Class 10 Science · Chapter 4

Carbon and its Compounds

442 Class 10 Science questions on Carbon and its Compounds from the NCERT textbook, NCERT Exemplar and CBSE board papers 2017–2026, each with its answer and a step-by-step solution.

CBSE Class 10 Science previous year papers, with answers →

Questions · page 13 of 23

1. Q241 CBSE 2023
   1. (i)Name the compound formed when ethanol is heated at 443 K in the presence of conc. ${\text{H}}_2{\text{SO}}_4$ and draw ​its electron dot structure. State the role of conc. ${\text{H}}_2{\text{SO}}_4$ in this reaction.
   2. (ii)What ​is ​hydrogenation ? Explain it with the ​help of a chemical equation. State the ​role of ​this reaction in industry.
   Show answer

   Ethene forms, with conc. ${\text{H}}_2{\text{SO}}_4$ as dehydrating agent; hydrogenation adds hydrogen to unsaturated compounds and hardens vegetable oils into vanaspati ghee.

   (i) The compound formed is ethene (ethylene), ${\text{CH}}_2={\text{CH}}_2$: ${\text{C}}_2{\text{H}}_5\text{OH}\underset{443\text{K}}{\overset{{\text{conc. H}}_2{\text{SO}}_4}{\to }}{\text{CH}}_2={\text{CH}}_2+{\text{H}}_2\text{O}$. In its electron dot structure each carbon shares one electron pair with each of its two hydrogen atoms and **two** pairs with the other carbon, which is the double bond (see the figure). The concentrated ${\text{H}}_2{\text{SO}}_4$ acts as a dehydrating agent — it removes a molecule of water from ethanol. (ii) Hydrogenation is the addition of hydrogen to an unsaturated hydrocarbon in the presence of a catalyst such as nickel or palladium, converting it into a saturated hydrocarbon: ${\text{CH}}_2={\text{CH}}_2+{\text{H}}_2\stackrel{\text{Ni}}{\to }{\text{CH}}_3-{\text{CH}}_3$. In industry it is used to harden unsaturated vegetable oils into solid saturated fats, which is how vanaspati ghee is made.

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2. Q242 CBSE 2023
   (a) How do the following conversions take place ? Write chemical equation for each : (i) Ethanol ​to ​Ethanoic acid (ii) ​Ethene to Ethane (b) Give one ​example each with ​chemical equation for the following ​reactions :

   1. (i)Combustion reaction
   2. (ii)Substitution reaction
   3. (iii)​Saponification reaction
   Show answer

   Oxidation gives ethanoic acid, addition of hydrogen gives ethane; methane shows combustion and substitution, and ethyl ethanoate shows saponification.

   (a)(i) Ethanol is **oxidised** to ethanoic acid by warming it with an oxidising agent such as alkaline potassium permanganate (or acidified potassium dichromate): ${\text{CH}}_3{\text{CH}}_2\text{OH}+2[\text{O}]\underset{\Delta }{\overset{{\text{alk. KMnO}}_4}{\to }}{\text{CH}}_3\text{COOH}+{\text{H}}_2\text{O}$. (a)(ii) Ethene is converted to ethane by an **addition** of hydrogen in the presence of a nickel catalyst: ${\text{CH}}_2={\text{CH}}_2+{\text{H}}_2\underset{573\text{K}}{\overset{\text{Ni}}{\to }}{\text{CH}}_3-{\text{CH}}_3$. (b)(i) Combustion — methane burns in air: ${\text{CH}}_4+2{\text{O}}_2⟶{\text{CO}}_2+2{\text{H}}_2\text{O}+\text{heat and light}$. (b)(ii) Substitution — methane reacts with chlorine in sunlight: ${\text{CH}}_4+{\text{Cl}}_2\stackrel{\text{sunlight}}{\to }{\text{CH}}_3\text{Cl}+\text{HCl}$. (b)(iii) Saponification — ethyl ethanoate is heated with sodium hydroxide: ${\text{CH}}_3{\text{COOC}}_2{\text{H}}_5+\text{NaOH}⟶{\text{CH}}_3\text{COONa}+{\text{C}}_2{\text{H}}_5\text{OH}$.

