Which among the following statements with respect to carbon is/are correct ? 1. Carbon forms the basis for all living organisms and many things we use 2. Carbon shows tetra-valency and the property of catenation 3. Carbon forms covalent bonds with itself and other elements 4. Carbon forms compounds containing triple and tetra bonds between carbon atoms Select the correct answer using the code given below :
- (a)1 only
- (b)1 and 2 only
- (c)1, 2 and 3
- (d)2 and 4
Correct — C, 1, 2 and 3. The first three statements are the standard textbook account of carbon and all hold. Carbon does form the basis of every living organism and of an enormous share of the materials in daily use; it is tetravalent, with four valence electrons and therefore four bonds, and it shows catenation, the ability to bond to further carbon atoms and build chains, branched chains and rings; and it bonds covalently both to itself and to hydrogen, oxygen, sulphur, nitrogen and chlorine. Statement 4 is the doctored one. Carbon atoms link by single, double and triple bonds, but a quadruple bond between two carbon atoms is not found in stable compounds, so 'triple and tetra bonds' fails and every option containing statement 4 goes with it.
- (a)1 only — Far too narrow. Statements 2 and 3 are as textbook-correct as statement 1 — tetravalency together with catenation is the standard explanation offered for the sheer number of carbon compounds, and covalent bonding is how carbon achieves it.
- (b)1 and 2 only — This drops statement 3, but covalent bonding is not an optional extra — it is the mechanism by which the tetravalency named in statement 2 is realised, and carbon's covalent bonds to hydrogen, oxygen, nitrogen, sulphur and chlorine are exactly what generate the families of organic compounds.
- (d)2 and 4 — It keeps the one false statement, 4, and simultaneously discards two true ones, 1 and 3. Even a candidate unsure about quadruple bonds should reject this option for throwing away the uncontroversial first statement.
Two properties between them account for the millions of known carbon compounds. Tetravalency means carbon has four valence electrons and forms four covalent bonds, so it can hold four other atoms at once. Catenation means carbon bonds strongly to other carbon atoms, so those four bonds can be spent building long chains, branched chains and rings. Because the carbon-carbon bond is short and strong, the resulting molecules are stable, which is why no other element rivals carbon here — silicon manages chains of only about seven or eight atoms, and those hydrides are very reactive.
This is a doctored-textbook item, and recognising the genre is the fastest route through it. Three of the four statements are lifted almost word for word from the chapter summary on carbon; the fourth takes another summary sentence and changes one word, so that 'double and triple bonds' becomes 'triple and tetra bonds'. The alteration is easy to miss because 'tetra' has just been made familiar by the word tetravalency two statements earlier. Keep the two ideas apart: carbon has a valency of four, meaning four bonds in total to any partners, but between one carbon atom and another the maximum is three.
- Carbon is tetravalent — it has four valence electrons and forms four covalent bonds.
- Catenation is carbon's ability to bond to other carbon atoms, giving straight chains, branched chains and rings.
- Carbon-carbon links may be single, double or triple; a quadruple bond between two carbon atoms is not found in stable compounds.
- Silicon catenates only to about seven or eight atoms and those compounds are very reactive, which is why carbon and not silicon supports so vast a family of compounds.
A carbon atom makes four bonds in all, but to another carbon atom at most three — which is where statement 4 fails.
- Letting the word tetravalency carry over into 'tetra bonds' between two carbon atoms; the two claims are different and only the first is true.
- Rejecting statement 1 as too vague to be examined — it is a verbatim textbook sentence and is marked correct.
- Assuming that a statement combination containing the maximum number of options must be right in a statements question.
Asked as a statements-and-code item built from a chapter summary, with one sentence altered by a single word.
Which one of the following is the correct sequence in increasing order of molecular weights of the hydrocarbons?
- (a) Methane, ethane, propane and butane
- (b) Propane, butane, ethane and methane
- (c) Butane, ethane, propane and methane
- (d) Butane, propane, ethane and methane
Answer(a) Methane, ethane, propane and butane
Shows catenation in action. Each step in this series adds one more carbon atom to the chain, which is only possible because carbon bonds to carbon.
Which one of the following statements is not correct?
- (a) Most carbon compounds are good conductors of electricity.
- (b) Bonding in organic compounds is covalent.
- (c) Graphite is used as a lubricant.
- (d) Diamond is an allotrope of carbon.
Answer(a) Most carbon compounds are good conductors of electricity.
Another NOT-correct item on the same chapter, and it turns on the consequence of statement 3 here — covalent bonding gives molecules with no free ions or electrons, so carbon compounds conduct poorly.
C₄H₈ belongs to the homologous series of
- (a) alkanes
- (b) alkenes
- (c) alkynes
- (d) cycloalkanes
Answer(b) alkenes
Applies the single-double-triple distinction directly — the formula fixes one double bond between two carbon atoms, placing the compound in the alkene series.
- practice — not a real PYQ
The maximum number of bonds that can be formed between two carbon atoms in a stable compound is
- (a)one
- (b)two
- (c)three
- (d)four
Answer(c) three — carbon-carbon links may be single, double or triple, as in ethane, ethene and ethyne, but not quadruple.
- practice — not a real PYQ
The property that allows carbon to form long chains, branched chains and rings of carbon atoms is called
- (a)catenation
- (b)tetravalency
- (c)allotropy
- (d)isomerism
Answer(a) catenation — the ability of an element to bond to further atoms of itself, shown by carbon to a degree no other element matches.