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1. CLASS : IX
CHAPTER : STRUCTURE OF ATOM

2. It is a glass tube which is sealed at both ends. Two
electrodes ,cathode and anode are placed inside the
tube which are connected to a source of high voltage
(greater than 10,000 volts ).The tube is also
connected to a vacuum pump to reduce the pressure.
When current of very high voltage and when pressure
is reduced to 0.001mm of Hg ,the glass walls of
discharge tube begins to glow.(fluorescence)
DISCHARGE TUBE EXPERIMENT

4. Cathode Rays
They are a beam of negatively charged particles
(electrons) discovered by J.J Thomson in 1897.
Properties of Cathode rays
• They travel in a straight line.
• They are made up of negatively charged particles
called electrons.
• They get deflected by electric and magnetic fields.
• They can do mechanical work since they possess
kinetic energy.
• Charge on an electron is 1.6 X10-19 coloumbs.
• Mass of an electron is 9.1 x10 -31 Kg (negligible)

5. • They are a beam of positively charged particles that originate
from anode. They were discovered by E. Goldstein. They are
also called canal rays because they pass through the
perforations of cathode.
Properties of anode rays(Canal rays )
• They travel in a straight line.
• They are called canal rays(because they pass through the
perforations of the cathode.
• They are made up of protons.
• The nature of anode rays depend upon the gas enclosed in
the discharged tube.
• They possess Kinetic energy.
• They get deflected by electric and magnetic fields.
Anode Rays

7. 1. How were cathode rays produced using a
discharge tube? Give four properties of cathode
rays.
2.What are canal rays? Give the four properties
of canal rays
ANSWER THE FOLLOWING

8. Discovery of sub atomic particles :-
In 1900, J.J. Thomson discovered the presence of the
negatively charged particles called electrons in the atom.
In 1886, E. Goldstein discovered new radiations in gas
discharge and called them canal rays. These rays were
positively charged. This later led to the discovery of the
positively charged particles called protons in the atom.
In 1932 Chadwick discovered the presence of particles
having no charge in the atom called neutrons.

9. Atoms have three types of sub atomic particles. They
are electrons, protons and neutrons.
Electrons are negatively charged (e-), protons are
positively charged (p+) and neutrons have no charge
(n).
The mass of an electron is 1/2000 the mass of a
hydrogen atom. The mass of a proton is equal to the
mass of a hydrogen atom and is taken as 1 unit. The
mass of a neutron is equal to the mass of a hydrogen
atom and is and is taken as 1 unit.
.
CHARGED PARTICLES IN MATTER

10. Thomson’s model of an atom :-
J.J Thomson’s model of an atom may be visualized like a
Christmas pudding of cake. The electrons are like dry
fruits which are embedded on the base which looks like a
positively charged sphere.
According to another view, it can be compared with
watermelon. The red edible part represents the positive
charge in which electrons are embedded in the same way
as seed in the watermelon.

11. J.J THOMSON’S MODEL OF AN ATOM
According to J.J Thomson
1. An atom may be regarded as a positively charged
sphere in which protons are present.
2.The negatively charged electrons may be regarded
as studded or embedded in this sphere.
3.The positive charges due to protons and negative
charges due to electrons balance each other. As a
result ,an atom as a whole is electrically neutral or it
has no net charge.
Limitations
1. J.J Thomson’s model of atom failed to explain how the positive charge holds on
the electrons inside the atom. It also failed to explain an atom’s stability. The
theory did not mention anything about the nucleus of an atom

12. RUTHERFORD’S GOLD FOIL
EXPERIMENT
Rutherford performed the famous α –scattering
experiment in the year 1911.For this, he selected a thin
foil of a heavy gold metal. It was about 1000 atoms
thick. He bombarded the same with high speed alpha
(α-) particles. These are in fact ,positively charged
helium nuclei (He2+ ) .Each such particles carries a
mass of 4u and a charge of +2 units.These were
emitted from a radio active element Radium closed in
a Lead box.