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3. Q243 CBSE 2023
   1. (i)What is a homologous series ​of ​carbon compounds? Write general formula for alkynes. Name ​and draw the ​electron dot structure of first homologue ​of this series.
   2. (ii)State the meaning of the functional group in an organic compound. Write the ​formula of the functional group present in alcohols and carboxylic acids.
   Show answer

   Alkynes: ${\text{C}}_n{\text{H}}_{2n-2}$; first member ethyne, ${\text{C}}_2{\text{H}}_2$, has five shared pairs; the functional groups are $-\text{OH}$ in alcohols and $-\text{COOH}$ in carboxylic acids.

   (i) A homologous series is a family of carbon compounds that have the same functional group (for hydrocarbons, the same kind of bond between carbons) and the same general formula, in which each member differs from the next by a $-{\text{CH}}_2-$ unit, that is, by $14$ u in molecular mass; the members show similar chemical properties and a gradual change in physical properties. The general formula of alkynes is ${\text{C}}_n{\text{H}}_{2n-2}$. The first member is ethyne (acetylene), ${\text{C}}_2{\text{H}}_2$, $\text{H}-\text{C}\equiv \text{C}-\text{H}$. In its electron dot structure each carbon shares one electron pair with a hydrogen atom and three electron pairs with the other carbon, so there are five shared pairs in all, as drawn in the figure. (ii) A functional group is an atom or a group of atoms, such as a heteroatom like oxygen or a group containing it, that gives a carbon compound its characteristic chemical properties whatever the length and nature of its carbon chain. Alcohols: $-\text{OH}$ (hydroxyl group). Carboxylic acids: $-\text{COOH}$ (carboxyl group).

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4. Q244 CBSE 2023
   1. (i)​It is observed that covalent compounds are bad conductors of ​electricity. ​Give reason.
   2. (ii)​Carbon can neither form ${\text{C}}^{4+}$ cation nor ${\text{C}}^{4-}$ anion. Why ?
   3. (iii)Draw ​electron dot structure of Ethanol.
   4. (iv)​Identify hetero atom(s) in the following ​compounds : (a) ${\text{CH}}_3{\text{CH}}_2-\underset{\underset{\text{O}}{\Vert }}{C}-{\text{CH}}_3$ (b) ${\text{CH}}_3{\text{CH}}_2\text{Cl}$
   Show answer

   Covalent compounds contain no free ions; carbon shares electrons rather than forming ${\text{C}}^{4+}$ or ${\text{C}}^{4-}$; the hetero atoms are O and Cl.

   (i) In covalent compounds the electrons are shared between atoms and no charged particles (ions) are formed; the shared electrons stay in the bonds. With no charged particles free to move, covalent compounds are generally bad conductors of electricity. (ii) Carbon (2, 4) would have to gain or lose four electrons. To form ${\text{C}}^{4-}$, a nucleus of six protons would have to hold ten electrons, which is difficult. To form ${\text{C}}^{4+}$, four electrons would have to be removed, which needs a large amount of energy and leaves six protons holding just two electrons. So carbon shares its valence electrons instead, forming covalent bonds. (iii) Ethanol, ${\text{CH}}_3{\text{CH}}_2\text{OH}$: each carbon shares four electron pairs (with three H and one C, and with one C, two H and the O); the oxygen shares one pair with carbon and one with hydrogen and keeps two lone pairs; each hydrogen shares one pair. In all, eight shared pairs and two lone pairs, twenty electrons (see the structure drawn). (iv) (a) Oxygen (O), in the ketone group C=O. (b) Chlorine (Cl).

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5. Q245 CBSE 2023
   Write the ​chemical equation ​for ​the following ​:

   1. (i)Combustion ​of methane
   2. (ii)Oxidation of ethanol
   3. (iii)Hydrogenation of ethene
   4. (iv)​Esterification Reaction
   5. (v)Saponification ​Reaction
   Show answer

   Methane burns to carbon dioxide and water; ethanol oxidises to ethanoic acid; ethene is hydrogenated; an ester forms by esterification and splits by saponification.

