CBSE Class IX Structure of Atom

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CBSE Class IX ( 9th) Science Chapter 4. Structure Of The Atom | esson E!ercises

Structure of The Atom

Introduction

Matters are made of tiny particles called atom. Atom is made of three particles; electron, protonand neutron. These particles are called fundamental particles of an atom or sub atomic particles.

Electron (e – ) - Electron is denoted by ‘e’ and is a negatively charged particle. The absolutecharge over an electron is equal to .!"#$% of negative charge and is considered equal to & .

The relative mass of electron is '()!. *ince the mass of an electron is very small, thus it isconsidered equal to #. Electrons revolve round the nucleus of atoms.

Proton (p+) - +roton is denoted by ‘p’ and is positively charged particle. The absolute chargeover proton is .!"# $% coulomb of positive charge and it is considered as unit positive charge.Thus absolute charge over a proton is equal to .

The absolute mass of a proton is equal to .!"# $- g and considered equal to as it is equal tothe mass of hydrogen atom. +roton is present in the nucleus of atom.

Neutron (n) – /eutron is denoted by ‘n’ and is a neutral particle.

The absolute mass of neutron is .! " # $- g. The relative mass of neutron is equal to . /eutronis presents in the nucleus of atom.

Nucleus – The centre of atom is called nucleus. /ucleus comprises of neutron and proton. /ucleus of an atom contains the 0hole mass of an atom.

Discovery of Electron

1n (%2; 3. 3. Thomson, a 4ritish physicist, proposed that atom contains at least one negativelycharged particle. 5ater this particle 0as named as electron. Thomson called those particles‘corpuscles’.

Discovery of Proton

Ernest 6oldstein in ((! discovered the presence of ne0 radiation in gas discharge tube even before the identification of electron. 7e called these rays as 8anal 9ays. 7is e"periment led tothe discovery of proton.

Discovery of Neutron



1n %)- 3. 8had0ic: discovered another subatomic particle called neutron. /eutron is present inthe nucleus of all atoms.

Thomson!s "odel of Atom

3. 3. Thomson proposed the model of atom similar to a 8hristmas +udding or similar to a 0atermelon. 7is model of atom is generally called plum and pudding model of atom.

7e proposed that electrons are embedded the 0ay blac: seeds of 0ater melon are embedded; in

the sphere of positive charge. According to Thomsona< An atom consists of positively charged sphere in 0hich electrons are embedded.

b< The quanta of negative and positive charges are equal. The equal number of negative chargeand positive charge ma:es an atom electrically neutral.

#utherford!s "odel of Atom

Ernest 9utherford in %#% 0ith his team bombarded very thin gold foil 0ith = & particles. 7efound that



a< Most of the = & particles passed 0ithout any hindrance.

b< *ome of the = & particles deflected from their original path at noticeable angle.

c< >ery fe0 of the = & particles bounced bac: at their original path.

?n the basis of his observation, he proposed the model of atom. The 9utherford@s Model ofAtom is as follo0s

a< Most of the part in an atom is empty.

b< There is a positively charged center in atom, 0hich contains nearly the 0hole mass of atom.The centre is called nucleus.

c< The siBe of nucleus is very small compared to an atom.

d< Electrons revolve round the nucleus.

The 9utherford’s E"periment is also :no0n as 6eiger$Marsden E"periment.

Dr$%&$c's of #utherford "odel

a< According to 9utherford’s Model, electron revolves round the positively charged nucleus0hich is not e"pected to be stable. 4ut a charged particle in an accelerated motion along acircular path 0ould lose energy because of radiation and finally 0ould fall into nucleus. This

ma:es an atom unstable 0hile atoms are quite stable.

1f atoms 0ere not stable no matter 0ould e"ist in nature.

b< 9utherford model could not solve the problem of atomic mass of atom as it proposed only thee"istence of protons in the nucleus.

7o0ever, the problem of atomic mass could be solved after the discovery of neutron.



ohr!s "odel of Atom

/eils 4ohr, a Canish physicist, in %) proposed model of atom 0hich rectified the problemsleft by 9utherford’s Model. 7e proposed that

a< Electrons revolve round the nucleus in a fi"ed orbit.

b< 7e called these orbits as ‘stationary orbit’.

c< Each stationary orbit is associated 0ith fi"ed amount of energy, thus electrons do not radiateenergy as long as they :eep on revolving around the nucleus in fi"ed orbit.