14. 2)Rutherford’s model of an atom
Rutherford’s alpha scattering experiment :-
Rutherford allowed a beam of fast moving alpha particles ( α –
particles) having positive charge to fall on a thin gold foil. He
observed that :-
i) Most of the α – particles passed straight through the gold foil.
ii) Some of the α – particles were slightly deflected by small angles.
iii) Very few α – particles appeared to rebound.

15. Conclusions from Rutherford’s
alpha scattering experiment
i) Most of the space inside an atom is empty. (because most of the α –
particles passed straight through the gold foil.)
ii) The atom had a small nucleus having positive charge. ( because
some of the α – particles having positive charge were slightly
deflected by small angles.)
iii) The size of the nucleus is very small compared to the size of the
atom. (because very few α – particles appeared to rebound and
most of the positive charge and mass of the atom is in the nucleus.)
Rutherford’s model of an atom :-
i) An atom has a positively charged nucleus at its centre and most of
the mass of the atom is in the nucleus.
ii) The electrons revolve around the nucleus in different orbits.
iii) The size of the nucleus is very small compared to the size of the
atom.

16. Defects of Rutherford’s
model of the atom
Any particle in a circular orbit would undergo
acceleration and during acceleration the charged
particle would radiate energy. So the revolving
electrons would lose energy and fall into the nucleus
and the atom would be unstable. We know that atoms
are stable.

17. c) Bohr’s model of an atom
i) An atom has a positively charged nucleus at its centre and most of
the mass of the atom is in the nucleus.
ii) The electrons revolve around the nucleus in special orbits called
discrete orbits.
iii) These orbits are called shells or energy levels and are represented
by the letters K, L, M, N etc. or numbered as 1, 2, 3, 4, etc.
iv) While revolving in the discrete orbits the electrons do not radiate
energy.
Shells or energy levels in an atom

18. Answer the following questions
1. State the postulates of Bohr’s model of atom?
2. What observation in a scattering experiment led Rutherford to make
the following conclusions:
i) Most of the space in an atom is empty.
ii) Centre is positively charged.
3. Atom is electrically neutral. Explain this statement based on Thomson’s
model of an atom
4. Write the maximum number of electrons that can be filled in K and L
shell of Bohr’s model of an atom.

19. Answers
1. i) Only certain special orbits known as discrete orbits of electrons
are allowed inside the atom.
ii) While revolving in discrete orbits, electrons do not radiate energy
2. i) Most of the alpha particles passed through the gold foil without
deflection.
ii) A small fraction of alpha particles got deflected.
3. According to Thomson’s model of atom, an atom consists of a
positively charged sphere and the electrons are embedded in it, the
magnitude of positive and the negative charges are equal, so the atom
is electrically neutral.
4. K shell:2, L shell :8

20. The distribution of electrons in the different shells was suggested by
Bohr and Bury. The following are the rules for filling electrons in the
different shells.
i) The maximum number of electrons in a shell is given by
the formula 2n2 where n is the number of the shell 1, 2, 3
etc.
First shell or K shell can have = 2n2 = 2 x 12
= 2x1x1 = 2 electrons
Second shell or L shell can have = 2n2 = 2 x 22
= 2x2x2 = 8 electrons
Third shell or M shell can have = 2n2 = 2 x 32 = 2x3x3
= 18 electrons
Fourth shell or N shell can have = 2n2 = 2 x 42 = 2x4x4
= 32 electrons and so on.
ii) The maximum number of electrons that can be filled in
the outermost shell is 8.
iii) Electrons cannot be filled in a shell unless the inner
shells are filled.
Distribution of electrons in different shells :-

21. OCTET AND DUPLET RULE
• Octet rule:
The maximum number of electrons that the outer most shell can hold is 8 electrons
because an atom is stable if it has 8 electrons in the outermost shell.
• Duplet rule : The first shell of an atom can hold a maximum of 2 electrons .
Note:
• Elements except noble gases (they already have 8 electrons in their outer shell) react to
get 8 electrons in the outer most shell to become stable.
• If an element has one, two or three electrons in their outer most shell it gives away the
electrons, becomes a cation (Positively charged atom ) and is metal.
• If an element has 5,6 and 7 electrons in the outermost orbit,it accepts the electrons and
becomes anion(negatively charged atom) and is non-metal.
• If an element has 4 electrons in the outermost orbit,it will share 4 electrons.It has
metalloid properties.