   (i) Combustion of methane: ${\text{CH}}_4+2{\text{O}}_2\to {\text{CO}}_2+2{\text{H}}_2\text{O}+\text{heat and light}$ (ii) Oxidation of ethanol: ${\text{CH}}_3{\text{CH}}_2\text{OH}\stackrel{{\text{alkaline KMnO}}_4+\text{heat}}{\to }{\text{CH}}_3\text{COOH}$ (or with acidified potassium dichromate and heat); balanced with the oxygen from the oxidising agent: ${\text{CH}}_3{\text{CH}}_2\text{OH}+2[\text{O}]\to {\text{CH}}_3\text{COOH}+{\text{H}}_2\text{O}$ (iii) Hydrogenation of ethene: ${\text{CH}}_2={\text{CH}}_2+{\text{H}}_2\stackrel{\text{Ni catalyst}}{\to }{\text{CH}}_3-{\text{CH}}_3$ (iv) Esterification (ethanoic acid + ethanol gives ethyl ethanoate + water): ${\text{CH}}_3\text{COOH}+{\text{C}}_2{\text{H}}_5\text{OH}\stackrel{{\text{conc. H}}_2{\text{SO}}_4}{\to }{\text{CH}}_3{\text{COOC}}_2{\text{H}}_5+{\text{H}}_2\text{O}$ (v) Saponification (the ester is split by sodium hydroxide into sodium ethanoate and ethanol): ${\text{CH}}_3{\text{COOC}}_2{\text{H}}_5+\text{NaOH}\to {\text{CH}}_3\text{COONa}+{\text{C}}_2{\text{H}}_5\text{OH}$; with a fat or oil the same reaction gives soap and glycerol.

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6. Q246 CBSE 2023
   (i) Butane had both Carbon – Carbon bonds as ​well as Carbon – ​Hydrogen bonds. Draw its structural ​formula and state the ​number of

   1. (1)C – H bonds and
   2. (2)C – C bonds in it. (ii) You have two carbon compounds with the molecular formula ${\text{C}}_3{\text{H}}_6\text{O}$. Name two compounds with this formula ​and ​also draw ​their ​structural formula.
   Show answer

   Butane has 10 C-H bonds and 3 C-C bonds; propanal and propanone both have the formula ${\text{C}}_3{\text{H}}_6\text{O}$.

   (i) Structural formula of butane, ${\text{C}}_4{\text{H}}_{10}$: ${\text{CH}}_3-{\text{CH}}_2-{\text{CH}}_2-{\text{CH}}_3$, drawn in full as four carbon atoms in a chain, each end carbon carrying three hydrogen atoms and each middle carbon two (see the structure). (1) Number of C–H bonds $=10$; (2) number of C–C bonds $=3$. (ii) Two compounds with the molecular formula ${\text{C}}_3{\text{H}}_6\text{O}$ are propanal, ${\text{CH}}_3-{\text{CH}}_2-\text{CHO}$ (an aldehyde), and propanone (acetone), ${\text{CH}}_3-\text{CO}-{\text{CH}}_3$ (a ketone); their full structural formulae are drawn with every bond shown. They are isomers of each other.

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7. Q247 CBSE 2023
   Explain why carbon forms compounds mainly ​by covalent bonds. Explain in brief two main reasons for carbon forming ​a large number of ​compounds. Why ​does carbon form ​strong ​bonds with most other elements ​?
   Show answer

   Carbon shares electrons because gaining or losing four is difficult; catenation and tetravalency give many compounds; the small carbon atom forms strong bonds.

   Covalent bonding: carbon (atomic number $6$, configuration $2,4$) has four valence electrons and would have to gain or lose four electrons to reach a noble-gas configuration. Gaining four to form ${\text{C}}^{4-}$ is difficult, because a nucleus with six protons would have to hold ten electrons; losing four to form ${\text{C}}^{4+}$ would need a large amount of energy, leaving six protons holding only two electrons. So carbon shares its four valence electrons with other atoms and forms covalent bonds. Two main reasons for the large number of carbon compounds: (i) Catenation: carbon atoms link with other carbon atoms by covalent bonds to form long chains, branched chains and rings, with single, double or triple bonds between them. (ii) Tetravalency: with a valency of four, each carbon atom can bond with four other atoms of carbon or of other elements such as hydrogen, oxygen, nitrogen, sulphur and chlorine. Strong bonds: the carbon atom is small, so its nucleus holds the shared pairs of electrons strongly; this makes carbon's bonds with most other elements strong and its compounds exceptionally stable.