The circular path around the nucleus is called orbit, energy level or shell. Energy level arerepresented by letter & D, 5, M, /, . and so on.

Therefore,

• st orbit is denoted by & D • -nd orbit is denoted by & 5

• )rd orbit is denoted by & M, and so on.

The orbits are denoted by , -, ), . and so on.

Distri&ution of Electrons in r&it or Shell

The distribution of electrons in an orbit is obtained by -n -, 0here ‘n’ is number of orbit.

Therefore,

Num&er of electrons in *-shell ie in ,st or&it

7ere n F

Therefore,

-n-

F - " -

F -

Thus, ma"imum number of electrons in D$shell i.e. st shell F -

Number of electrons in L-shell, i.e. in 2nd orbit

7ere n F -, therefore,



-n- F - " -- F (

Thus, ma"imum number of electrons in 5$shell F (

/umber of electrons in M$shell, i.e. in )rd orbit

7ere n F ), therefore,

-n- F - " )- F (

Thus, ma"imum number of electrons in M$shell F (

/umber of electrons in /$shell, i.e. in th shell

7ere n F , therefore,

-n-

F - " -

F )-Thus, ma"imum number of electrons in /$shell F )-

1n similar 0ay ma"imum number of electrons in any shell can be calculated.

Atomic Num&er

Atomic number is the fundamental properties of an atom. Every atom is identified by its uniqueatomic number. Atomic number is denoted by ‘B’.

Atomic num&er is eu$l to the num&er of protons present in $n $tom

*ince an atom is electrically neutral, thus number of protons and number of electrons are equal toma:e an atom electrically neutral.

Atomic num&er . Num&er of protons . Num&er of electrons

E/$mple –

The atomic number of 7ydrogen is , helium is -, lithium is ), beryllium is , boron is G, carbonis !, nitrogen is 2, o"ygen is (, etc.

S$mple e/ercise

< Atomic number of calcium is -#. 8alculate the number of electrons and protons in calcium.

*olution

*ince, Atomic number F /umber of protons F /umber of electrons



Therefore,

/umber of electrons in calcium F -#

/umber of protons in calcium F -#

-< /umber of protons in sodium atom is , find the atomic number and number of electrons ina sodium atom.

*olution,

*ince, Atomic number F /umber of protons F /umber of electrons

Therefore,

Atomic number of sodium F

/umber of electrons in sodium F

Mass Number or Atomic Mass

Mass number of an atom is defined as the sum of the number of protons and number of neutrons.Mass number is nearly equal to the atomic mass of an atom. *ince, protons and neutrons residein the nucleus, thus they are also :no0n as nucleons.

This means

Mass number of an atom F /umber of protons /umber of neutrons

E"ample

< Atomic mass of aluminium is -2 u and atomic number is ), find the number of protons andnumber of neutrons in aluminium.

*olution

*ince,

Atomic number F )

Therefore, number of proton F )



He :no0 that, Atomic mass Mass number< F /umber of protons /umber of neutrons

Therefore,

-2 u F ) n

?r, n F -2 & ) F

Therefore, number of proton F ) and number of neutron F

-< The atomic number of carbon is ! and number of neutron is equal to !. Iind the atomic massor mass number of carbon.

*olution

*ince atomic number of carbon F !

Therefore, number of proton F !

/o0, Atomic mass F number of proton number of neutron

?r, Atomic mass or mass number F ! ! F - u

Thus, mass number or atomic mass of carbon F -u

Arr$n0ement of electrons in $n $tom – Electronic 1onfi0ur$tion

The ma"imum number of electrons can be obtained by -n-

; 0here ‘n’is the orbit number. Thusafter :no0ing the ma"imum number of electrons for a particular shell, the arrangement ofelectrons in an atom can be identified. 1t is called 4ohr 4ury *chemes.

#ules to %rite the electronic confi0ur$tion of $n $tom

a< Ma"imum number of electrons in an orbit is calculated by -n-, 0here ‘n’ is number of orbitand may be equal to , -, ) , . . . .

b< Electrons occupy the ne"t orbit only after filling the inner orbit completely.

c< The ma"imum number of electrons in outermost orbit 0ill not be more than (.

