Critical Thinking Solutions: Structure of the Atom | Science Class 9 PDF Download

 Page 1


Solutions: Critical Thinking for Str ucture of the
A t o m
This do cumen t pro vides detailed solutions and explanations for the critical thinking ques-
tions in Chapter 4: Structure of the A tom from the NCER T Class 9 Science textb o ok.
The answ ers clarify concepts suc h as atomic mo dels, subatomic particles, electron config-
uration, and isotop es, fostering analytical skills for Class 9 studen ts. Chemical form ulas
and mathematical expressions use prop er subscripts and sup erscripts for clarit y .
1 Solutions for Analyzing A tomic Mo dels
1.1 Questions and Answ ers
• Wh y did R utherford’s atomic mo del c hallenge Thomson’s plum pudding
mo del? Analyze the k ey evidence from R utherford’s gold foil exp erimen t
that led to the disco v ery of the n ucleus, as describ ed in the NCER T
textb o ok.
R utherford’s atomic mo del c hallenged Thomson’s plum pudding mo del, whic h pro-
p osed that atoms are a uniform p ositiv e sphere with em b edded electrons. The
NCER T textb o ok describ es R utherford’s gold foil exp erimen t, where alpha parti-
cles w ere fired at a thin gold foil. Most particles passed through, but some w ere
deflected at large angles, indicating a small, dense, p ositiv ely c harged n ucleus. This
con tradicted Thomson’s mo del, as a diffuse p ositiv e c harge w ould not cause suc h
deflections. The evidence of a concen trated n ucleus led R utherford to prop ose that
atoms ha v e a cen tral n ucleus with orbiting electrons, rev olutionizing atomic theory .
• Consider Bohr’s mo del of the atom, whic h in tro duced fixed electron
orbits. Wh y is this mo del an impro v emen t o v er R utherford’s mo del,
and what limitations do es it still ha v e in explaining atomic b eha vior?
Bohr’s mo del impro v ed R utherford’s b y addressing the instabilit y of orbiting elec-
trons, whic h, p er R utherford’s mo del, should emit energy and spiral in to the n u-
cleus. The NCER T textb o ok explains that Bohr prop osed fixed orbits with sp ecific
energy lev els where electrons do not radiate energy . This explained the stabilit y
of atoms and sp ectral lines (e.g., h ydrogen’s emission sp ectrum). Ho w ev er, Bohr’s
mo del is limited: it applies mainly to h ydrogen-lik e atoms and fails to explain com-
plex atoms, electron in teractions, or quan tum b eha viors lik e w a v e-particle dualit y ,
requiring m o dern quan tum mec hanics for a complete description.
• Supp ose a classmate argues that Dalton’s atomic mo del is en tirely ob-
solete due to mo dern disco v eries. Ho w w ould y ou defend the relev ance
of Dalton’s mo del, using its con tributions to the concept of atoms in
c hemical reactions?
Dalton’s atomic mo del is not en tirely obsolete. The NCER T textb o ok highligh ts
that Dalton prop osed atoms as indivisible, unique units for eac h elemen t, com bin-
ing in fixed ratios during c hemical reactions (e.g., 2 H
2
+ O
2
--? 2 H
2
O). This
laid the foundation for stoic hiometry and c hemical form ulas. While mo dern disco v-
eries (e.g., subatomic particles) refined the mo del, Dalton’s ideas remain relev an t
1
Page 2


Solutions: Critical Thinking for Str ucture of the
A t o m
This do cumen t pro vides detailed solutions and explanations for the critical thinking ques-
tions in Chapter 4: Structure of the A tom from the NCER T Class 9 Science textb o ok.
The answ ers clarify concepts suc h as atomic mo dels, subatomic particles, electron config-
uration, and isotop es, fostering analytical skills for Class 9 studen ts. Chemical form ulas
and mathematical expressions use prop er subscripts and sup erscripts for clarit y .