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8. Q248 CBSE 2023
   1. (i)​What ​are ​soaps ? Explain the mechanism ​of cleansing ​action of soap with the help of a labelled diagram.
   2. (ii)Detergents are better than soaps. ​Justify.
   Show answer

   Soaps are sodium or potassium salts of long-chain carboxylic acids that clean by forming micelles around oily dirt; detergents work in hard water.

   (i) Soaps are sodium or potassium salts of long-chain carboxylic acids, for example sodium stearate, ${\text{C}}_{17}{\text{H}}_{35}\text{COONa}$. A soap molecule has an ionic end, $-{\text{COO}}^{-}{\text{Na}}^{+}$, which is hydrophilic and dissolves in water, and a long hydrocarbon tail, which is hydrophobic and dissolves in oil. Most dirt is oily and does not dissolve in water. In soapy water the soap molecules form micelles around the oily dirt: the hydrocarbon tails point into the oil and the ionic ends face the water, so the dirt is held at the centre of the micelle. The micelles stay suspended in the water as an emulsion, kept apart by the repulsion between their like-charged surfaces, and when the cloth is rinsed they carry the dirt away. (Labelled diagram: soap molecule with ionic head and hydrocarbon tail; micelle with oily dirt at the centre, tails inside and heads facing the water.) (ii) Hard water contains calcium and magnesium ions. Soap reacts with these to form an insoluble scum, $2{\text{C}}_{17}{\text{H}}_{35}\text{COONa}+{\text{Ca}}^{2+}\to ({\text{C}}_{17}{\text{H}}_{35}\text{COO}{)}_2\text{Ca}+2{\text{Na}}^{+}$, so a lot of soap is wasted before it lathers. Detergents, which are generally sodium salts of sulphonic acids or ammonium salts with chloride or bromide ions, have charged ends that do not form insoluble precipitates with calcium and magnesium ions. They therefore remain effective in hard water, which is why detergents are better than soaps for washing in hard water.

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9. Q249 CBSE 2023
   1. (i)Draw two structural isomers of butane.
   2. (ii)Draw the ​structures of propanol and propanone.
   3. (iii)​Name the third homologue ​of : (a) alcohols (b) aldehydes (iv) Name the ​following : (a) [the ​structure ​shown in the figure] (b) ${\text{CH}}_3-{\text{CH}}_2\text{CH}={\text{CH}}_2$ (v) ​Show ​the covalent bond formation in nitrogen molecule.

   Show answer

   Isomers n-butane and isobutane; third homologues propanol and propanal; benzene and but-1-ene; nitrogen forms a triple bond sharing three electron pairs.

   (i) The two structural isomers of butane, ${\text{C}}_4{\text{H}}_{10}$, are n-butane, ${\text{CH}}_3-{\text{CH}}_2-{\text{CH}}_2-{\text{CH}}_3$ (a straight chain), and isobutane (2-methylpropane), ${\text{CH}}_3-\text{CH}({\text{CH}}_3)-{\text{CH}}_3$ (a branched chain); their full structural formulae are drawn in the figure. (ii) Propanol: ${\text{CH}}_3-{\text{CH}}_2-{\text{CH}}_2-\text{OH}$; propanone: ${\text{CH}}_3-\text{CO}-{\text{CH}}_3$, with the oxygen double-bonded to the middle carbon (see the figure). (iii) (a) Propanol, ${\text{C}}_3{\text{H}}_7\text{OH}$ (after methanol and ethanol); (b) propanal, ${\text{C}}_2{\text{H}}_5\text{CHO}$ (after methanal and ethanal). (iv) (a) Benzene, ${\text{C}}_6{\text{H}}_6$; (b) but-1-ene (1-butene), ${\text{C}}_4{\text{H}}_8$. (v) Each nitrogen atom (2, 5) has five valence electrons and needs three more. The two atoms share three pairs of electrons, forming a triple bond, $\text{N}\equiv \text{N}$, and each keeps one unshared pair, so each is surrounded by eight electrons (see the electron dot figure).