22. Valency
• Valency : The capacity of an element to combine with other atoms to
form a compound is valency. It is therefore the number of electrons
given, accepted or shared during bond formation
• The electrons present in the outermost shell of an atom are called
valence electrons.
• Calculation of Valency :
i ) For elements having valence electrons 1,2 or 3 ,valency is equal to
the number of valence electrons.
ii) For elements having valence electrons 4,5,6 or 7 valency is equal to
8-number of valence electrons.
Valency=8 - number of valence elctrons
Eg: In Na atom no of
valence electrons =1
So valency =1

23. • ANSWER THE FOLLOWING QUESTIONS
1. Find out the valency of the atoms represented by the figure (a) and (b)
(a) (b)
2. Helium atom has 2 electrons in its valence shell but its valency is not .Explain.
3.Elements A and B have atomic numbers 18 and 16 respectively. Which of
these two would be more reactive and Why ?

24. • ANSWERS
1. a) Valency = 0 , Argon (Ar) atom, it has already 8 electrons in
the valence shell. So it is stable.
b) Valency = 1, Chlorine (Cl) atom .valence electons =7
valency = 8-valence electrons
=8-7=1
2. Helium atom has only one shell (K shell) which can have maximum 2
electrons. Thus ,its shell is already complete called duplet. It can neither
lose electrons nor gain electrons. Hence its valency is zero. It is called
noble gas or inert gas.
3. Electronic configuration of A (Z=18) =2,8,8
Electronic configuration of B (Z=16) =2,8,6
Element A has completely filled outer most shell. It would be least
reactive. Element B has incomplete outermost shell (M shell has 6
electrons) only. Therefore, element B would be more reactive than A.

25. LETS POLL !!!
1. Number of valence electrons in Cl- ion is
a) 16 b) 8
c) 17 d) 18
2. An atom with 3 protons and 4 neutrons will have a valency of
a) 3 b) 7
c) 1 d) 4

26. ELECTRONIC CONFIGURATION OF FIRST 20 ELEMENTS
Name of
element
Symb
ol
Atomic
Number
Number
of
Protons
Number
of
Neutrons
Num
ber of
Electro
ns
Distribution
Of Electrons
K L M N
Val
enc
y
Hydrogen H 1 1 - 1 1 - - - 1
Helium He 2 2 2 2 2 - - - 0
Lithium Li 3 3 4 3 2 1 - - 1
Beryllium Be 4 4 5 4 2 2 - - 2
Boron B 5 5 6 5 2 3 - - 3
Carbon C 6 6 6 6 2 4 - - 4
Nitrogen N 7 7 7 7 2 5 - - 3
Oxygen O 8 8 8 8 2 6 - - 2
Fluorine F 9 9 10 9 2 7 - - 1
Neon Ne 10 10 10 10 2 8 - - 0
Sodium Na 11 11 12 11 2 8 1 - 1
Magnesium Mg 12 12 12 12 2 8 2 - 2

27. Name of
element
Symb
ol
Atomic
Number
Number
of
Protons
Number
of
Neutrons
Number
of
Electro
ns
Distribution
Of Electrons
K L M N
Valen
cy
Aluminium Al 13 13 14 13 2 8 3 - 3
Silicon Si 14 14 14 14 2 8 4 - 4
Phosphorus P 15 15 16 15 2 8 5 - 3,5
Sulphur S 16 16 16 16 2 8 6 - 2
Chlorine Cl 17 17 18 17 2 8 7 - 1
Argon Ar 18 18 22 18 2 8 8 - 0
Potassium K 19 19 20 19 2, 8, 8, 1 1
Calcium Ca 20 20 20 20 2, 8, 8, 2 2

