds of carbon are more than three million which is much larger than the compounds formed by all other elements put together.
BONDING IN CARBON COMPOUNDS
Carbon forms covalent bonds in its compounds with other atoms. In each compound, the valency of carbon is four. That is, carbon has a tetravalent character. But what is covalent bond and what is the meaning of tetravalent ?
Why does a carbon atom form only covalent bond?
The atomic number of carbon is 6 and the first shell contains just two electrons and the second shell (Outermost shell) contains four electrons.
Carbon atom can attain the noble gas configuration by sharing its valence electrons with other atoms of carbon or with atoms of other elements and form covalent bond.
A chemical bond formed between two atoms of the same element or two atoms of different elements by sharing of electrons is called a covalent bond.
Fig: Carbon forming covalent bond in Methane (CH4)Necessary conditions of the formation of covalent bond:
- The combining atoms should have nonmetallic character.
- The combining atoms should contain 4 to 7 electrons in their respective valence shell.
- In hydrogen there is only 1 valence electron, but it also forms covalent bond.
- The combining atoms need 1, 2, 3 or 4 electrons to complete their octet (hydrogen completes its duplet).
- The combining atoms should contribute equal number of electrons to form pair of electrons to be shared.
- After sharing the pair of electrons each combining atoms should attain stable electronic configuration like its nearest noble gas.
CLASSIFICATION OF COVALENT BOND
On the basis of the number of electrons shared by two combining atoms, the covalent bond are of three types.
Single Covalent Bond: A single covalent bond is formed by the sharing of one pair of electrons between the two atoms. It is represented by one short line (---) between the two atoms.
Example: H-H, Cl -Cl, H-Cl, CH3-CH3.
Double Covalent Bond: A double covalent bond is formed by the sharing of two pairs of electrons between the two combining atoms. It is represented by putting (=) two short lines between the two bonded atoms.
Examples: O = O (O2), CO2 (O = C = O), H2C = CH2
Triple covalent bond: A triple bond is formed by the sharing of three pair of electrons between the two combining atoms. It is represented by putting three short line (≡) between two bonded atoms.
Example: N2 (N≡N), CH≡CH.
Fig: N-N triple covalent bond
Formation of single covalent compounds
Formation of hydrogen molecule (H2):
A molecule of hydrogen is composed of two H-atoms. The electronic configuration of H-atom is.
H-H Bond in terms of energy shells (orbits)
Formation of chlorine molecule (Cl2):
The atomic number of chlorine is 17, thus there are 17 electrons in an atom of chlorine.
Electronic configuration of Cl atom
Electronic configuration of Ar atom
Chlorine atom needs one electron more to complete its octet
Cl-Cl bond in terms of energy shell orbits
Formation of hydrochloric acid (HCl): H atom has one valence electron. It needs 1 electron more to complete its duplet and chlorine atom has 7 valence electrons. It needs 1 electron more to complete its octet and acquire stable electronic configuration. (2, 8, 8) like the noble gas argon.
Formation of oxygen (O2): The atomic number of O atom is 8. There are 6 electron in the valence shell of oxygen atom and it needs 2 more electrons to attain the nearest stable inert gas Neon (2, 8) configuration:
Formation of nitrogen molecule (N2): The atomic number of nitrogen is 7 and its electronic configuration is K(2), L(5). It needs 3 electrons more to complete its octet like noble gas neon (2, 8).
Formation of ammonia molecule (NH3): The atomic number of N is 7. It's electronic configuration is 2, 5 there are 5 electrons in its valence shell. It needs 3 electrons more to complete its octet like noble gas neon (2, 8).
Formation of H2O molecule: The electronic configuration of hydrogen is K (1) and that of oxygen is K(2) L(6). Thus, each hydrogen atom requires one and oxygen atom requires two more electrons to achieve the stable electronic configuration.
Formation of CO2 molecule: The atomic number of C is 6 and the electronic configuration of C is K(2), L(4) and that of oxygen is K(2), L(6) thus each carbon atom requires 4 and oxygen atom requires two more electrons to achieve the stable electronic configuration.
Fig: Formation of CO2
Formation of CH4 molecule: Methane is a covalent compound containing 4 covalent bonds. It contains one carbon atom and four hydrogen atoms covalently bonded to the central carbon atom.
Fig: Formation of Ethene
Formation of Acetylene or ethyne molecule (C2H2):
Carbon atom forms a triple covalent bond with another carbon atom and a single covalent bond with hydrogen atom.
Fig: formation of ethyne molecule
Q. What would be the electron dot structure of a molecule of sulphur which is made up of eight atoms of sulphur?
Ans. The eight atoms of sulphur are joined together in the form of a puckered ring. This is also known as the crown shape.
Non polar and polar covalent compounds:
Non polar covalent bond: A covalent bond formed between two atoms of the same element or same electronegativity is called a non-polar covalent bond. Example : H2, N2, O2, Cl2 etc.
Polar covalent bond: The covalent bond formed between the atoms of two elements having different electronegativities is called a polar covalent bond. Molecule in which the atoms are bonded by a polar covalent bond are called polar molecules.
Note: In a polar covalent bond, the shared pair of electrons lies more toward the atom which is more electronegative.
Example: HCl, H2O & NH3
Characteristics of covalent bond and covalent compounds:
1. Characteristics of covalent bond:
Covalent bond is formed by mutual sharing of electrons.
Note: Shared pair of electrons is also called as bonding pair of electrons.
2. Characteristics of covalent compounds:
Physical State: The covalent compounds are generally gases or liquids, but compounds with high molecular masses are solids.
Example: Solid: Urea, Glucose, Naphthalene.
Liquids: Water, ethanol, benzene.
Gases: Methane, chlorine, hydrogen, oxygen
Melting and boiling points: Covalent compounds have low melting and low boiling points because intermolecular forces (cohesive forces) in covalent compounds are weaker than those in ionic compounds.
Note: Some exception like diamond and graphite which are covalent solids have very high M.P. & B.P.
Solubility: Covalent compounds generally dissolve readily in organic solvents but they are less soluble in water.
For example: Naphthalene which is an organic compound dissolves readily in organic solvents like ether but is insoluble in water. However some covalent compounds like urea, glucose, sugar etc. are soluble in water. Some polar covalent compounds like ammonia and hydrochloric acid are soluble in water.
Conductivity: Covalent compounds do not conduct electricity because they contain neither the ions nor free electrons necessary for conduction, So they do not conduct electricity
For example: Covalent compounds like glucose, alcohol, carbon tetrachloride do not conduct electricity.
Differences between ionic and covalent compounds:
The chemical compounds which are present in living organisms (plant and animal) are called organic compounds. The belief that formation of organic compounds was possible only in plants and animals led the scientists of early days to propose that the Vital Force was necessary for the formation of such compounds. But the experimental work of Friedrich Wohler (German chemist) denied the idea of vital force when he prepared urea in his laboratory. (urea is an organic compound and waste product of urine).