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10. Q250 CBSE 2023
    (i) What is saponification ? Differentiate ​between soaps and detergents on the basis of the following ​:

    1. (1)​Their chemical composition
    2. (2)Their mechanism in hard water (ii) Explain ​the formation of micelles between oily ​dirt and ​soap molecules. Also draw its diagram.
    Show answer

    Saponification is alkaline hydrolysis of an ester; soaps form scum in hard water, detergents do not; soap molecules trap oily dirt in micelles.

    (i) Saponification is the alkaline hydrolysis of an ester: an ester heated with an alkali such as sodium hydroxide gives an alcohol and the sodium salt of the carboxylic acid, for example ${\text{CH}}_3{\text{COOC}}_2{\text{H}}_5+\text{NaOH}⟶{\text{CH}}_3\text{COONa}+{\text{C}}_2{\text{H}}_5\text{OH}$. When the ester is a fat or oil, it gives glycerol and soap, which is why the reaction is used to make soap. (1) Chemical composition: soaps are sodium or potassium salts of long-chain carboxylic acids (ionic end $-{\text{COO}}^{-}$); detergents are generally sodium salts of sulphonic acids (ionic end such as $-{\text{SO}}_3^{-}$) or ammonium salts with chloride or bromide ions, also with long hydrocarbon chains. (2) Mechanism in hard water: soap reacts with the calcium and magnesium ions of hard water to form an insoluble precipitate (scum), so it lathers poorly and more soap is needed; the charged ends of detergents do not form insoluble precipitates with calcium and magnesium ions, so detergents remain effective in hard water. (ii) A soap molecule has an ionic (hydrophilic) end and a long hydrocarbon (hydrophobic) tail. In water, the tails of many soap molecules dissolve in the oily dirt while their ionic ends stay in the water, forming a cluster, a micelle, with the dirt at its centre and the ionic ends on its surface. The micelles stay suspended in the water, because their charged surfaces repel one another, so the dirt is rinsed away with the water. Diagram: a drop of oily dirt surrounded by soap molecules with their tails in the oil and their ionic heads outwards, in the water.

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11. Q251 CBSE 2023
    1. (i)Write ​the ​name and general formula of a chain of hydrocarbons in which an addition reaction with hydrogen can take place. Stating the essential condition required for an ​addition reaction to occur, ​write the chemical equation giving the name ​of ​the reactant and product of such a reaction. How ​is an addition reaction different from a substitution reaction ?
    2. (ii)Write the structure of benzene.
    Show answer

    Alkenes, ${\text{C}}_n{\text{H}}_{2n}$, add hydrogen with a nickel catalyst (ethene gives ethane); benzene is a six-carbon ring with alternate single and double bonds.

    (i) Name: alkenes, the unsaturated hydrocarbons with a carbon-carbon double bond; general formula ${\text{C}}_n{\text{H}}_{2n}$. Essential condition: the hydrocarbon must be unsaturated, with a double (or triple) bond between carbon atoms, and the addition of hydrogen is carried out in the presence of a catalyst such as nickel or palladium. Equation: ${\text{CH}}_2={\text{CH}}_2$ (ethene) $+{\text{H}}_2\stackrel{\text{Ni}}{\to }{\text{CH}}_3-{\text{CH}}_3$ (ethane). Difference: in an addition reaction atoms are added across the double or triple bond of an unsaturated compound and a single product is formed; in a substitution reaction an atom of a saturated compound is replaced by another atom, as in ${\text{CH}}_4+{\text{Cl}}_2\stackrel{\text{sunlight}}{\to }{\text{CH}}_3\text{Cl}+\text{HCl}$. (ii) Benzene, ${\text{C}}_6{\text{H}}_6$: a hexagonal ring of six carbon atoms with alternate single and double bonds, each carbon carrying one hydrogen atom (see the figure).

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12. Q252 CBSE 2022
    (a) Write the molecular formula of ​the ​following carbon compounds :

    1. (i)​Methane
    2. (ii)Propane (b) Carbon ​compounds have low ​melting and boiling ​points. Why ​?
    Show answer

    Methane is ${\text{CH}}_4$, propane is ${\text{C}}_3{\text{H}}_8$; weak forces between molecules give low melting and boiling points.