28. Atomic structure of the first eighteen elements :-
H He
Li Be B C N O F Ne
Na Mg Al Si P S Cl Ar

29. Atomic number(Z) =19
Electronic configuration
=2,8,8,1
Atomic number(Z)=20
Electronic
configuration
=2,8,8,2
ATOMIC STRUCTURE OF POTASSIUM (K) AND CALCIUM (Ca)

30. Atomic number and Mass
number
a) Atomic number (Z) :-
The atomic number of an element is the number of protons present in the
nucleus of the atom of the element.
All the atoms of an element have the same atomic number.
Eg :- Hydrogen – Atomic number = 1 (1 proton)
Helium - Atomic number = 2 (2 protons)
Lithium - Atomic number = 3 (3 protons)
b) Mass number (A) :-
The mass number of an element is the sum of the number of protons and
neutrons (nucleons) present in the nucleus of an atom of the element.
The mass of an atom is mainly the mass of the protons and neutrons in the
nucleus of the atom.
Eg :- Carbon (A= 12) (6 protons + 6 neutrons) Mass = 12u
Aluminium (A= 27) (13 protons + 14 neutrons) Mass = 27u
Sulphur (A= 32) (16 protons + 16 neutrons) Mass = 32u

31. Atomic number and Mass number
In an neutral atom
Atomic number (z) = no of protons
OR
no of electrons
Mass number (A) = no of protons + no of neutrons
Eg: The atom of an element has 9 protons ,9electrons and
10 neutrons.

32. In the notation of an atom the atomic number
and mass number are written as
Eg.
ATOMIC SYMBOL NOTATION

33. LETS POLL !!!
1. Which of the two will be chemically more reactive, Neon (Ne) with atomic
number 10 or Chlorine (Cl) with atomic number 17 ?
a) Chlorine b) Neon
c) Both are equally reactive d) Can’t say
2. The nucleons are
a) protons and electrons b) neutrons and electrons
c) protons and neutrons d) none of these

34. ANSWER THE FOLLOWING QUESTIONS
1. The atom of an element has 9 protons ,9 electrons and 10
neutrons.
i) What is the atomic number of the element ?
ii) What is the mass number of the element ?
Iii) Name the element and give its electronic configuration.
iv) Predict the valency of the element .
2. The element Sulphur has atomic number 16,and mass number
32.State the number of protons, electrons and neutrons in it. Give
the arrangement of the electrons in different energy shells. What
is the valency of the element ?
3.
Mass
number
Atomic
number
No of
Protons
No of
electrons
Name of
the
element
24 ---- 12 ---- ----
35` ---- ----- 17 ----

35. ANSWERS
1. i) the atomic number of the element =No of protons =9
Ii) the mass number of the element = No of protons + No of
neutrons = 9+10 =19
Iii) the element with Z =9 is Fluorine (F) and its electronic
configuration = 2,7
iv) the valency of the element is 1 and is calculated as 8-7=1
2. No of protons = atomic number =16
No of electrons = No of protons = 16
No of neutrons = Mass no –no of protons
=32-16=16
Electronic arrangement =2,8,6, Valency =8-6 =2
3.
Mass
number
Atomic
number
No of
Protons
No of
electrons
Name of
the
element
24 12 12 12 Mg
35` 17 17 17 Cl

36. Isotopes
Isotopes are atoms of the same element having the same atomic
numbers but different mass numbers.
Eg :- Hydrogen has three isotopes. They are Protium, Deuterium (D)
and Tritium (T).
ISOTOPES OF HYDROGEN
ISOTOPES OF CARBON

