EXPLANATIONS

Objective Questions

10. (c) The structural formula of pentane C H

12 is

1. (c) Carbon exists in the atmosphere in the form of carbon

dioxide gas (CO )

in air (only 0.03%). Carbon also occurs in

⏐

the earth’s crust in the form of minerals like carbonates. It also

H⎯C⎯

C⎯

C⎯H

occurs in the form of fossil fuels, organic compounds, wood,

⏐

cotton and wool, etc.

2. (b) Oxygen atom has six (6) valence electrons. Thus, to

It contains 16 covalent bonds.

complete its octet, it forms double bond with another oxygen

11. (a) C H

8 isanalkanebecauseitresembleswiththegeneral

atom to get O₂ molecule as

formula of alkane, i.e. C H

6 isbenzenewhichisa

cyclic ring having double bonds in alternate carbon atoms

C H

2 resembleswiththegeneralformulaofalkyneand

or O==O

C H

8 isanalkenebecausenumberofH-atomsaredoubleof

that of carbon atoms.

Shared electrons

12. (a) ‘Eth’ represents 2 carbon atoms and ‘yne’ shows presence

(covalent bond

of a triple bond. Thus, ethyne has the structural formula as

formation)

H⎯C ≡≡ C⎯ H. It is also known as acetylene.

3. (d) Electronic configuration of N (atomic number 7) is^KL .

13. (c) Unsaturated hydrocarbons have double or triple bond in

their structure. Both (ii) and (iv) have double carbon-carbon

Therefore, it needs three more electrons to complete its

bonds in their structures.

octet. Each nitrogen atom shares three electrons to form a

14. (c) Benzene molecule contains alternate single and double

molecule of N₂ as

bonds. Its formula is C H

In structure (a), double bonds are not at alternate positions.

In structure (b), the formula is C₆H

and in structure (d),

the formula is C H

15. (a) Structure (i) is n-butane.

4. (c) Carbon has 4 electron in its valence shell, while hydrogen

Structure (iii) is iso-butane.

has one electron in its valence shell.

Since, molecular formula is same, only structures are

To complete their octet and duplet respectively, they form

different. So, (i) and (iii) are isomers while structure (ii) and

covalent bonds. Carbon utilises its 4 valence electron and

(iv) have molecular formula C H

forms 4 covalent bonds with 4 hydrogen atoms, using one

16. (a)BothA and Rare true and Risthe correct explanationofA.

valence electron with each hydrogen atom.

Covalent compounds consist of molecules and not ions

5. (a) A molecule of ammonia (NH₃) has only single bonds and

which can transfer charge.

these are covalent bonds.

17. (b) Both A and R are true but R is not the correct explanation

Lone pair

of A.

Diamond is not good conductor of electricity because of the

H×

× H or H — N — H

absence of free electrons.

18. (a)BothA and Rare true and Risthe correct explanationofA.

A graphite crystal consists of various layers of carbon atoms

6. (b) 8 covalent bonds are formed in S molecule.

in which each carbon atom is joined to three other atoms by

strong covalent bonds. The various layers of carbon atoms in

graphite are held together by weak van der Waals’ forces

making it slippery to touch.

19. (a)BothA and Rare true and Risthe correct explanationofA.

Catenation is the bonding of atoms of the same element into

Crown shaped (S 8) molecule

a series, called as chain.

7. (d) Graphite can not be used for making insulated plates, as

Catenation occurs more readily with carbon, which forms

it is a good conductor of electricity.

strong covalent bond with other C-atoms to form long chains

8. (d) C, Si and Gebelongs to group 14 and their valency is 4.

and structures.

But oxygen has electronic configuration 2, 6. So, its valency

20. (a)BothA and Rare true and Risthe correct explanationofA.

is 2. Hence, it does not show tetravalency.