Electronic confi0ur$tion of 2ydro0en

Atomic number of hydrogen F

Therefore number of electrons F



Ma"imum number of electrons in st orbit F -

*ince, hydrogen has only one electron, therefore, it 0ill reside in st orbit.

Thus electronic configuration of hydrogen

/umber of orbit present in hydrogen F

Electronic configuration of Helium

Atomic number of helium F -

Therefore number of electrons F -

Therefore, electronic configuration of helium is

/umber of orbit in helium atom F

Electronic confi0ur$tion of 3ithium

Atomic number of 5ithium F )

Therefore number of electrons F )

*ince the ma"imum number of electrons in st orbit is equal to -, therefore, after accommodating- electrons in st orbit, the third electron 0ill go in -nd orbit.

Thus, electronic configuration of lithium is

/umber of orbit in 5ithium atom F ).

Electronic configuration of 4eryllium

Atomic number of beryllium F .

Therefore number of electrons F .



Thus, electronic configuration of 4eryllium is

/umber of orbit in beryllium F -

Electronic configuration of 4oron

Atomic number of boron F G

Therefore number of electrons F G

Thus, electronic configuration of boron is

/umber of orbit in boron F -

Electronic configuration of 8arbon

Atomic number of carbon F !

Therefore number of electrons F !

Thus, electronic configuration of carbon is

/umber of orbit in carbon F -

Electronic 1onfi0ur$tion of Elements - 4rom Nitro0en (N) to Sodium (N$)

Electronic configuration of /itrogen

Atomic number of nitrogen F 2.

Therefore number of electrons F 2



Thus, electronic configuration of nitrogen is

/umber of orbit in nitrogen F -

Electronic confi0ur$tion of /y0en

Atomic number of o"ygen F (.

Therefore number of electrons F (.

Thus, electronic configuration of o"ygen is

/umber of orbit in o"gyen F -

Electronic confi0ur$tion of 4luorine

Atomic number of fluorine F %

Therefore number of electrons F %

Thus, electronic configuration of fluorine is

/umber of orbit in fluorine F -

Electronic configuration of Neon

Atomic number of neon F #

Therefore number of electrons F #

Thus, electronic configuration of neon is



/umber of orbit in neon F -

Electronic confi0ur$tion of Sodium

Atomic number of sodium F


*ince, in -nd orbit the ma"imum number of electrons is equal to ( and there are electrons insodium atom, thus the eleventh electron 0ill go in third orbit.

Thus, electronic configuration of sodium is

/umber of orbit in sodium F )

Electronic 1onfi0ur$tion of Elements - 4rom "$0nesium to 1$lcium

Electronic configuration of Magnesium

Atomic number of magnesium F -

Therefore number of electrons F -

Thus, electronic configuration of magnesium is

/umber of orbit in magnesium F ).

Electronic confi0ur$tion of Aluminium

Atomic number of aluminium F ).

Therefore number of electrons F ).



Thus, electronic configuration of aluminium is

/umber of orbit in aluminium F )

Electronic confi0ur$tion of Silicon

Atomic number of silicon F


Thus, electronic configuration of silicon is

/umber of orbit in silicon F )

Electronic configuration of Phosphorous (P

Atomic number of phosphorous F G


Thus, electronic configuration of phosphorous is

/umber of orbit in phosphorous F )

Electronic confi0ur$tion of Sulphur (S)

Atomic number of sulphur F !




Thus, electronic configuration of sulphur is

/umber of orbit in sulphur F )

Electronic configuration of 8hlorine 8l<

Atomic number of chlorine F 2

Therefore number of electrons F 2

Thus, electronic configuration of chlorine is

/umber of orbit in chlorine F )

Electronic configuration of Argon Ar<

Atomic number of argon F (

Therefore number of electrons F (

Thus, electronic configuration of argon is

/umber of orbit in argon F )

Electronic configuration of +otassium D<

Atomic number of potassium F %

Therefore number of electrons F %



*ince, ma"imum number of electrons in outermost orbit 0ill not be more than (, thus the%thelectron of potassium atom 0ill reside in th orbit.

