Class 10 Bonding in carbon-the covalent bond

### Topics to be covered

=> General introduction
=> Bonding in carbon-the covalent bond
=> Versatile nature of carbon
=> Saturated and unsaturated carbon compounds
=> Chains, branches and rings

### GENERAL INTRODUCTION

color{green}(★) The amount of carbon present in the earth’s crust and in the atmosphere is quite low.

color{green}(★) The earth’s crust has only color{red}(0.02%) carbon in the form of minerals (like carbonates, hydrogencarbonates, coal and petroleum) and the atmosphere has color{red}(0.03%) of carbon dioxide.

color{green}(★) In spite of this small amount of carbon available in nature, carbon seems to have an immense importance in our life.

### BONDING IN CARBON – THE COVALENT BOND

color{green}(★) Reactivity of elements depends upon their tendency to attain a completely filled outer shell ( noble gas configuration).

color{green}(★) Elements forming ionic compounds achieve this by either gaining or losing electrons from the outermost shell.

color{green}(★) In the case of carbon, it has four electrons in its outermost shell and needs to gain or lose four electrons to attain noble gas configuration.

color{green}(★) If it were to gain or lose electrons –

(i) It could gain four electrons forming color{red}(C^(4–)) anion. But it would be difficult for the nucleus with six protons to hold on to ten electrons, that is, four extra electrons.

(ii) It could lose four electrons forming color{red}(C^(4+)) cation. But it would require a large amount of energy to remove four electrons leaving behind a carbon cation with six protons in its nucleus holding on to just two electrons. Carbon overcomes this problem by sharing its valence electrons with other atoms of carbon or with atoms of other elements. The shared electrons color{red}("‘belong’") to the outer shells of both the atoms and lead to both atoms attaining the noble gas configuration. Similarly other elements can attain stability by sharing of electrons.
For eg:

color{green}(★) The atomic number of hydrogen is 1. Hence hydrogen has one electron in its color{red}(K) shell and it requires one more electron to fill the color{red}(K) shell. So two hydrogen atoms share their electrons to form a molecule of hydrogen, color{red}(H_2).

The shared pair of electrons is said to constitute a single bond between the two hydrogen atoms. A single bond is also represented by a line between the two atoms.

color{green}(★) The atomic number of chlorine is 17.

color{green}(★) In the case of oxygen there are six electrons in its color{red}(L) shell (the atomic number of oxygen is eight) and it requires two more electrons to complete its octet. So each atom of oxygen shares two electrons with another atom of oxygen to give us the structure shown in Fig. 4.3. The two electrons contributed by each oxygen atom give rise to two shared pairs of electrons. This is said to constitute a double bond between the two atoms.

color{green}(★) A molecule of water showing the nature of bonding between one oxygen atom and two hydrogen atoms.

color{green}(★) Nitrogen has the atomic number 7. In order to attain an octet, each nitrogen atom in a molecule of nitrogen contributes three electrons giving rise to three shared pairs of electrons. This is said to constitute a triple bond between the two atoms.

color{green}(★) The electron dot structure for color{red}(NH_3) molecule.

color{green}(★) METHANE:

1 A compound of carbon.

2 It is widely used as a fuel and is a major component of bio-gas and Compressed Natural Gas (CNG).

3 It is also one of the simplest compounds formed by carbon.

4 It has a formula color{red}(CH_4) .This is because hydrogen has a valency of 1 and Carbon is tetravalent because it has four valence electrons. Now in order to achieve noble gas configuration, carbon shares these electrons with four atoms of hydrogen.

So the bonds which are formed by the sharing of an electron pair between two atoms are known as covalent bonds.

color{green}("𝐏𝐑𝐎𝐏𝐄𝐑𝐓𝐈𝐄𝐒 𝐎𝐅 𝐂𝐎𝐕𝐀𝐋𝐄𝐍𝐓 𝐁𝐎𝐍𝐃𝐒:")

color{green}(★) Covalently bonded molecules are seen to have strong bonds within the molecule, but intermolecular forces are small.

color{green}(★) This gives rise to the low melting and boiling points of these compounds.

color{green}(★) Since the electrons are shared between atoms and no charged particles are formed, such covalent compounds are generally poor conductors of electricity.