37. APPLICATIONS OF RADIO ISOTOPES

38. AVERAGE ATOMIC MASS
Actually , isotopes are found in nature in some particular
ratio and on this basis, average atomic mass of an isotope
can be calculated,
eg: atomic mass of Chlorine is 35.5 .
The two isotopes of Chlorine 35Cl, 37Cl are present in nature
in the ratio of 3:1 respectively. Therefore, average atomic
mass of isotope can be calculated as
= (35 x 3) + (37x1) = 35.5 u
4

39. AVERAGE ATOMIC MASS
Q1. If the element Bromine is in the form two isotopes
which are 79Br(49%) and 81Br(50.3%) ,then calculate the
average atomic mass of Bromine.
Percentage of Br-79 isotope =49.7
Percentage of Br-81 isotope =50.3
Average atomic mass of Bromine
= atomic mass of 1st isotope x its % + atomic mass of 2nd isotope x its %
100
= (79x49.7) + (81 x50.3)
100
= 39.263+ 40.743 =80.00u

40. LETS POLL !!!
1. Which isotope is used in the nuclear power plants to generate electricity ?
a) Uranium 235 b) Iodine 131
c) Cobalt 60 d) Uranium238
2. The isotope used to remove the brain tumors and treatment of cancer
is
a) U-235 b) Na -24
c) Iodine d) Co-60

41. Isobars
Isobars are atoms of different elements having different atomic
numbers but same mass numbers.
These pairs of elements have the same number of nucleons.
Eg :- Calcium (Ca) – atomic number - 20 and Argon (Ar) – atomic
number 18 have different atomic numbers but have the same mass
numbers – 40.
Eg.1
Eg.2

42. Ions are formed when atoms lose or gain electrons in
order to fulfil the octet rule and have full valence
electron shells. When they lose electrons, they
become positively charged and are named cations.
When they gain electrons, they are negatively charged
and are named anions.
There are two types of ions
1.Cations or positive ions
2. Anions or negative ions
FORMATION OF IONS (CATIONS AND ANIONS)
There are two types of ions
1.Cations or positive ions
2. Anions or negative ions

44. POSITIVEIONS
(CATIONS)
A positive ion gets formed in case atom of an element loses one or
more electrons. For example, the electronic distribution in Sodium
(Na) is 2,8,1.It loses one electron to form a positive ion with electronic
distribution 2,8.Since for an ion, the number of electrons (2+8=10) is
one less than the number of protons (11),it has one unit positive
charge. It may be represented as follows.

45. NamingCations
A metal that has
lost electrons to
become an ion has
the same name as
theelement
Eg:Ca2+ = calciumion
Eg:Na+ =Sodium ion

46. Negative Ions or Anions
A negative ion gets formed in case atom of an
element gains one or more electrons. For example,
electronic distribution in Fluorine (F) is 2,7.It gains
one electron to form a negative ion with electronic
distribution 2,8.Since in the ion, the number of
electrons(2+8=10) is one more than the number of
protons (9) ,it has one unit negative charge. It may
be represented as follows

47. Naming
Anions A nonmetal that has
gained electrons to
become an ion has
the the same name
as the element but
with the ending
changed to -ide
Eg:Cl- = chloride ion

48. Naming Anions
nitrogen à
oxygen à
fluorine à
phosphorous à
sulphur
à chlorine
à bromine
à iodine
à
nitride
oxide
fluoride
phosphide
sulphide
chloride
bromide
iodide

49. A metal atom that has lost electrons (cation) and a nonmetal
atom that has gained electrons (anion) will have the same
number of electrons as its nearest noble gas.

50. Cations
The cations Na+, Mg2+, and Al3+ all have the same
numberofelectronsasatomsofneon.
Thisrelationshipisknownasbeing
isoelectronic(havingthesamenumberof electrons).

51. Cations
An metal atom that has lost
electrons (cation) will have
the same number of electrons
as its nearest noblegas.

