Carbon and its Compounds
Carbon's tetravalency and catenation, expressed through covalent bonding, let it form millions of stable compounds that underpin all life and most materials we use.
Science & Technology fundamentals recur in Prelims, where carbon's allotropes (diamond, graphite, fullerene C-60), CNG/biogas chemistry and the saturated–unsaturated distinction are favourite factual hooks. For GS-III, this base supports questions on energy (natural gas, hydrocarbons), advanced/nano materials and industrial chemistry. Even basic chemistry questions in CSAT/Prelims draw on covalent vs ionic properties.
Understand the chapter
Carbon — Scarce in Nature, Central to Life
Although carbon is only 0.02% of the earth's crust and 0.03% of the atmosphere, it is the backbone of all living structures and of everyday materials like food, clothes, medicines and fuels. Its outsized importance comes from how it bonds, not from its abundance. Burning a carbon compound yields carbon dioxide, the standard test confirming carbon's presence.
- Crust: 0.02% carbon as carbonates, hydrogencarbonates, coal, petroleum
- Atmosphere: 0.03% as carbon dioxide
- All living structures are carbon-based
- Test: combustion of a carbon compound produces CO2
The Covalent Bond — Carbon Shares, Never Transfers
Carbon (atomic number 6) has four valence electrons. A 6-proton nucleus cannot easily hold the four extra electrons of a C4- anion, and forming a C4+ cation needs enormous energy. Carbon resolves this by sharing electron pairs with other atoms, so both atoms attain a noble-gas configuration — this shared-pair link is the covalent bond.
- Carbon is tetravalent: 4 valence electrons
- Both C4- and C4+ formation are energetically unfavourable
- Sharing pairs lets both atoms reach octet/noble-gas configuration
- Shared electrons belong to the outer shells of both atoms
Single, Double and Triple Bonds
The number of shared electron pairs decides the bond type. Hydrogen (H2) shares one pair (single bond), oxygen (O2) shares two pairs (double bond) and nitrogen (N2) shares three pairs (triple bond). Methane (CH4), one of carbon's simplest compounds, forms four single C-H bonds and is a major component of biogas and CNG.
- Single bond = 1 shared pair (H2), shown by a line
- Double bond = 2 shared pairs (O2)
- Triple bond = 3 shared pairs (N2)
- Methane CH4: tetravalent carbon shares with 4 hydrogen atoms
Why Covalent Compounds Behave Differently
Carbon compounds typically have low melting and boiling points and are poor conductors of electricity. The bonds within a molecule are strong, but the forces between molecules are weak, so little energy is needed to pull molecules apart. Because no ions or charged particles form, covalent compounds do not conduct like ionic compounds, which conduct in molten or dissolved states.
- Strong intramolecular bonds, weak intermolecular forces
- Low melting/boiling points (e.g., methane melts at 90 K)
- Poor electrical conductivity — no ions produced
- Contrast: ionic compounds have high m.p./b.p. and conduct in solution/molten state
Allotropes of Carbon
The same carbon atoms, bonded differently, give allotropes with very different physical but identical chemical properties. In diamond each carbon bonds to four others in a rigid three-dimensional lattice, making it the hardest known substance; in graphite each carbon bonds to three others in hexagonal layers, making it soft, slippery and a good conductor. Fullerenes — first identified as C-60 — arrange carbon in a football/geodesic-dome shape.
- Diamond: each C bonded to 4 C, rigid 3-D, hardest substance, non-conductor
- Graphite: each C bonded to 3 C, hexagonal layers, conducts electricity, slippery
- Fullerene (C-60/Buckminsterfullerene): football shape, named after architect Buckminster Fuller
- Synthetic diamonds: pure carbon under very high pressure and temperature
Versatility — Catenation and Tetravalency
Two properties explain why carbon forms millions of compounds, more than all other elements combined. Catenation is carbon's unique ability to bond with other carbon atoms, building long chains, branches and rings linked by single, double or triple bonds. Tetravalency lets carbon bond with four other atoms at once, and its small atomic size makes these bonds exceptionally strong and stable.
- Catenation: self-linking into chains, branches and rings
- Tetravalency: bonds with up to 4 atoms (C, O, H, N, S, Cl, etc.)
- Small atomic size lets the nucleus hold shared pairs strongly, giving strong bonds
- Silicon catenates only up to 7-8 atoms and is very reactive
Organic Compounds and Hydrocarbon Families
Carbon compounds were once thought to need a 'vital force' from living systems, until Friedrich Wohler synthesised urea from ammonium cyanate in 1828. Hydrocarbons (only C and H) divide into saturated alkanes (single bonds) and unsaturated alkenes (double bonds) and alkynes (triple bonds); unsaturated compounds are more reactive. Compounds with the same molecular formula but different structures are structural isomers.