### VERSATILE NATURE OF CARBON

color{green}(★) Carbon has the unique ability to form bonds with other atoms of carbon, giving rise to large molecules. This property is called catenation. These compounds may have long chains of carbon, branched chains of carbon or even carbon atoms arranged in rings.

color{green}(★) In addition, carbon atoms may be linked by single, double or triple bonds. Compounds of carbon, which are linked by only single bonds between the carbon atoms are called saturated compounds. Compounds of carbon having double or triple bonds between their carbon atoms are called unsaturated compounds.

color{green}(★) Silicon forms compounds with hydrogen which have chains of upto seven or eight atoms, but these compounds are very reactive.

color{green}(★) The carbon-carbon bond is very strong and hence stable. This gives us the large number of compounds with many carbon atoms linked to each other.

color{green}(★) Since carbon has a valency of four, it is capable of bonding with four other atoms of carbon or atoms of some other mono-valent element.

color{green}(★) Carbon forms compounds with oxygen, hydrogen, nitrogen, sulphur, chlorine and many other elements which gives rise to compounds with specific properties which depend on the elements other than carbon present in the molecule.

color{green}(★) The bonds that carbon forms with most other elements are very strong making these compounds exceptionally stable. One reason for the formation of strong bonds by carbon is its small size. This enables the nucleus to hold on to the shared pairs of electrons strongly. The bonds formed by elements having larger atoms are much weaker.

color{red}("JUST FOR CURIOUS")

The two characteristic features seen in carbon, that is, tetravalency and catenation, put together give rise to a large number of compounds. Many have the same non-carbon atom or group of atoms attached to different carbon chains. These compounds were initially extracted from natural substances and it was thought that these carbon compounds or organic compounds could only be formed within a living system. That is, it was postulated that a ‘vital force’ was necessary for their synthesis. Friedrich Wöhler disproved this in 1828 by preparing urea from ammonium cyanate. But carbon compounds, except for oxides of carbon, carbonate and hydrogencarbonate salts continue to be studied under organic chemistry.

### Saturated and Unsaturated Carbon Compounds

All the carbon compounds which contain just carbon and hydrogen are called hydrocarbons.

color{green}("𝐒𝐀𝐓𝐔𝐑𝐀𝐓𝐄𝐃 𝐇𝐘𝐃𝐑𝐎𝐂𝐀𝐑𝐁𝐎𝐍𝐒:")

color{green}(★) The saturated hydrocarbons are hydrocarbons in which carbon atoms are linked together by single bonds only.

color{green}(★) color{red}(underset("Methane " (CH_4))( H - underset(underset(H)(|)) overset(overset(H)(|))C - H)) , color{red}(underset(" Ethane" (C_2 H_6))(H - underset(underset(H)(|)) overset(overset(H)(|))C - underset(underset(H)(|))overset(overset(H)(|))C- H))
 color{green}(★) All saturated hydrocarbons are known as alkanes.

color{green}("𝐔𝐍𝐒𝐀𝐓𝐔𝐑𝐀𝐓𝐄𝐃 𝐇𝐘𝐃𝐑𝐎𝐂𝐀𝐑𝐁𝐎𝐍𝐒:")

color{green}(★) The hydrocarbons containing multiple bonds between two carbon atoms are called unsaturated hydrocarbons.

color{green}(★) Unsaturated hydrocarbons can be further divided into two categories:

color{green}("𝟏 𝐀𝐥𝐤𝐞𝐧𝐞:") Hydrocarbons having atleast one double bond between two carbon atoms are called alkenes.

color{green}("𝟐 𝐀𝐥𝐤𝐲𝐧𝐞𝐬:") Hydrocarbons having atleast one triple bond between two carbon atoms are called alkynes.

color{red}(underset("Ethyne"(C_2H_2))(H - C equiv C - H))

### Chains, Branches and Rings

Formula of saturated compounds of carbon and hydrogen

color{green}("𝐈𝐒𝐎𝐌𝐄𝐑𝐒:") The organic compounds which have same molecular formulae but different structures are called isomers.

color{green}("𝐂𝐘𝐂𝐋𝐈𝐂 𝐒𝐓𝐑𝐔𝐂𝐓𝐔𝐑𝐄:")

color{green}(★) When a series of atoms is connected to form a loop or ring, it leads to the formation of cyclic structure.

color{green}(★) In cycloalkane all the carbon-carbon bonds are single.

color{green}(★) General formulae of cycloalkanes color{red}(C_n H_2 n).

color{green}("𝐁𝐄𝐍𝐙𝐄𝐍𝐄:")

color{green}(★) It is an organic compound with formulae color{red}(C_6H_6).

color{green}(★) Here the carbon atoms are present with alternating single and double bonds.