Atomic number 6 (2, 4): tetravalent (valency 4). It forms covalent bonds by sharing electrons.
Catenation: carbon atoms bond to each other to form long chains, branches and rings. Tetravalency and catenation give millions of compounds. Carbon forms single, double and triple bonds.
Covalent compounds:low melting and boiling points, usually non-conductors, soluble in organic solvents.
Allotropes of carbon
Allotrope
Structure and properties
Diamond
Each carbon bonded to four others, a rigid 3-D lattice; hardest natural substance; very high refractive index; poor conductor
Graphite
Layers of hexagons; soft, slippery; good conductor; used in pencils, electrodes and lubricants (not lead)
Fullerene
C-60, football-shaped; found by Kroto, Smalley, Curl
Graphene
A single layer of graphite; very strong and conductive
Saturated hydrocarbons burn with a clean blue flame; unsaturated ones burn with a yellow, sooty flame.
Methane is the main component of natural gas, biogas and marsh gas; LPG is propane and butane; CNG is mostly methane.
Homologous series: a family of compounds with the same functional group, whose members differ by a CH2 unit (mass difference 14 u). Properties change gradually.
Functional group: an atom or group that decides the compound's properties.
Functional group
Name
Example
-OH
Alcohol
Methanol, ethanol
-CHO
Aldehyde
Methanal, ethanal
>C=O
Ketone
Propanone (acetone)
-COOH
Carboxylic acid
Ethanoic (acetic) acid
Halogen (-Cl, -Br)
Halo-alkane
Chloromethane
IUPAC naming: prefix words meth-, eth-, prop-, but-, pent-, hex- (1 to 6 carbons); endings: -ane (alkane), -ene (alkene), -yne (alkyne), -ol (alcohol), -al (aldehyde), -one (ketone), -oic acid (carboxylic acid). Isomers have the same molecular formula but different structures (butane and isobutane).
Chemical properties
Combustion:CH4 + 2O2 -> CO2 + 2H2O + heat. Incomplete combustion gives carbon monoxide and soot.
Oxidation: ethanol is oxidised to ethanoic acid by alkaline potassium permanganate or acidified potassium dichromate.
Addition reaction: unsaturated hydrocarbons add hydrogen in the presence of nickel catalyst, giving saturated ones. This is the basis of hydrogenation of vegetable oil into vanaspati ghee.
Substitution reaction: a hydrogen atom is replaced (alkanes with chlorine in sunlight).
With sodium gives hydrogen; dehydration with conc. H2SO4 gives ethene
Reacts with carbonates to release CO2; with alcohol forms an ester (sweet smell)
Denatured alcohol is poisonous ethanol made unfit to drink (with methanol and a dye). Methanol is poisonous; it can cause blindness and death.
Pure ethanoic acid freezes at 16.6 C and is called glacial acetic acid.
Esterification: acid + alcohol, in the presence of concentrated sulphuric acid, gives an ester + water. Esters smell fruity (perfumes, flavours).
Fermentation:yeast converts sugar into ethanol and CO2.
Soaps and detergents
Soap:sodium or potassium salts of long-chain fatty acids, made by saponification (fat or oil + NaOH). Glycerol is a by-product.
Micelle: soap molecules have a water-loving (hydrophilic) head and an oil-loving (hydrophobic) tail; they form spherical micelles around dirt.
Soaps do not work well in hard water (they form scum); detergents (sulphonate or sulphate salts) work in hard water. Some detergents are not biodegradable and pollute water.
Prepared for exam revision from the NCERT Science textbooks for Classes 6 to 10 and standard sources. Space missions, awards and records change, so check official sources for recent facts. Found a mistake? Tell us.