Thus, electronic configuration of potassium is

/umber of orbit in potassium F

Electronic configuration of 8alcium 8a<

Atomic number of calcium F -#

Therefore number of electrons F -#

Thus, electronic configuration of calcium is

/umber of orbit in calcium F

5$lency

Jou have read that noble gases have fully filled outermost shell. Cue to this, they are stable andthey do not react 0ith other elements. ?ther elements also tend to attain stable configuration bycompleting the octet in their outermost orbit. This is important to note that, the number ofelectrons in the outermost orbit of an element is closer to octet. An element can lose or gainelectron in order to complete the octet. This tendency of losing or gaining electrons impartsvalency to an element.

5et us ta:e e"ample of hydrogen. 7ydrogen can readily lose or gain an electron. *o, its valencyis one. /o0, let us ta:e e"ample of 7ydrochloric Acid 78l<. ?ne atom of chlorine combines0ith one atom of hydrogen to form hydrochloric acid. 1n this case, hydrogen loses one electronand thus gets charge. ?n the other hand, chlorine gains an electron and thus gets & charge.*o, valency of hydrogen and chlorine are one.

5$lency c$n &e defined $s com&inin0 c$p$city of $n $tom



E"ample &

7ydrogen molecule $ 7ydrogen has only one electron in its outermost orbit, thus it requires onemore electrons to complete its outermost orbit. Therefore, in order to complete outermost orbit,hydrogen shares one electron 0ith another hydrogen atom and form 7- hydrogen molecule<.

1n the case of 5i8l 5ithium chloride< $ 5ithium has three electrons in its outermost orbit andchlorine has seven electrons in its outermost orbit. Thus in order to ma:e outermost orbitcompletely filled lithium loses one electrons and chlorine gains one electron. After losing oneelectron, lithium has t0o electrons in its outermost orbit and after gaining one electron, chlorinehas eight electrons in its outermost orbit. And they form 5i8l 5ithium chloride<

N$me6 Sym&ol6 Atomic num&er6 Num&er of electrons6 Distri&ution of electrons in shells(electronic confi0ur$tion) $nd 5$lency of some elements (4rom 2ydro0en to1$lcium)

!sotopes

Elements having same atomic number but different atomic masses are :no0n as 1sotopes.

E"ample &

8arbon$-, 8arbon$), 8arbon$ are thee isotopes of carbon atom. 7ere -, ) and are theatomic masses of isotopes of carbon respectively. *ince, atomic number is the unique property ofan atom, thus the atomic number of carbon is ! even in the case of three types of carbonisotopes<



7ydrogen $ , Ceuterium & -, Tritium $) are three isotopes of hydrogen.

The isotopes of hydrogen are 0ritten as

Kse of 1sotopes

8arbon & is used in carbon dating.

An isotope of uranium is used as fuel in nuclear reactor.

An isotope of cobalt is used in treatment of cancer.

An isotope of iodine is used in treatment of goitre.

1sobars

Atoms having same atomic mass and different atomic numbers are :no0n as 1sobars.

E"ample &

4oth the elements have same atomic mass equal to # but different atomic numbers, i.e. argonhas atomic number equal to ( and calcium has atomic number equal to -#.

7uestion , 1omp$re the properties of electrons6 protons $nd neutronsAns%er

Electrons Protons Neutron

Electrons are /egatively charged particlesElectrons are present in outershells 0ith in an atom andrevolve around the nucleus in0ell$defined orbits or discreteorbits

The mass of an electron isabout ' -### timesthe mass of anhydrogen atom.A electron is represented as @e$@

+rotons are +ositively charged particles.+rotons are present in thenucleus of all atoms /umber of protons determinesthe atomic number of an

elementThe mass of a proton is ta:en asone unit and equals to neutron.A proton is represented as @p@

/eutrons do not carry any chargeand are neutral. /eutrons are present in thenucleus of all atoms, e"cepthydrogenThe mass of a neutron is ta:en as

one unit and equals to that of proton.A neutron is represented as @n@

7uestion 8 9h$t $re the limit$tions of :: Thomson;s model of the $tom<Ans0er Thomson proposed that an atom consists of a positively charged sphere and the electronsare



embedded in it.The negative and positive charges are equal in magnitude. *o, the atom as a 0hole iselectrically neutral.Thomson@s model of the atom fails to e"plain 9utherford@s =$particle scatteringe"periment in 0hich most of the fast moving =$particles passed straight through thegold foil.?nly*ome of the =$particles 0ere deflected by the foil by small angles.Hhich clearly established atom hasa lot of empty space and positive charge is concentrated in a very small volume 0ithin the atom.