1 Solutions for Analyzing A tomic Mo dels
1.1 Questions and Answ ers
• Wh y did R utherford’s atomic mo del c hallenge Thomson’s plum pudding
mo del? Analyze the k ey evidence from R utherford’s gold foil exp erimen t
that led to the disco v ery of the n ucleus, as describ ed in the NCER T
textb o ok.
R utherford’s atomic mo del c hallenged Thomson’s plum pudding mo del, whic h pro-
p osed that atoms are a uniform p ositiv e sphere with em b edded electrons. The
NCER T textb o ok describ es R utherford’s gold foil exp erimen t, where alpha parti-
cles w ere fired at a thin gold foil. Most particles passed through, but some w ere
deflected at large angles, indicating a small, dense, p ositiv ely c harged n ucleus. This
con tradicted Thomson’s mo del, as a diffuse p ositiv e c harge w ould not cause suc h
deflections. The evidence of a concen trated n ucleus led R utherford to prop ose that
atoms ha v e a cen tral n ucleus with orbiting electrons, rev olutionizing atomic theory .
• Consider Bohr’s mo del of the atom, whic h in tro duced fixed electron
orbits. Wh y is this mo del an impro v emen t o v er R utherford’s mo del,
and what limitations do es it still ha v e in explaining atomic b eha vior?
Bohr’s mo del impro v ed R utherford’s b y addressing the instabilit y of orbiting elec-
trons, whic h, p er R utherford’s mo del, should emit energy and spiral in to the n u-
cleus. The NCER T textb o ok explains that Bohr prop osed fixed orbits with sp ecific
energy lev els where electrons do not radiate energy . This explained the stabilit y
of atoms and sp ectral lines (e.g., h ydrogen’s emission sp ectrum). Ho w ev er, Bohr’s
mo del is limited: it applies mainly to h ydrogen-lik e atoms and fails to explain com-
plex atoms, electron in teractions, or quan tum b eha viors lik e w a v e-particle dualit y ,
requiring m o dern quan tum mec hanics for a complete description.
• Supp ose a classmate argues that Dalton’s atomic mo del is en tirely ob-
solete due to mo dern disco v eries. Ho w w ould y ou defend the relev ance
of Dalton’s mo del, using its con tributions to the concept of atoms in
c hemical reactions?
Dalton’s atomic mo del is not en tirely obsolete. The NCER T textb o ok highligh ts
that Dalton prop osed atoms as indivisible, unique units for eac h elemen t, com bin-
ing in fixed ratios during c hemical reactions (e.g., 2 H
2
+ O
2
--? 2 H
2
O). This
laid the foundation for stoic hiometry and c hemical form ulas. While mo dern disco v-
eries (e.g., subatomic particles) refined the mo del, Dalton’s ideas remain relev an t
1
for understanding reaction ratios and conserv ation of mass, forming the basis for
c hemical c alculations still used to da y , suc h as predicting reaction pro ducts.
2 Solutions for Ev aluating Subatomic P articles
2.1 Questions and Answ ers
• Wh y is the n um b er of protons in an atom (e.g.,
12
6
C with 6 protons) more
critical than the n um b er of neutrons in defining an elemen t’s iden tit y?
Discuss the r ole of protons in c hemical prop erties.
The n um b er of protons, or atomic n um b er, defines an elemen t’s iden tit y , as p er the
NCER T textb o ok. F or
12
6
C, the atomic n um b er 6 (6 protons) iden tifies it as carb on,
determining its c hemical prop erties via the n um b er of electrons in a neutral atom
(also 6). Electrons go v ern c hemical b onding, so carb on forms sp ecific comp ounds
(e.g., CO
2
). Neutrons affect mass but not c hemical b eha vior, as isotop es lik e
14
6
C
ha v e the same c hemical prop erties despite differen t neutron coun ts. Protons are
critical b ecause they dictate electron arrangemen t and reactivit y .
• In a neutral atom, the n um b er of protons equals the n um b er of electrons.
What w ould happ en to the atom’s prop erties if it gains or loses electrons,
forming ions lik e Na
+
or Cl
–
? Analyze the implications for c hemical
reactivit y .