The alchohols have general formula of C H

2n + 1

OH. So, the

9. (b) Friedrich Wohler accidently prepared urea from

alcohols have the series of formula from the different

ammonium cyanate and the synthesis of urea discarded the

compounds with different between the succeeding and

vital force theory.

preceding molecules being a — CH₂ — unit.

21.

(i)

(b)The structuresofthe following givencompoundsare:

(II) is the structure of diamond and (III) is the structure

I. SO₂

II. N₂

of Buckminster fullerene as their structure resembles

with geodesic domes.

O== S== O

N≡≡ N

(iv)

(b) In graphite, only three valence electrons are used

III. HCl

IV. NH₃

for bond formation and hence fourth electron is free to

••

H⎯Cl

H⎯ N⎯ H

move which makes it a good conductor of electricity.

⏐

(v)

(d) Coke is an example of amorphous form of carbon

which is obtained as a residue in destructive distillation

∴ I, III and IV do not contain a triple bond. Hence,

of coal.

option (b) is correct.

Subjective Questions

(ii)

(a) O₂ contains a double bond between it’s atoms.

1. Carbon shares it’s valence electrons with other atoms of

The structures of the given compounds are :

carbon or with atoms of other elements in order to complete

(a) O₂ ⇒O == O

it’s octet. These shared electrons belong to the outermost

shells of both atoms and in this way, both atoms attain the

(b) N₂ ⇒ N ≡≡ N

nearest noble gas configuration. This type of bonding is

called covalent bonding.

⏐

2. The bonds that are formed by sharing electrons are known

⏐

as covalent bond. In covalent bonding, both atoms share the

valence electrons, i.e. the shared electrons belong to the

valence shells of both the atoms. CH Cl

is called

chloromethane, which contains 1 carbon atom, 3 hydrogen

(d) H O

⇒

atoms and 1 chlorine atom.

H H

K L

(iii)

Electronic configuration of carbon, 6 = 2,

contribute one electron and thus share single electron

pair to form single covalent bond. As electrons are

Electronic configuration of hydrogen, 1 = 1

shared by both atoms, they acquire inert gas

K L M

configuration of argon atom in valence shell.

Electronic configuration of chlorine,17= 2

Carbon atom has four outermost electrons, each hydrogen

Sharing of

atom has one electron and chlorine has seven outermost

Cl Cl

or Cl Cl or Cl — Cl

electrons

electrons. Carbon shares its four outermost electrons with

3 hydrogen atoms and 1 chlorine atom to form CH Cl

One shared electron pair

follows

(iv)

to complete their octet is known as the covalency of

that atom. Therefore, the covalency of nitrogen is

three because it needs three electrons to complete it’s

3H + C + Cl

or H—C—Cl

octet.

(v)

(a) The shared pair of electrons constitute a single bond

between the two H-atoms, which is represented by a

single line between two H-atoms.

3. Atomic number of C = 6

K L

Electronic configuration =

^× H

or H—H

Atomic number of O = 8

K L

Electronic configuration =

Single bond showing H molecule

To attain the stable electronic configuration, carbon needs

22.

(i)

4 electrons, while oxygen needs 2 electrons. So, in CO₂, each

which in turn, are bonded to four more atoms. Thus, X

oxygen atom share two electrons from carbon. Thus, oxygen

is a giant molecule and has a structure similar to that of

diamond. Substance X is not a compound as it consists

and carbon both complete their octet.

of only one type of atoms.

Before

After

(ii)

(a) In the structure of diamond, carbon atoms are held

combination

combination Shared electrons

together by single covalent bonds as this is a rigid

three-dimensional network structure because each

C O

O C O

carbon atom is bonded to four other carbon atoms.

(iii)

(d) (I) is the structure of graphite crystal which

O==C==O

consists of layers of carbon atoms or sheets of carbon

Carbon dioxide (CO )

molecule

atoms.