7uestion = 9h$t $re the limit$tions of #utherford;s model of the $tom<Ans0er As per 9utherford@s nuclear model of an atom , an atom has a very small siBed nucleus 0ith positive charge inside and has electrons revolving around this nucleus in 0ell$defined orbits. /early allthe mass of an atom resides in the nucleus.9utherford@s model of the atom failed to e"plain the the stability of the atom. As any particle in acircular orbit 0ould undergo acceleration. Curing acceleration, revolving electron as charged particles 0ould lose energy and finally fall into the nucleus. This may lead to a collapsed atomicstructure, resulting in very unstable atoms.8ontrarily atoms are mostly stable.

7uestion > Descri&e ohr;s model of the $tom

Ans0er . /eils 4ohr put for0ard the follo0ing postulates about the model of an atomi< ?nly certain special orbits :no0n as discrete orbits of electrons, are allo0ed inside the atom.ii< Hhile revolving in discrete orbits the electrons do not radiate energy. These orbits or shells are called energy levels.These orbits or shells are represented by the lettersD,5,M,/, or the numbers, nF,-,),,.

7uestion ? 1omp$re $ll the proposed models of $n $tom 0iven in this ch$pterAns%er

T2"SN;S "odel of AnAtom

#@T2E#4#D;S "odel ofAn Atom

2#; S "odel of An Atom

i< An atom consists of a positively charged sphere andthe electrons are embedded init.ii< The negative and positivecharges are equal in magnitude.*o, the atom as a 0hole is

electrically neutral.

i< There is a positively chargedcentre in an atom called thenucleus. /early all the mass ofan atom resides in the nucleus.ii< The electrons revolvearound the nucleus in 0ell$defined orbits.iii< The siBe of the nucleus is

very small as compared to thesiBe of the atom.

i< ?nly certain special orbits:no0n as discrete orbits ofelectrons, are allo0ed inside theatom.ii< Hhile revolving in discreteorbits the electrons do not radiateenergy.

7uestion Summ$rise the rules for %ritin0 of distri&ution of electrons in v$rious shells for thefirst ei0hteen elementsAns0er The follo0ing rules are follo0ed for 0riting the distribution of electrons in different energylevels or shells for the first eighteen elements



i< The ma"imum number of electrons present in a shell is given by the formula -n-, 0here @n@ is theorbitnumber or energy level inde", ,-,),.7ence the ma"imum number of electrons in different shellsare asfollo0sfirst orbit or D$shell 0ill be F - L - F -,second orbit or 5$shell 0ill be F - L -- F (,third orbit or M$shell 0ill be F - L )-F (,fourth orbit or /$shell 0ill be F - L -F )-,ii< The ma"imum number of electrons that can be accommodated in the outermost orbit is (.iii< Electrons are not accommodated in a given shell, unless the inner shells are filled. That is, theshells are filled in a step$0ise manner.

Distri&ution of electrons in v$rious shells for the first ei0hteen elements

Element Sym&ol Atomic Num&erNum&er ofElectron

Distri&ution of electronsD 5 M /

7ydrogen7elium5ithium4eryllium4oron8arbon /itrogen?"ygenIluorine /eon*odium

MagnesiumAluminium*ilicon+hosphorus*ulphur 8hlorineArgon

77e5i4e48 /?I /e /a

MgAl*i+*8lA

-)G!2(%#

-)G!2(

-)G!2(%#

-)G!2(

----------

-------

$$-)G!2((

(((((((

$$$$$$$$$$

-)G!2(

$$$$$$$$$$$

$$$$$$$

7uestion B Define v$lency &y t$'in0 e/$mples of silicon $nd o/y0enAns%er The valency may be defined as the combining capacity of the atoms of elements, ortheir tendency to form molecules 0ith atoms of the same or different elements. The electrons in an

atom are arranged in different shells'orbits. The electrons present in the outermost shell of an atomare :no0n as the valence electrons. The outermost shell of an atom can accommodate a ma"imum of( electrons. All atoms of various elements have tendency to have a fully$filled outermost shell 0ith (electrons or octet . This is achieved through reaction 0ith atoms of other elements, by sharing,gaining or losing electrons.• 1n case of *ilicon, it has electrons in the outer most shell, thus it is deficient by electronsto have a fully filled outer most shell. 7ence 1ts valency is .