    (a) (i) Methane: ${\text{CH}}_4$ (ii) Propane: ${\text{C}}_3{\text{H}}_8$ (b) Most carbon compounds, such as methane, ethanol and chloroform, are covalent compounds made of separate molecules. The atoms inside each molecule are held by strong covalent bonds, but the forces of attraction between the molecules are weak. Melting or boiling only has to separate the molecules from one another, so little heat energy is needed and the melting and boiling points are low, much lower than those of ionic compounds, whose oppositely charged ions attract one another strongly.

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13. Q253 CBSE 2022
    “Carbon ​prefers to share its valence electrons with ​other atoms of carbon ​or with atoms ​of other elements rather than ​gaining or ​losing the valence electrons in order to attain ​noble gas configuration.” ​Give reasons to justify this statement.
    Show answer

    Gaining or losing four electrons is unfavourable for carbon, so it shares its four valence electrons and forms covalent bonds.

    Carbon (atomic number $6$, configuration $2,4$) has four valence electrons, and reaching a noble gas configuration by forming an ion is unfavourable either way: (1) Gaining four electrons would give the ${\text{C}}^{4-}$ anion, but it would be difficult for a nucleus with six protons to hold on to ten electrons, that is, four extra electrons. (2) Losing four electrons would give the ${\text{C}}^{4+}$ cation, but that would require a large amount of energy, leaving six protons holding on to just two electrons. So carbon shares its four valence electrons with other carbon atoms or with atoms of other elements, forming covalent bonds (as in ${\text{CH}}_4$), and attains the noble gas configuration without forming an ion.

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14. Q254 CBSE 2022
    Draw ​the electron dot structure of the molecules of ​

    1. (a)Oxygen, and
    2. (b)Nitrogen. ​The ​atomic ​numbers ​of oxygen and nitrogen are 8 ​and 7 respectively.
    Show answer

    ${\text{O}}_2$ has a double bond (two shared pairs); ${\text{N}}_2$ has a triple bond (three shared pairs); every atom has eight electrons.

    (a) Oxygen, ${\text{O}}_2$: each O atom (2, 6) has six valence electrons and needs two more, so the two atoms share two pairs of electrons, a double bond, $\text{O}=\text{O}$. In the electron dot structure the two shared pairs sit between the two O symbols and each O keeps two lone pairs, so each O is surrounded by $4+4=8$ electrons. (b) Nitrogen, ${\text{N}}_2$: each N atom (2, 5) has five valence electrons and needs three more, so the two atoms share three pairs of electrons, a triple bond, $\text{N}\equiv \text{N}$. The three shared pairs sit between the two N symbols and each N keeps one lone pair, so each N is surrounded by $2+6=8$ electrons. The figure shows both structures, with one atom's electrons as dots and the other's as crosses.

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15. Q255 CBSE 2022
    What is ​a functional group ? ​Write the ​formula ​of functional group ​

    1. (a)alcohol, and ​
    2. (b)ketone.
    Show answer

    A functional group gives a compound its characteristic properties; alcohol is $-\text{OH}$ and ketone is $>\text{C}=\text{O}$.

    A functional group is an atom, or a group of atoms containing a heteroatom (such as oxygen, nitrogen or a halogen), that replaces hydrogen in a hydrocarbon chain and gives the compound its characteristic chemical properties, whatever the length and nature of the carbon chain. (a) Alcohol: $-\text{OH}$ (the hydroxyl group), as in ethanol, ${\text{CH}}_3{\text{CH}}_2\text{OH}$. (b) Ketone: $>\text{C}=\text{O}$ (a carbonyl group whose carbon is bonded to two carbon atoms; also written $-\text{CO}-$), as in propanone, ${\text{CH}}_3{\text{COCH}}_3$.

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16. Q256 CBSE 2022
    What ​are Covalent ​Compounds ​? ​List two ​properties of covalent compounds.
    Show answer

    Covalent compounds have atoms held by shared electron pairs; they generally have low melting and boiling points and are poor conductors of electricity.

    Covalent compounds are compounds in which the atoms are held together by covalent bonds, that is, by sharing pairs of electrons between atoms; no electrons are transferred and no ions are formed. Examples: methane (${\text{CH}}_4$), water (${\text{H}}_2\text{O}$), ethanol (${\text{C}}_2{\text{H}}_5\text{OH}$). Two properties of covalent compounds: 1. They generally have low melting and boiling points, because the forces of attraction between their molecules are weak (the bonds within each molecule are strong). For example, methane melts at $90$ K and boils at $111$ K. 2. They are generally poor conductors of electricity, because they contain no ions (charged particles) to carry the current.