52. LETS POLL !!!
1. Four elements W,X,Y and Z contain 8,11,9 and 17 protons per atom
respectively. The element which cannot form an anion is most likely to be
a) W b) X
c) Y d) Z
2. The atomic number of four elements A,B,C and D are 12 ,13,15 and 3
respectively. The element which cannot form a cation is
a) A b) B
c) C d) D

53. Sample Protons Neutrons Electrons
A 19 21 19
B 17 18 17
C 17 20 17
D 18 22 18
ANSWER THE FOLLOWING
1. On the basis of the number of protons, neutrons and electrons in the
samples given below.
Identify:
a) Write the electronic distribution in atom of element B ?
b) Is element B a metal or a non metal ? Why ?
c) Which two elements form a pair of Isotopes ?
d) Which two elements form a pair of Isobars ?

54. 2. a) Write the electronic configuration of Al3+ and O2- and draw
their schematic atomic structure.
3. The composition of two atomic particles are given :
a) What is the mass number of X?
b) What is the mass number of Y?
c) What is the relation between X and Y?
d) Which element/elements do they represent?
X Y
Protons 8 8
Neutrons 8 9
Electrons 8 8

55. 4. An atom of an element has 6 electrons in the outermost M shell. Write
a) Electronic configuration
b) Name of this element
5. The electronic configuration of K shell is 2, 8, 8, 1 instead of 2, 8, 9
though the M shell can accommodate up to 18 electrons. Explain?
6. Show the electron distribution in magnesium atom and a magnesium
ion diagrammatically and also write their atomic number?
7. Give reason for the following :
a) Isotopes of element are chemically similar
b) Ions are more stable than atoms

56. 1. 1. a) The electronic distribution of element B is 2,8,7
b) the element B is a non metal (Cl). Its atom needs only one electron to have the
configuration of nearest noble gas element Argon( 2,8,8)
c) Elements B and C form pair of isotopes since both of them have same number of
electrons 17 and same atomic number (Z=17)
d) The elements A and D represent pair of isobars since both of them have same mass
number (A = 40)
2. Al3+ = 2,8
O2- = 2,8
3.a) Mass number of X = 8 + 8 = 16
b) Mass number of Y = 8 + 9 = 17
c) Atomic particles represents a pair of isotopes
d) The element with atomic number 8 is Oxygen
4. a) Electronic configuration =2,8,6
b) Name of this element = Sulphur
5. M shell cannot have more than 8 electrons in case if it happens to be outermost or
valence shell in an atom. Therefore, the correct electronic configuration of the element of
the K shell is 2, 8, 8, 1.
ANSWERS

57. 6. Mg atom
No of electrons = 12, no of protons = 12
Mg2+ion
No of electrons = 10, no of protons = 12
7. a) Isotopes of an element have same atomic number but different mass
number. Hence, they have same number of electrons. Chemical properties are
related to the number of valence electrons. Thus, isotopes have similar chemical
properties.
b) Ions are more stable because their valence shell is fulfilled and therefore no
sharing of electrons takes place. While atoms do not have fulfilled valence shell
and so, Ions are considered more stable.

58. ANNEXURE
https://www.youtube.com/watch?v=4QAzu6fe8rE -discharge tube
experiment- cathode ray experiment
https://www.youtube.com/watch?v=wS51eOCoJCg – Anode ray
experiment
https://www.youtube.com/watch?v=GLp9ihOYjjM The Atom song
https://www.youtube.com/watch?v=lLwnACfo7hY -J.J Thomson
model of an atom
https://www.youtube.com/watch?v=kHaR2rsFNhg –Rutherfords
alpha scattering experiment
https://www.youtube.com/watch?v=4z46Bs3fRCY –Rutherford’s
model of atom
https://www.youtube.com/watch?v=fm2C0ovz-3M - Rutherford’s
model of atom failure, Bhor’s model of an atom
https://www.youtube.com/watch?v=lVSF2lP4oBA –Valency
https://www.youtube.com/watch?v=EIAaGHK5pjA&t=23s –formation
of ions-Cations and Anions

59. THANK YOU