• 1n case of ?"ygen, it has ! electrons in the outer most shell, thus it is deficient by - electronsto have a fully filled outer most shell. 7ence 1ts valency is -.

7uestion C E/pl$in %ith e/$mples(i) Atomic num&er6 (ii) "$ss num&er6 (iii) Isotopes $nd ( iv) Iso&$rs ive $ny t%o uses of

isotopesAns%er (i) Atomic num&er Atomic number of an element is defined as the number of protons present inits atom. 1t is denoted by @@.

(ii) "$ss num&er The mass number is defined as the sum of the total number of protons andneutrons present in the nucleus of an atom.

(iii) Isotopes 1sotopes are defined as the atoms of the same element, having the same atomic number but different mass numbers.

( iv) Iso&$rs Atoms of different elements 0ith different atomic numbers, 0hich have the samemass number, are :no0n as isobars.

T%o uses of isotopes i< An isotope of uranium is used as a fuel in nuclear reactors.ii< An isotope of cobalt is used in the treatment of cancer.

7uestion N$+ h$s completely filled * $nd 3 shells E/pl$inAns%er /a represent *odium 0ith atomic number . Hhich means it has protons. /umber ofElectrons is equal to number of protons. *o *odium has electrons in the outer shells distributed as- electrons in @:@ shell, ( electron in @5@ shell and electron in @M@ shell. The distribution of electron

in the shells is governed by the formula i.e. -n

-

. Hhere @n@ represents the number of shells. The firstshell is represented as @D@, second shell as @5@, third shell as @M@, and fourth shell as @/@ and so on.There for, as per the formula above, the @D@ shell i.e. st shell 0ill contain - " - F- electrons and@5@ shell 0ill contain - " -- F ( electrons.Hhich means it has completely filled @D@ and @5@ shells.

7uestion ,F If &romine $tom is $v$il$&le in the form of6 s$y6 t%o isotopes

2%

r (>BG))G $nd (

r (?F=G).)G c$lcul$te the $ver$0e $tomic m$ss of &romine $tomAns%er The average atomic mass of bromine atom 2% L %.2 < ' ## ( L G#.)<' ## F)%-!.) #2.) < ' ## F (###.! ' ##F (#.#! u



7uestion ,, The $ver$0e $tomic m$ss of $ s$mple of $n element H is ,8 u 9h$t $re thepercent$0es of isotopes

!

H ( $nd

(

H( in the s$mple<Ans%er *uppose the percentage of

!

H in 0iven s$mple . /(:. The percentage of(

H in 0iven s$mple . ,FF-/(

As given average atomic mass of a sample of an element N F !.- u

FO ! L " '##< P( L ##$"< '##Q F !.- FO !" (##$("< ' ## F !.-FO !" (##$("< F !.- L ##

FO $-" (## F !-#FO $-" F !-#$(##F $-" F $(#FO " F %#

:. The percentage of !

N in given sample F " F %#(

:. The percentage of!

N in given sample F ##$" F##$%# F #( 7uestion ,8 If . =6 %h$t %ould &e the v$lency of the element< Also6 n$me the elementAns%er 6iven F )0e :no0 that F Atomic /umber F /umber of protons F /umber of electrons:. /umber of electrons F )



Cistrubution of electrons is given by the formula -n- 0here n is the number of orbit or shellrepresented by letters D, 5, M, / etc7ence for first orbit i.e. @:@ the number of electrons F - L F -for second orbit i.e. @5@ the number of electron F total electrons $ numbers of electrons in @D@ F )$-F>alency of element is as it 0ill readily lose its outermost valance electrons than to gather 2 more tohave a fully filled outermost orbit.The element is 5ithium

7uestion ,= 1omposition of the nuclei of t%o $tomic species H $nd J $re 0iven $s under

H JProtons . Neutrons . Cive the m$ss num&ers of H $nd J 9h$t is the rel$tion &et%een the t%o species<

Ans%er He :no0 that

Mass number of an atom F /umber of +rotons number of /eutrons Atomic /umber of an atomF /umber of +rotons:. Atomic /umber of N F !:. Atomic /umber of y F !:. Mass number of N F !! F -:. Mass number of y F !( F N and J have similar Atomic /umber !< but have different mass numbers, therefor both areisotopes of same element.