4. (i) The formation of calcium chloride with the help of

6. Covalent compounds have low melting and boiling points

electron dot structure.

due to small intermolecular forces of attraction between the

Element

Atomic

Electronic

atoms.

number

configuration

(i) Valency of each carbon atom in an alkane is four.

Calcium (Ca)

2, 8, 8, 2

(ii) Valency of each carbon atom in an alkyne is four.

8. Covalent compounds are poor conductors of electricity

Chlorine (Cl)

2, 8, 7

because covalent bonds are formed by sharing of electrons

between atoms.

Ca²⁺

So, they don’t have a free electron that is required for

electricity transfer (electricity is the flow of free electrons).

Thus, they are bad conductors.

+2Cl− ⎯→

CaCl

9. In the structure of diamond, all the four valence electrons of

carbon are involved in the formation of covalent bonds.

Two valence electrons of calcium attack the valency of

Thus, no free electrons are available.

two chlorine to attain the noble gas configuration.

Whereas, in the structure of graphite, three electrons in the

(ii)

Ionic compounds do not conduct electricity in solid state

valence shell of carbon are involved in covalent bond

but conduct electricity in molten and aqueous state

formation and the fourth electron is free to move.

because in solid state, there is no free ion to move and

Therefore, graphite is a good conductor of electricity.

pass electricity. Whereas in the molten and aqueous

state, there is free ions to move and pass electricity.

10. Diamond has a giant structure that consists of carbon atoms

in which each carbon atom is bonded to four other carbon

K L

(i)

Electronic configuration of carbon, C(6) is 2,

atoms forming a rigid three-dimensional network structure,

which is responsible for it’s hardness.

K L M

Electronic configuration of chlorine, Cl(17

)

is 2,

So, a lot of energy is required to break the network

of strong covalent bonds. That’s why it has high melting

To attain the electronic configuration of the nearest noble

point.

gas,carbonneeds4electronand chlorine needs 1electron.

11. The main factors that enables carbon to form large number

So, with chlorine, carbon forms carbon tetrachloride.

of compounds are

Electron dot structure and structural formula ofCCl₄ is as

follows

(i) Catenation The tendency of carbon to form chains of

identical atoms is known as catenation. Carbon forms

long chains by combining with other carbon atoms

through covalent bonds.

orCl—C—Cl

(ii) Tetravalency It has 4 valence electrons, so it can form

4 covalent bonds with four different atoms, or two

double bonds or a single and a triple bond with other

atoms. This tendency helps carbon to form a large

number of compounds.

Carbon tetrachloride (CCl

)

(iii) Tetravalency Carbon forms strong bonds with most of

K L

(ii)

Electronic configuration of oxygen, O(8) is 2,

other elements like H, O, N, S, Cl etc., due to it’s

small size which helps it to attract more number of

With oxygen, carbon forms carbon dioxide. To attain the

electrons.

electronic configuration of the nearest noble gas, carbon

12. The hydrocarbons in which all the carbon atoms are

needs 4 electrons and oxygen needs 2 electrons.

connected by only single bonds are called saturated

Therefore, in CO₂, each oxygen atom shares 2 electrons

hydrocarbons or alkanes or paraffins. The general formula of

with carbon. Electronic configuration of carbon (6) is

these compounds is C H

2n+ 2 .

K L

where, n = number of carbon atoms in one molecule of a

hydrocarbon.

The electron dot structure and structural formula of CO₂

Amongst, the given compounds, only C₄

and C₆

is as follows:

belongs to the formula of C H

10 and

C H

14 aresaturatedhydrocarbons.

orOCO

13. General formula of cycloalkane = C H

In cyclopentane, n = 5

∴ Formula of cyclopentane, C H

5 ×2

Carbon dioxide (CO )

Electron dot structure of cyclopentane

The structures of possible isomers of butane (C H

)

are

H H

⏐

H⎯C⎯C⎯C⎯C⎯H

H⎯C⎯C⎯C⎯H

×H

⏐

butane

H⎯C⎯H

×H

(I)

⏐

H H H

Iso-butane

(II)

14.