In a neutral atom, protons equal electrons, ensuring no net c harge. Losing elec-
trons forms p ositiv e ions (e.g., Na
+
loses one electron, lea ving 11 protons and 10
electrons), while gaining electrons forms negativ e ions (e.g., Cl
–
gains one, with 17
protons and 18 electrons), as p er the NCER T textb o ok. Ions are more reactiv e:
Na
+
readily donates its p ositiv e c harge to form comp ounds lik e NaCl, and Cl
–
ac-
cepts electrons. This increased reactivit y driv es ionic b onding, critical for forming
salts a nd other comp ounds.
• A classmate claims that neutrons ha v e no role in an atom b ecause they
are neutral. Refute this claim b y explaining the significance of neutrons
in atomic stabilit y , using e xamples lik e
14
6
C and
12
6
C.
Neutrons are crucial for atomic stabilit y , despite b eing neutral. The NCER T text-
b o ok explains that neutrons con tribute to the n ucleus’s mass and stabilize it b y
coun teracting proton repulsion. F or
12
6
C (6 protons, 6 neutrons), the neutron-
proton balance ensures stabilit y . In
14
6
C (6 protons, 8 neutrons), extra neutrons
mak e it radioactiv e, as the im balance leads to deca y . Neutrons th us influence n u-
clear stabilit y and isotop e b eha vior, refuting the claim, as their presence or absence
affects whether an atom is stable or radioactiv e.
3 Solutions for In terpreting Electron Configuration
3.1 Questions and Answ ers
• Wh y do es the electron configuration of nitrogen (N, atomic n um b er
7) follo w the 2,5 pattern in its shells? Analyze ho w this configuration
influences nitrogen’ s c hemical b eha vior, suc h as forming N
2
.
2
Page 3


Solutions: Critical Thinking for Str ucture of the
A t o m
This do cumen t pro vides detailed solutions and explanations for the critical thinking ques-
tions in Chapter 4: Structure of the A tom from the NCER T Class 9 Science textb o ok.
The answ ers clarify concepts suc h as atomic mo dels, subatomic particles, electron config-
uration, and isotop es, fostering analytical skills for Class 9 studen ts. Chemical form ulas
and mathematical expressions use prop er subscripts and sup erscripts for clarit y .
1 Solutions for Analyzing A tomic Mo dels
1.1 Questions and Answ ers
• Wh y did R utherford’s atomic mo del c hallenge Thomson’s plum pudding
mo del? Analyze the k ey evidence from R utherford’s gold foil exp erimen t
that led to the disco v ery of the n ucleus, as describ ed in the NCER T
textb o ok.
R utherford’s atomic mo del c hallenged Thomson’s plum pudding mo del, whic h pro-
p osed that atoms are a uniform p ositiv e sphere with em b edded electrons. The
NCER T textb o ok describ es R utherford’s gold foil exp erimen t, where alpha parti-
cles w ere fired at a thin gold foil. Most particles passed through, but some w ere
deflected at large angles, indicating a small, dense, p ositiv ely c harged n ucleus. This
con tradicted Thomson’s mo del, as a diffuse p ositiv e c harge w ould not cause suc h
deflections. The evidence of a concen trated n ucleus led R utherford to prop ose that
atoms ha v e a cen tral n ucleus with orbiting electrons, rev olutionizing atomic theory .
• Consider Bohr’s mo del of the atom, whic h in tro duced fixed electron
orbits. Wh y is this mo del an impro v emen t o v er R utherford’s mo del,
and what limitations do es it still ha v e in explaining atomic b eha vior?
Bohr’s mo del impro v ed R utherford’s b y addressing the instabilit y of orbiting elec-
trons, whic h, p er R utherford’s mo del, should emit energy and spiral in to the n u-
cleus. The NCER T textb o ok explains that Bohr prop osed fixed orbits with sp ecific
energy lev els where electrons do not radiate energy . This explained the stabilit y
of atoms and sp ectral lines (e.g., h ydrogen’s emission sp ectrum). Ho w ev er, Bohr’s
mo del is limited: it applies mainly to h ydrogen-lik e atoms and fails to explain com-
plex atoms, electron in teractions, or quan tum b eha viors lik e w a v e-particle dualit y ,
requiring m o dern quan tum mec hanics for a complete description.