7uestion ,> 4or the follo%in0 st$tements6 %rite T for True $nd 4 for 4$lsea< 3.3. Thomson proposed that the nucleus of an atom contains only nucleons.b< A neutron is formed by an electron and a proton combining together. Therefore, it is neutral.c< The mass of an electron is about -### times that of proton.d< An isotope of iodine is used for ma:ing tincture iodine, 0hich is used as a medicine.Ans%er a< 3.3. Thomson proposed that the nucleus of an atom contains only nucleons.4<b< A neutron is formed by an electron and a proton combining together. Therefore, it is neutral.4<c< The mass of an electron is about ' -### times that of proton.T<d< An isotope of iodine is used for ma:ing tincture iodine, 0hich is used as a medicine.T<

Put tic' (K) $0$inst correct choice $nd cross (L) $0$inst %ron0 choice in uestions ,?6 ,$nd ,B

7uestion ,? #utherford!s $lph$-p$rticle sc$tterin0 e/periment %$s responsi&le for thediscovery of($) Atomic Nucleus(&) Electron(c) Proton(d) NeutronAns%er a< Atomic /ucleus R<



b< Electron L<c< +roton L<d< /eutron L<

7uestion , Isotopes of $n element h$ve ($) the s$me physic$l properties (&) different chemic$lproperties (c) different num&er of neutrons (d) different $tomic num&ersAns%er a< the same physical properties L<b< different chemical properties L<c< different number of neutrons R<d< different atomic numbers. L<

7uestion ,B Num&er of v$lence electrons in 1l – ion $re ($) , (&) C (c) ,B (d) ,CAns%er a< ! L<b< ( R<c< 2 L<

d< ( L<

7uestion ,C 9hich one of the follo%in0 is $ correct electronic confi0ur$tion of sodium<a< -,( b< (,-, c< -,,( d< -,(,.

Ans%er (d) 86C6,

7uestion , 1omplete the follo%in0 t$&le

AtomicNum&er

"$ssNum&er

Num&er ofNeutrons

Num&er ofProtons

Num&er ofElectrons

N$me of theAtomic Species

% $ # $ $ $# )- $ $ $ *ulphure

$ - $ - $ $

$ - $ $ $

$ # # $

Ans%er

Atomic

Num&er

"$ss

Num&er

Num&er of

Neutrons

Num&er of

Protons

Num&er of

Electrons

N$me of the Atomic

Species% % # % % Iluorine

! )- ! ! ! *ulphure

- - - - - Magnesium

- Ceuterium

# +rotium



Suestions E"plain 0ith e"amples i< atomic number ii< mass number iii< isotopes and iv< isobars. 6ive

any t0o uses of isotopes.

Ans. i< Atomic number The number of protons in the nucleus of an atom is called its atomic number.E"ample 8hlorine has 2 protons in its nucleus, therefore, its atomic number is 2.

ii< Mass number The number of protons and neutrons present in the nucleus of an atom is :no0n as its mass

number.

E"ample *odium has protons and - neutrons in its nucleus, therefore, its mass number is -< F -).

iii< 1sotopes Atoms of the same element having the same atomic number but different mass numbers are

called isotopes.

E"ample 8hlorine has t0o isotopes )G28l and )2

2 8l , in 0hich atomic number is 2 for both, but mass

numbers are )G and )2.

iv< 1sobars Atoms of different elements having the same mass number, but different atomic numbers are

:no0n as isobars.

E"ample 8alcium and argon atoms have the same mass number #, but different atomic numbers -# and (respectively.

@ses of Isotopes

. 1sotope of uranium -)G%-K is used as a nuclear fuel in atomic reactors.

-. 8obalt & !#, an isotope of cobalt is used in the treatment of cancer.

).1sotopes of iodine is used to treat goitor

Suestions /a has completely filled D and 5 shells. E"plain.

Ans. /a loose electron to form sodium ion /a . *o, /a ion has one electron less than /a atom i.e., it has

# electrons. Therefore, Electronic configuration of /a F -,( completely filled :-< and(< l shells

Suestions 1f bromine atom is available in the form of, say, t0o isotopes 2%)G 4r %.2< and(

)G 4r G#.)<,

calculate the average atomic mass of bromine atom.