(i)

Carbon is a versatile element because of its properties.

It shows the property of catenation due to which it

The first three members of alkane series are:

forms a large number of compounds. Carbon is

(i) CH₄ (methane)

tetravalent. Due to this, it forms covalent compounds

(ii) C H

(ethane)

only.

(iii) C H

(propane)

(ii)

Saturated hydrocarbons contain carbon-carbon single

In the above members of alkane series, it is not possible to

bonds. e.g. Methane (CH )

4 ,

ethane (C H )

6 .

have different arrangements of carbon atoms, because

Unsaturated hydrocarbons contain atleast one

branching is not possible from either first or last carbon.

carbon-carbon double or triple bond. e.g. Propene

Thus, we cannot have isomers of first three members of

(C H )

6 .

alkane series.

(iii)

Functional group is an atom or group of atoms joined in

17. There are four isomers possible for the molecular formula

a specific manner which is responsible for the

C H O

. These are as follows:

characteristics chemical properties of the organic

H H

compounds.

•

••

•

(i) CH CH CHO

or CH CH

—C

^⏐ →H••

•

C ••O

•

Examples are alcohols ( ⎯OH), aldehyde group

••

⏐⏐

(⎯CHO), carboxylic group (⎯COOH) , ketone (⎯CO)

H H

etc.

15.

(i)

Molecular formula of ethane is C H

Its electron dot

••

•

(ii) CH

⎯

C⎯

→ H••C

•

^• H

structure is

••

⏐⏐

•

H H

•

••O H

••

H C C

••

•

H H

(iii) CH

CH ==CH ⎯OH

→ H••C

C •••C •O

^• H

3⎯

•

••

H H

(ii)

Molecular formula of ethene is C H

Its electron dot

structure is

••

• •

•

(iv) CH

CH⎯CH ⎯OH

→ H••C

•

C ••C

•

^• H

H H

••

H H H

18. An atom or a group of atoms present in a molecule which

C C

largely determines it’s chemical properties is called functional

group.

H H

Compound Structural formula Functional group

(i) Ethanol

⎯OH

(iii)

Molecular formula of ethyne is C H

Its electron dot

(C H OH)

⏐

(Alcoholic)

structure is

H⎯C

⎯C

⎯OH

⏐

(ii) Ethanoic acid

H O

(CH COOH)

⏐

⏐⏐

H⎯C⎯C⎯OH

⎯C ⎯OH

16. Isomers are those molecules which have same molecular

⏐

(Carboxylic acid)

formula but different structural formula, i.e. show different

properties.

19. Application of homologous series are as follows

shell. So, it requires 2 electrons to complete it’s octet for

^● All members of homologous series shows similar chemical

attaining noble gas configuration. Hence, it shares two

electrons with another atom of oxygen to make a molecule of

properties and generally prepared through one common

oxygen.

method, e.g. all alkenes are prepared by dehydration of

By doing so, both the atoms of oxygen get 8 electrons in their

corresponding alcohols.

outermost shell. Thus, a double bond is formed between two

^● The physical properties of the members change gradually,

oxygen atoms which consists of four electrons.

i.e. show gradation in properties as the number of carbon

Before

After

atom per molecule increased.

combination

20. A homologous series is the family of organic compounds

Shared electrons

having the same functional group, similar chemical

properties but the successive (adjacent) members of the

O or O

⎯

series are differ by a CH₂ unit or 14 mass units.

Consecutive members of the homologous series of alcohols

Oxygen atoms

Oxygen molecule

are

CH OH⎤

Formation of Sulphur Molecule (S )

⎥Theydifferby⎯CH2 unit.

C H OH5

⎦

The atomic number of sulphur is 16 and electronic

configuration is 2, 8, 6. It also has 6 electrons in it’s outermost

The physical properties are determined by alkyl group/

hydrocarbon part/part other than the functional group.

shell and requires 2 electrons to complete it’s octet state.