• Supp ose a classmate argues that Dalton’s atomic mo del is en tirely ob-
solete due to mo dern disco v eries. Ho w w ould y ou defend the relev ance
of Dalton’s mo del, using its con tributions to the concept of atoms in
c hemical reactions?
Dalton’s atomic mo del is not en tirely obsolete. The NCER T textb o ok highligh ts
that Dalton prop osed atoms as indivisible, unique units for eac h elemen t, com bin-
ing in fixed ratios during c hemical reactions (e.g., 2 H
2
+ O
2
--? 2 H
2
O). This
laid the foundation for stoic hiometry and c hemical form ulas. While mo dern disco v-
eries (e.g., subatomic particles) refined the mo del, Dalton’s ideas remain relev an t
1
for understanding reaction ratios and conserv ation of mass, forming the basis for
c hemical c alculations still used to da y , suc h as predicting reaction pro ducts.
2 Solutions for Ev aluating Subatomic P articles
2.1 Questions and Answ ers
• Wh y is the n um b er of protons in an atom (e.g.,
12
6
C with 6 protons) more
critical than the n um b er of neutrons in defining an elemen t’s iden tit y?
Discuss the r ole of protons in c hemical prop erties.
The n um b er of protons, or atomic n um b er, defines an elemen t’s iden tit y , as p er the
NCER T textb o ok. F or
12
6
C, the atomic n um b er 6 (6 protons) iden tifies it as carb on,
determining its c hemical prop erties via the n um b er of electrons in a neutral atom
(also 6). Electrons go v ern c hemical b onding, so carb on forms sp ecific comp ounds
(e.g., CO
2
). Neutrons affect mass but not c hemical b eha vior, as isotop es lik e
14
6
C
ha v e the same c hemical prop erties despite differen t neutron coun ts. Protons are
critical b ecause they dictate electron arrangemen t and reactivit y .
• In a neutral atom, the n um b er of protons equals the n um b er of electrons.
What w ould happ en to the atom’s prop erties if it gains or loses electrons,
forming ions lik e Na
+
or Cl
–
? Analyze the implications for c hemical
reactivit y .
In a neutral atom, protons equal electrons, ensuring no net c harge. Losing elec-
trons forms p ositiv e ions (e.g., Na
+
loses one electron, lea ving 11 protons and 10
electrons), while gaining electrons forms negativ e ions (e.g., Cl
–
gains one, with 17
protons and 18 electrons), as p er the NCER T textb o ok. Ions are more reactiv e:
Na
+
readily donates its p ositiv e c harge to form comp ounds lik e NaCl, and Cl
–
ac-
cepts electrons. This increased reactivit y driv es ionic b onding, critical for forming
salts a nd other comp ounds.
• A classmate claims that neutrons ha v e no role in an atom b ecause they
are neutral. Refute this claim b y explaining the significance of neutrons
in atomic stabilit y , using e xamples lik e
14
6
C and
12
6
C.
Neutrons are crucial for atomic stabilit y , despite b eing neutral. The NCER T text-
b o ok explains that neutrons con tribute to the n ucleus’s mass and stabilize it b y
coun teracting proton repulsion. F or
12
6
C (6 protons, 6 neutrons), the neutron-
proton balance ensures stabilit y . In
14
6
C (6 protons, 8 neutrons), extra neutrons
mak e it radioactiv e, as the im balance leads to deca y . Neutrons th us influence n u-
clear stabilit y and isotop e b eha vior, refuting the claim, as their presence or absence
affects whether an atom is stable or radioactiv e.