- Saturated = single bonds only (alkanes, e.g., ethane C2H6); less reactive
- Unsaturated = double/triple bonds (alkene ethene C2H4, alkyne ethyne C2H2); more reactive
- Structural isomers: same formula, different structure (butane C4H10)
- Rings: cyclohexane C6H12 and benzene C6H6
Key terms
- Covalent bond
- A bond formed by sharing an electron pair between two atoms so both attain noble-gas configuration.
- Catenation
- Carbon's ability to bond with other carbon atoms, forming long chains, branches and rings.
- Tetravalency
- Carbon's capacity to bond with four other atoms, due to its four valence electrons.
- Allotropes
- Different physical forms of the same element, e.g., diamond, graphite and fullerene of carbon.
- Saturated compounds
- Carbon compounds containing only single carbon-carbon bonds (alkanes); generally less reactive.
- Unsaturated compounds
- Carbon compounds with double or triple carbon-carbon bonds (alkenes/alkynes); more reactive.
- Hydrocarbons
- Compounds made up of only carbon and hydrogen.
- Structural isomers
- Compounds with the same molecular formula but different structural arrangements.
- Fullerene
- A carbon allotrope with atoms in a closed football-like cage; the first identified was C-60.
- Organic compounds
- Carbon compounds (except carbides, oxides of carbon, carbonates and hydrogencarbonates) studied under organic chemistry.
Must-know facts exam-ready
- Atomic number of carbon is 6; it has 4 valence electrons and is tetravalent
- Earth's crust holds 0.02% carbon; the atmosphere holds 0.03% carbon dioxide
- Diamond is the hardest known substance; graphite conducts electricity though it is a non-metal
- In diamond each carbon bonds to 4 others (3-D); in graphite to 3 others in hexagonal layers
- Fullerene C-60 was the first identified; named after US architect Buckminster Fuller
- Friedrich Wohler prepared urea from ammonium cyanate in 1828, disproving the 'vital force' theory
- Methane (CH4) is a major component of biogas and CNG
- Alkanes = saturated (single bonds); alkenes = double bonds; alkynes = triple bonds
- H2 has a single bond, O2 a double bond, N2 a triple bond
- Known carbon compounds number in the millions — more than all other elements combined
- Silicon catenates only up to 7-8 atoms and those compounds are very reactive
- First six alkanes: methane CH4, ethane C2H6, propane C3H8, butane C4H10, pentane C5H12, hexane C6H14
Memory tricks remember it for good
Traps to avoid
- Diamond and graphite have identical chemical properties but very different physical properties — both are pure carbon, not different elements
- Graphite conducts electricity even though it is a non-metal, while diamond does NOT conduct — students often reverse this
- Catenation is not exclusive to carbon — silicon also catenates, but only up to 7-8 atoms and very reactively
- 'Saturated' means single bonds (alkanes), not 'highly reactive' — saturated compounds are actually less reactive than unsaturated ones
- Covalent compounds have low (not high) melting/boiling points and are poor conductors — the opposite of ionic compounds
- Carbon does not form C4+ or C4- ions; it shares electrons (covalent), it does not transfer them
Exam focus
🧠 Prelims angles
- Allotropes of carbon and their distinguishing properties (diamond hardest, graphite conducts, fullerene C-60)
- Methane as the main component of biogas and CNG; hydrocarbon fuels
- Telling alkanes/alkenes/alkynes apart by bond type and reactivity
- Covalent vs ionic compound properties — melting point and electrical conductivity
- Wohler's 1828 urea synthesis and the fall of the 'vital force' theory
- Catenation and tetravalency as the basis of carbon's versatility
✍️ Mains angles GS-III
- Carbon-based advanced materials (fullerenes, nanotubes, graphene) and their strategic value for India's technology and energy sectorsExtend the chapter's allotrope chemistry to GS-III S&T applications in electronics, energy storage and defence materials
- Hydrocarbons and India's energy transition — the role of natural gas (CNG) and biogas as cleaner fuelsUse methane's role in CNG/biogas to discuss the gas-based economy, emissions and cleaner-fuel policy
- Carbon chemistry, combustion and the atmospheric carbon balanceLink the 0.03% atmospheric CO2 and combustion of carbon compounds to environmental and climate themes
Last-minute revision tick as you recall
- Carbon: atomic number 6, tetravalent, bonds by sharing electrons (covalent), never by transfer
- Catenation + tetravalency = millions of carbon compounds, most of any element
- Single/double/triple bond = 1/2/3 shared pairs (H2 / O2 / N2)
- Allotropes: diamond (hard, 4 bonds), graphite (conducts, 3 bonds), fullerene (C-60)
- Covalent compounds: low melting/boiling points, poor conductors, no ions
- Saturated = alkanes (single, less reactive); unsaturated = alkenes/alkynes (more reactive)
- Hydrocarbons = C + H only; structural isomers = same formula, different structure
- Wohler 1828: urea from ammonium cyanate ended the 'vital force' theory
- Crust 0.02% C, atmosphere 0.03% CO2; methane is key in CNG and biogas
Distilled from NCERT Class 10 · Science (Class 10) for UPSC. Always cross-check facts with the original NCERT.