Ans The average atomic mass of bromine atom F %.2 of 2% G#.) of (F )%.-!)#.2) F (#.##! u F

(# u,

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Questions: The average atomic mass of a sample of an element N is !.- u. Hhat are the percentages of

isotopes ! ( N and (( N in the sampleU

Ans 5et the percentage of isotope ! ( N be y. then, the percentage of isotope (( N 0ill be ## V y< .

Therefore, !.- F ! " y'## ( " ##$y<'##

!.- F !y'## (## $ ##y<'##

!.- "## F !y(## $ (y

!-# $(## F $-y

$(#'$- F y F %#

*o, ! ( N F %# and ( ( N F #

Questions: 1f F ), 0hat 0ould be the valency of the elementU Also, name the element.

Ans F ), 0e mean that the atomic number of the element is ). 1ts electronic configuration is -, . 7ence, the

valency of the element is Therefore, the element 0ith F ) is lithium.

"ultiple 1hoice 7uestions , / ? . ? m$r's1. Which of the following correctly represent the electronic distribution in the Mg atom?

(a) 3 ! 1 (b) " ! " (c) 1 ! 3 (d) ! " "

". #utherford$s %alpha (&) particles scattering e'periment$ resulted in to discoery of

(a) lectron (b) *roton (c) +ucleus in the atom (d) ,tomic mass

3. -he ion of an element has 3 positie charges. Mass number of the atom is " and the number of

neutrons is 1/. What is the number of electrons in the ion?

(a) 13 (b) 10 (c) 1/ (d) 1

/. lements with alency 1 are

(a) ,lways metals (b) always metalloids (c) either metals or non2metals (d) always non2metals

. -he first model of an atom was gien by

(a) +. 4ohr (b) . 5oldstein (c) #utherford (d) 6.6. -homson

,ns: 1. (b) ". (c) 3. (b) /. (c) . (d)

7ery 8hort ,nswer Questions " ' 9 10 mars

. ;s it possible for the atom of an element to hae one electron one proton and no neutron? ;f so

name the

element.



,ns: <es it is true for hydrogen atom which is represented as 1=1

. Why did #utherford select a gold foil in his &>ray scattering e'periment?

,ns it is because gold has high malleability can be hammered into thin sheet

!. Will l23 and l23 hae different alences?

,ns: +o ;t is because these are isotopes of chlorine that hae same atomic number but different

mass

number

@. alculate the number of neutrons present in the nucleus of an element A which is represented as31 X15

,ns:31 X

15 .indicate that +o. of proton91 and mass number 931

Mass number 9 +o. of protons B +o. of neutrons 9 31

+umber of neutrons 9 31> number of protons 9 31>1 9 1

10. -he atomic number of calcium and argon are "0 and 1! respectiely but the mass number of

both these elements is /0. What is the name gien to such a pair of elements?

,ns: ;sobars

8hort ,nswer Questions (3 mars) 3 ' 9 1 mars

11. Why do =elium +eon and ,rgon hae a Cero alency?

,ns: =elium +eon and ,rgon hae " ! and ! electron in outermost cell so they are haing no need

to gain

or loss electrons .=ence they hae Cero alency.

1". ;n what way the #utherford proposed atomic model?

,ns: #utherford proposed a model in which electrons reole around the nucleus in well2defined

orbits.-here is a positiely charged centre in an atom called the nucleus. =e also proposed that the siCe of

the nucleus is ery small as compared to the siCe ofthe atom and nearly all the mass of an atom is

centered in the

nucleus.

13. ;n what way the -homson proposed atomic model?



,ns: -homson proposed the model of an atom to be similar to a hristmas pudding. -he electrons

are

studded lie currants in a positiely charged sphere lie hristmas pudding and the mass of the

atom was

supposed to be uniformly distributed.

1/. What were the drawbacs of #utherford$s model of an atom?

,ns: -he orbital reolution of the electron is not e'pected to be stable. ,ny particle in a circular orbit

would undergo acceleration and the charged particles would radiate energy. -hus the reoling

electron would lose energy and finally fall into the nucleus. ;f this were so the atom should be highly

unstable and hence matter would not e'ist in the form that we now.

1. What are the limitations of 6.6. -homson$s model of an atom?

,ns. -he maDor limitation of 6.6. -homson$s model is that it does not e'plain how positiely charged

particles are shielded from negatiely charged particles without getting neutraliCed.

CBSE Class IX Structure of Atom

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