3 Solutions for In terpreting Electron Configuration
3.1 Questions and Answ ers
• Wh y do es the electron configuration of nitrogen (N, atomic n um b er
7) follo w the 2,5 pattern in its shells? Analyze ho w this configuration
influences nitrogen’ s c hemical b eha vior, suc h as forming N
2
.
2
Nitrogen (N, atomic n um b er 7) has 7 electrons, distributed as 2 in the first shell
(K) and 5 in the second shell (L), follo wing the 2n
2
rule(K : 2×1
2
= 2,L : 2×2
2
=
8,butonly5needed),aspertheNCERTtextbook.The5valenceelectronsmakenitrogenhighlyreactive,seeking3moreelectronstoachieveastableoctet.ThisleadstotheformationofN
2
,wheretwonitrogenatomssharethreeelectronpairs(triplebond),achievingstability,whichexplainsN
2
sinertnessintheatmosphere.
• Consider the maxim um electron capacit y of shells (e.g., 2n
2
forthen-thshell).Whymightthisrulelimitthenumberofelementsintheperiodictable?Discussitsimplicationsforelementslikeneon(Ne,atomicnumber10).
The 2n
2
rule(n = shellnumber)limitselectroncapacitypershell(e.g.,K : 2×1
2
= 2,L : 2×
2
2
= 8).TheNCERTtextbooknotesthatelementsaredefinedbytheiratomicnumber(protons =
electronsinneutralatoms).Asatomicnumberincreases,electronsfillhighershells,buttheperiodictablesstructureisconstrainedbystableconfigurations.Neon(Ne,atomicnumber10)has2,8electrons,fillingtheLshell,makingitstable(inertgas).Thefinitecapacityofshellslimitsthenumberofstableelectronarrangements,influencingtheperiodictablessizeandelementproperties.
• Supp o se y ou need to predict the c hemical reactivit y of an elemen t based on
its electron configuration. Wh y is the n um b er of v alence electrons (e.g., 1
for Na) more imp ortan t than inner shell electrons? Ev aluate using examples
from the c hapter.
V a lence electrons, in the outermost shell, determine c hemical reactivit y , as p er the
NCER T textb o ok, b ecause they participate in b onding. So dium (Na, atomic n um b er
11) has 1 v alence electron (2,8,1), making it highly reactiv e as it loses this electron to
form Na
+
. Inner electrons (2, 8) are stable and shielded, not participating in reactions.
Similarly , c hlorine (Cl, 2,8,7) gains one electron to form Cl
–
, driv en b y its 7 v alence
electrons. V alence electrons dictate b onding tendencies, explaining wh y elemen ts lik e Na
and Cl form ionic comp ounds lik e NaCl.
4 Solutions for Applying the Concept of Isotop es
4.1 Questions and Answ ers
• Wh y do isotop es lik e
12
6
C and
14
6
C ha v e similar c hemical prop erties but
differen t ph ysical prop erties? Analyze the role of neutrons in these dif-
ferences, as explained in the NCER T textb o ok.
Isotop es
12
6
C (6 protons, 6 neutrons) and
14
6
C (6 protons, 8 neutrons) ha v e the same
n um b er of protons and electrons, giving iden tical c hemical prop erties, as electrons
determine b onding, p er the NCER T textb o ok. Ho w ev er, differen t neutron coun ts
affect n uclear mass, leading to differen t ph ysical prop erties lik e mass and stabilit y .
14
6
C is radioactiv e due to excess neutrons, undergoing b eta deca y , while
12
6
C is stable.
Neutrons influence n uclear b eha vior, causing v ariations in ph ysical prop erties lik e
densit y or radioactiv e deca y rates.
• In a real-w orld application, suc h as carb on dating using
14
6
C, wh y is the
concept of isotop es critical? Discuss ho w the radioactiv e deca y of
14
6
C
affects its use in ar c haeology .
The concept of isotop es is critical for carb on dating, as explained in the NCER T
textb o ok .
14
6
C, a radioactiv e isotop e, deca ys with a half-life of ab out 5730 y ears.
