Carbon, Valency & Covalent Bonds – Recap സംയോജകത · സഹസംയോജകബന്ധനം
This whole chapter is about compounds of carbon. To understand them you need four ideas: how the electrons of a carbon atom are arranged, what valency means, why carbon shares electrons instead of giving them away or taking more, and how shared pairs make single, double and triple bonds. You met these ideas in Chapters 2 and 3. This page builds them again from the start, so nothing is assumed.
- An atom has a tiny nucleus (protons and neutrons) with electrons moving in shells around it. The atomic number is the number of protons, which equals the number of electrons in the atom.
- If you can already write carbon as 2,4, say why its valency is 4 and draw H–C≡C–H with every carbon having four bonds, do the sorter and the quiz, then go to page 2.
1. The carbon atom (Table 8.1)
Carbon has the symbol C and atomic number 6, so a carbon atom has 6 electrons. The first shell holds at most 2, so the other 4 go into the second shell. The electron configuration (ഇലക്ട്രോൺ വിന്യാസം) is 2,4. The electrons in the outermost shell decide how an atom combines. Carbon has 4 outer electrons, and a full second shell needs 8 (an octet, അഷ്ടകം).
The valency (സംയോജകത) of an atom is its combining power: how many electrons it gives, takes or shares to get a full outer shell. Carbon needs 4 more electrons, so its valency is 4. Hydrogen (1 electron, needs 1 more to reach 2 like helium) has valency 1. Oxygen (2,6) has valency 2, nitrogen (2,5) has valency 3 and chlorine (2,8,7) has valency 1.
The number of lines from an atom always equals its valency: hydrogen and chlorine 1, oxygen 2, nitrogen 3, carbon 4.
2. Why carbon shares its electrons
Sodium (2,8,1) easily gives away its one outer electron and chlorine (2,8,7) easily takes one; that makes ions and an ionic bond. Carbon is stuck in the middle. To reach a full shell it would have to lose 4 electrons or gain 4, and both are far too hard. So carbon does the third thing: it shares its 4 outer electrons with other atoms. Each shared pair of electrons is a covalent bond (സഹസംയോജകബന്ധനം).
3. Covalent bonds: from electron dots to lines
When two atoms share one pair of electrons they form a single bond (ഏകബന്ധനം), drawn as one line: H–H. Two shared pairs make a double bond (ദ്വിബന്ധനം), two lines: O=O. Three shared pairs make a triple bond (ത്രിബന്ധനം), three lines: N≡N. In every case each atom ends up with a full outer shell: 2 electrons for hydrogen, 8 for the others. A drawing in which every bond is shown as a line is a structural formula (ഘടനാവാക്യം); page 2 is all about it.
Electron colours: carbon amber, hydrogen green, oxygen red, nitrogen blue, chlorine purple. Electrons are all the same; the colours only show where each one came from.
4. Carbon joined to carbon: single, double and triple bonds (Fig 8.2)
Carbon atoms can share electrons with each other, and they can share one, two or three pairs. Whatever the bond between the carbons, each carbon still has exactly four bonds in total. The bonds that are not used between the carbons are free to join other atoms, such as hydrogen. Fig 8.2 of the textbook shows the three possibilities with their free bonds as short open lines.
A double bond counts as 2 of carbon's four bonds and a triple bond as 3. So a carbon with a double bond can hold only 2 more atoms, and a carbon with a triple bond only 1 more. That is why ethene has only four hydrogen atoms (C₂H₄) and ethyne only two (C₂H₂).
Count the bonds at each carbon (single – counts 1, double = counts 2, triple ≡ counts 3) and each hydrogen written next to it.
5. Why carbon is special
Because carbon has four bonds and can bond strongly to other carbon atoms, carbon atoms join into chains of almost any length, chains with branches and closed rings, using single, double or triple bonds. They also bond to hydrogen, oxygen, nitrogen and the halogens. This ability of the atoms of an element to join each other is called catenation (കാറ്റിനേഷൻ); carbon has it more than any other element (page 8). The result: there are about ten times as many carbon compounds as compounds of all the other elements put together.
Silicon is in the same group as carbon and also has 4 outer electrons, but silicon–silicon bonds are much weaker, so silicon forms only short chains. In nature silicon is found joined to oxygen (sand is SiO₂), not in long chains of its own atoms.
- Carbon: symbol C, atomic number 6, electron configuration 2,4, valency 4 (Table 8.1).
- Losing or gaining 4 electrons is too hard, so carbon shares electrons: it forms covalent bonds (സഹസംയോജകബന്ധനം).
- One shared pair = single bond (–), two pairs = double bond (=), three pairs = triple bond (≡).
- Every carbon atom has exactly four bonds: a double bond counts 2, a triple bond 3.
- Carbon atoms join each other into chains, branches and rings: catenation (കാറ്റിനേഷൻ).
- Drawing a carbon with 3 or 5 bonds ✗ – always count: exactly 4.
- Writing carbon's configuration as 2,8,4 or 4,2 ✗ – 6 electrons: 2,4.
- “A double bond is one bond, so it counts 1” ✗ – it is two shared pairs and counts 2.
- “Carbon forms C⁴⁺ ions” ✗ – in its compounds in this chapter carbon only shares electrons.