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17. Q257 CBSE 2022
    “Carbon ​cannot ​form ionic ​bonds.” ​Justify this ​statement.
    Show answer

    Carbon cannot easily gain or lose four electrons to form ions, so it shares electrons and forms covalent bonds.

    Carbon has atomic number $6$ and electronic configuration $2,4$, so it has four electrons in its outermost shell. To form an ionic bond it would have to become an ion by transferring four electrons, and neither way is favourable: 1. It could gain four electrons to form the anion ${\text{C}}^{4-}$. But it would be difficult for a nucleus with only six protons to hold on to ten electrons, that is, four extra electrons. 2. It could lose four electrons to form the cation ${\text{C}}^{4+}$. But removing four electrons needs a very large amount of energy, and it would leave six protons holding on to just two electrons. So the carbon atom does not form ionic bonds. Instead it shares its four valence electrons with other atoms and forms covalent bonds; in methane (${\text{CH}}_4$), for example, it shares one pair of electrons with each of four hydrogen atoms.

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18. Q258 CBSE 2022
    1. (i)Write ​the ​name and draw ​the ​structure of a saturated ​hydrocarbon ​with four carbon atoms.
    2. (ii)Write the number of single covalent bonds present in ​this ​compound.
    Show answer

    Butane, ${\text{C}}_4{\text{H}}_{10}$, has 13 single covalent bonds: 3 C-C bonds and 10 C-H bonds.

    (i) Butane, ${\text{C}}_4{\text{H}}_{10}$: ${\text{CH}}_3-{\text{CH}}_2-{\text{CH}}_2-{\text{CH}}_3$, four carbon atoms in a chain joined by single bonds, each carbon completing its four bonds with hydrogen atoms (structure drawn in the figure). (ii) 13 single covalent bonds: 3 C–C bonds and 10 C–H bonds.

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19. Q259 CBSE 2022
    Draw ​the ​electron dot ​structure ​of ​the ​following :

    1. (a)Cyclohexane
    2. (b)Butane
    Show answer

    Cyclohexane is a ring of six carbons with 18 shared pairs; butane is a chain of four carbons with 13 shared pairs.

    (a) Cyclohexane, ${\text{C}}_6{\text{H}}_{12}$: six carbon atoms in a ring. Between each pair of neighbouring carbons there is one shared pair of electrons, and each carbon shares one pair with each of its two hydrogen atoms — $6$ C–C and $12$ C–H shared pairs, $18$ in all, with every carbon completing an octet and every hydrogen a duplet. (b) Butane, ${\text{C}}_4{\text{H}}_{10}$ (${\text{CH}}_3-{\text{CH}}_2-{\text{CH}}_2-{\text{CH}}_3$): four carbon atoms in a chain; each end carbon shares a pair with one carbon and three hydrogens, and each middle carbon with two carbons and two hydrogens — $3$ C–C and $10$ C–H shared pairs, $13$ in all. The two structures are drawn in the figures, with carbon's electrons as dots and hydrogen's as crosses.

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20. Q260 CBSE 2022
    Carbon ​forms ​compounds mainly ​by ​covalent bonding. ​Why ​?
    Show answer

    Carbon cannot easily gain or lose four electrons to form ions, so it shares its four valence electrons and forms covalent bonds.

    Carbon (atomic number $6$, configuration $2,4$) has 4 valence electrons and needs 4 more to attain a noble gas configuration (an octet). It could gain 4 electrons to form ${\text{C}}^{4-}$, but it would be difficult for a nucleus with only 6 protons to hold 10 electrons; or it could lose 4 electrons to form ${\text{C}}^{4+}$, but that needs a large amount of energy. So carbon does not readily form ions. Instead it shares its 4 valence electrons with other atoms (of carbon or of other elements), so that each atom attains a noble gas configuration; bonds formed by sharing electrons are covalent bonds, as in methane, ${\text{CH}}_4$. Hence carbon forms its compounds mainly by covalent bonding.

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