Living organisms absorb
14
6
C, main taining a constan t ratio with
12
6
C. After death,
14
6
C deca ys without replenishmen t, and the remaining amoun t indicates age. By
measuring the
14
6
C/
12
6
C ratio in artifacts, arc haeologists estimate time since death,
relying on the distinct n uclear prop erti es of isotop es, making
14
6
C essen tial for dating
ancien t remains.
• Ev aluate the claim: “The study of atomic structure is purely theoretical
3
Page 4


Solutions: Critical Thinking for Str ucture of the
A t o m
This do cumen t pro vides detailed solutions and explanations for the critical thinking ques-
tions in Chapter 4: Structure of the A tom from the NCER T Class 9 Science textb o ok.
The answ ers clarify concepts suc h as atomic mo dels, subatomic particles, electron config-
uration, and isotop es, fostering analytical skills for Class 9 studen ts. Chemical form ulas
and mathematical expressions use prop er subscripts and sup erscripts for clarit y .
1 Solutions for Analyzing A tomic Mo dels
1.1 Questions and Answ ers
• Wh y did R utherford’s atomic mo del c hallenge Thomson’s plum pudding
mo del? Analyze the k ey evidence from R utherford’s gold foil exp erimen t
that led to the disco v ery of the n ucleus, as describ ed in the NCER T
textb o ok.
R utherford’s atomic mo del c hallenged Thomson’s plum pudding mo del, whic h pro-
p osed that atoms are a uniform p ositiv e sphere with em b edded electrons. The
NCER T textb o ok describ es R utherford’s gold foil exp erimen t, where alpha parti-
cles w ere fired at a thin gold foil. Most particles passed through, but some w ere
deflected at large angles, indicating a small, dense, p ositiv ely c harged n ucleus. This
con tradicted Thomson’s mo del, as a diffuse p ositiv e c harge w ould not cause suc h
deflections. The evidence of a concen trated n ucleus led R utherford to prop ose that
atoms ha v e a cen tral n ucleus with orbiting electrons, rev olutionizing atomic theory .
• Consider Bohr’s mo del of the atom, whic h in tro duced fixed electron
orbits. Wh y is this mo del an impro v emen t o v er R utherford’s mo del,
and what limitations do es it still ha v e in explaining atomic b eha vior?
Bohr’s mo del impro v ed R utherford’s b y addressing the instabilit y of orbiting elec-
trons, whic h, p er R utherford’s mo del, should emit energy and spiral in to the n u-
cleus. The NCER T textb o ok explains that Bohr prop osed fixed orbits with sp ecific
energy lev els where electrons do not radiate energy . This explained the stabilit y
of atoms and sp ectral lines (e.g., h ydrogen’s emission sp ectrum). Ho w ev er, Bohr’s
mo del is limited: it applies mainly to h ydrogen-lik e atoms and fails to explain com-
plex atoms, electron in teractions, or quan tum b eha viors lik e w a v e-particle dualit y ,
requiring m o dern quan tum mec hanics for a complete description.
• Supp ose a classmate argues that Dalton’s atomic mo del is en tirely ob-
solete due to mo dern disco v eries. Ho w w ould y ou defend the relev ance
of Dalton’s mo del, using its con tributions to the concept of atoms in
c hemical reactions?
Dalton’s atomic mo del is not en tirely obsolete. The NCER T textb o ok highligh ts
that Dalton prop osed atoms as indivisible, unique units for eac h elemen t, com bin-
ing in fixed ratios during c hemical reactions (e.g., 2 H
2
+ O
2
--? 2 H
2
O). This
laid the foundation for stoic hiometry and c hemical form ulas. While mo dern disco v-
eries (e.g., subatomic particles) refined the mo del, Dalton’s ideas remain relev an t
1
for understanding reaction ratios and conserv ation of mass, forming the basis for
c hemical c alculations still used to da y , suc h as predicting reaction pro ducts.
2 Solutions for Ev aluating Subatomic P articles
2.1 Questions and Answ ers
• Wh y is the n um b er of protons in an atom (e.g.,
12
6
C with 6 protons) more
critical than the n um b er of neutrons in defining an elemen t’s iden tit y?