So, each sulphur atom shares two electrons, 1 with each

The chemical properties are determined by functional

group such as ⎯OH group.

adjoining sulphur atom by single covalent bonds and thus,

complete it’s octet.

21. Add —CH₂ group to each compound to obtain next

homologous.

Before

After

combination

Compounds

Homologue compounds

CH₂ O

C H O,C H O,C H O

C H

8...

C H COOH

,C H COOH,

C H COOH

22. Atomic number of carbon is six. This means that it has four

electrons in its outermost shell and it needs four more

electrons to attain noble gas electronic configuration. It

8 covalent bonds are formed

does not form C4+ cation, as the removal of four valence

electrons will require a huge amount of energy.

The cation formed will have six protons and two electrons.

This makes it highly unstable. Carbon is unable to form

C4−anion as its nucleus with six protons will not be able to

hold ten electrons due to its small size. Thus, carbon

Crown shaped (S 8) molecule

achieves noble gas electronic configuration by sharing its

Eight sulphur atoms form a puckered ring or crown structure

four electrons either with same or different other atoms,

to form an eight atom molecule.

i.e. it forms covalent compounds.

24. Covalent compounds are those compounds which are formed

(i) Covalent compounds does not have free ions, due to

by sharing of valence electrons between the atoms. e.g.

this they are bad conductors of electricity in solid,

Hydrogen molecule is formed by mutual sharing of electrons

molten or aqueous state.

between two hydrogen atoms.

(ii) Covalent compounds are formed by covalent bonds and

They are different from ionic compounds as ionic compounds

it has been found that the intermolecular force of

are formed by the complete transfer of electrons from one

attraction in covalent compounds are weak.

atom to another, e.g. NaCl is formed when one valence

Thus, low amount of energy is required to break these

electron of sodiu m gets completely transferred to outer shell

force of attraction. Hence, their melting and boiling

of chlorine atom. The characteristic properties of covalent

points are quite low.

compounds are

23. Formation of Oxygen Molecule (O )

(i) They are generally insoluble or less soluble in water but

The atomic number of oxygen is 8 and electronic

soluble in organic solvents.

configuration is 2, 6, i.e. has 6 electrons in it’s outermost

(ii) They have low melting and boiling points.

(iii) They do not conduct electricity as they do not contain

H O

ions.

⏐

⏐⏐

⏐

(v)

H⎯C⎯C⎯ ⎯C⎯H

(iv) They are volatile in nature.

⏐

25. Organic compounds are called as hydrocarbons because

they are made up of only the elements-carbon (C) and

⏐⏐

hydrogen (H).

This compound contains ⎯

⎯

functional group

General formula for the homologous series of alkanes is

which belongs to ketone.

C H

2n + 2 whichareclassifiedassaturatedhydrocarbonsor

27.

(i)

Compound B has molecular formula as C H

10 and

alkanes. First member of the alkane family is “methane”.

contains five number of carbon atoms, i.e. n = 5.

⏐

It resembles with the general formula of alkene which

H⎯ C⎯ H

is C H₂

n n.

⏐

So, the name of this compound is pent + ene = pentene.

When five number of carbon atoms are present, it is

General formula for the homologous series of alkenes is

named as “pent”.

2n.whichareclassifiedinthecategoryofunsaturated

hydrocarbons. They are known as alkenes or olefins. First

Saturated

Unsaturated hydrocarbons

(ii)

member of the alkene family is “ethene”.

hydrocarbons

These hydrocarbons

These hydrocarbons contains at

^"C

==C

are linked by only

least one double or triple bond

General formula for the homologous series of alkynes is

single covalent

along with single bonds.

bond.

They are divided into two

C H

2n − 2

and they are also in the category of unsaturated

hydrocarbons. First member of the alkyne family is “ethyne”.