Discuss the r ole of protons in c hemical prop erties.
The n um b er of protons, or atomic n um b er, defines an elemen t’s iden tit y , as p er the
NCER T textb o ok. F or
12
6
C, the atomic n um b er 6 (6 protons) iden tifies it as carb on,
determining its c hemical prop erties via the n um b er of electrons in a neutral atom
(also 6). Electrons go v ern c hemical b onding, so carb on forms sp ecific comp ounds
(e.g., CO
2
). Neutrons affect mass but not c hemical b eha vior, as isotop es lik e
14
6
C
ha v e the same c hemical prop erties despite differen t neutron coun ts. Protons are
critical b ecause they dictate electron arrangemen t and reactivit y .
• In a neutral atom, the n um b er of protons equals the n um b er of electrons.
What w ould happ en to the atom’s prop erties if it gains or loses electrons,
forming ions lik e Na
+
or Cl
–
? Analyze the implications for c hemical
reactivit y .
In a neutral atom, protons equal electrons, ensuring no net c harge. Losing elec-
trons forms p ositiv e ions (e.g., Na
+
loses one electron, lea ving 11 protons and 10
electrons), while gaining electrons forms negativ e ions (e.g., Cl
–
gains one, with 17
protons and 18 electrons), as p er the NCER T textb o ok. Ions are more reactiv e:
Na
+
readily donates its p ositiv e c harge to form comp ounds lik e NaCl, and Cl
–
ac-
cepts electrons. This increased reactivit y driv es ionic b onding, critical for forming
salts a nd other comp ounds.
• A classmate claims that neutrons ha v e no role in an atom b ecause they
are neutral. Refute this claim b y explaining the significance of neutrons
in atomic stabilit y , using e xamples lik e
14
6
C and
12
6
C.
Neutrons are crucial for atomic stabilit y , despite b eing neutral. The NCER T text-
b o ok explains that neutrons con tribute to the n ucleus’s mass and stabilize it b y
coun teracting proton repulsion. F or
12
6
C (6 protons, 6 neutrons), the neutron-
proton balance ensures stabilit y . In
14
6
C (6 protons, 8 neutrons), extra neutrons
mak e it radioactiv e, as the im balance leads to deca y . Neutrons th us influence n u-
clear stabilit y and isotop e b eha vior, refuting the claim, as their presence or absence
affects whether an atom is stable or radioactiv e.
3 Solutions for In terpreting Electron Configuration
3.1 Questions and Answ ers
• Wh y do es the electron configuration of nitrogen (N, atomic n um b er
7) follo w the 2,5 pattern in its shells? Analyze ho w this configuration
influences nitrogen’ s c hemical b eha vior, suc h as forming N
2
.
2
Nitrogen (N, atomic n um b er 7) has 7 electrons, distributed as 2 in the first shell
(K) and 5 in the second shell (L), follo wing the 2n
2
rule(K : 2×1
2
= 2,L : 2×2
2
=
8,butonly5needed),aspertheNCERTtextbook.The5valenceelectronsmakenitrogenhighlyreactive,seeking3moreelectronstoachieveastableoctet.ThisleadstotheformationofN
2
,wheretwonitrogenatomssharethreeelectronpairs(triplebond),achievingstability,whichexplainsN
2
sinertnessintheatmosphere.
• Consider the maxim um electron capacit y of shells (e.g., 2n
2
forthen-thshell).Whymightthisrulelimitthenumberofelementsintheperiodictable?Discussitsimplicationsforelementslikeneon(Ne,atomicnumber10).
The 2n
2
rule(n = shellnumber)limitselectroncapacitypershell(e.g.,K : 2×1
2
= 2,L : 2×
2
2
= 8).TheNCERTtextbooknotesthatelementsaredefinedbytheiratomicnumber(protons =
electronsinneutralatoms).Asatomicnumberincreases,electronsfillhighershells,buttheperiodictablesstructureisconstrainedbystableconfigurations.Neon(Ne,atomicnumber10)has2,8electrons,fillingtheLshell,makingitstable(inertgas).Thefinitecapacityofshellslimitsthenumberofstableelectronarrangements,influencingtheperiodictablessizeandelementproperties.
• Supp o se y ou need to predict the c hemical reactivit y of an elemen t based on
its electron configuration. Wh y is the n um b er of v alence electrons (e.g., 1
for Na) more imp ortan t than inner shell electrons? Ev aluate using examples
from the c hapter.
V a lence electrons, in the outermost shell, determine c hemical reactivit y , as p er the
NCER T textb o ok, b ecause they participate in b onding. So dium (Na, atomic n um b er
11) has 1 v alence electron (2,8,1), making it highly reactiv e as it loses this electron to
form Na
+
. Inner electrons (2, 8) are stable and shielded, not participating in reactions.
Similarly , c hlorine (Cl, 2,8,7) gains one electron to form Cl
–
, driv en b y its 7 v alence
electrons. V alence electrons dictate b onding tendencies, explaining wh y elemen ts lik e Na
and Cl form ionic comp ounds lik e NaCl.
4 Solutions for Applying the Concept of Isotop es
4.1 Questions and Answ ers
• Wh y do isotop es lik e
12
6
C and
14
6
C ha v e similar c hemical prop erties but
differen t ph ysical prop erties? Analyze the role of neutrons in these dif-
ferences, as explained in the NCER T textb o ok.
Isotop es
12
6
C (6 protons, 6 neutrons) and
14
6
C (6 protons, 8 neutrons) ha v e the same
n um b er of protons and electrons, giving iden tical c hemical prop erties, as electrons
determine b onding, p er the NCER T textb o ok. Ho w ev er, differen t neutron coun ts
affect n uclear mass, leading to differen t ph ysical prop erties lik e mass and stabilit y .
14
6
C is radioactiv e due to excess neutrons, undergoing b eta deca y , while
12
6
C is stable.
Neutrons influence n uclear b eha vior, causing v ariations in ph ysical prop erties lik e
densit y or radioactiv e deca y rates.
• In a real-w orld application, suc h as carb on dating using
14
6
C, wh y is the
concept of isotop es critical? Discuss ho w the radioactiv e deca y of
14
6
C
affects its use in ar c haeology .
The concept of isotop es is critical for carb on dating, as explained in the NCER T
textb o ok .
14
6
C, a radioactiv e isotop e, deca ys with a half-life of ab out 5730 y ears.
Living organisms absorb
14
6
C, main taining a constan t ratio with
12
6
C. After death,
14
6
C deca ys without replenishmen t, and the remaining amoun t indicates age. By
measuring the
14
6
C/
12
6
C ratio in artifacts, arc haeologists estimate time since death,
relying on the distinct n uclear prop erti es of isotop es, making
14
6
C essen tial for dating
ancien t remains.
• Ev aluate the claim: “The study of atomic structure is purely theoretical
3
and has no practical applications. ” Pro vide a coun terexample from a
real-w orld con text, suc h as medical imaging or n uclear energy , to refute
this claim.
The claim is incorrect. A coun terexample is medical imaging using isotop es lik e
99 m
43
T c (tec hnetium-99m), whic h emits gamma ra ys for diagnostic scans (e.g., b one
imaging). The NCER T textb o ok notes that atomic structure, including isotop es,
enables suc h applications. Understanding n uclear prop erties allo ws precise use of
radioactiv e isotop es. Similarly , n uclear energy relies on uranium isotop es (
235
92
U)
for fission. These applications sho w that atomic structure is vital in medicine and
energy , di rectly impacting daily life, refuting the claim.
Conclusion
These solutions pro vide clear explanations for the critical thinking questions, reinforcing
concepts of atomic structure, subatomic particles, electron configuration, and isotop es.
They help Class 9 studen ts connect theoretical science to real-w orld applications, enhanc-
ing analytical skills and aligning with CBSE learning ob jectiv es for Chapter 4.
4