CLASS:9th                                CHAPTER #2

STRUTURE OF ATOM            

The word atom was first introduced by Greek philosopher Democritus. He gave the concept that matter is made up of very small indivisible particles called atoms. The origin of word atom was Greek word atomos means indivisible, uncuttable (a– not, tomos-a cutting)

Q: what is Rutherford’s atomic model? , what were main defects in the Rutherford’s atomic model?



  Rutherford’s atomic model shows the existence of nucleus in the atom, nature of charge on the nucleus   and the magnitude of charge on the nucleus.
  • Alpha particles.
  • Gold foil. (0.0004 cm thick)
  • Zinc sulphide screen.(ZnS)
  In his experiments, Rutherford bombarded alpha particles on very thin metallic foils such as gold foil of 0.0004cm thickness. He obtained alpha particles (He2+) from a radioactive element radium or polonium. A  In order to record experimental observations, he made use of circular screen coated with zinc sulphide.
1.    He observed that most of the alpha particles were pass through the foil undeflected.

2.     Very few particles were deflected when passed through the foil.

3.     One particle out of 1 million particles was deflected greater than 90o.

4.     Few particles were deflected at different angles.

5.    Massive alpha particles were not deflected by electrons.



From these experiments Rutherford proposed a planetary model for an atom. He drew the following conclusions.

1.       Since most of the a-particles pass through the foil undeflected, therefore, most of the space occupied by the atom is empty.

2.       The deflection of a few particles proved that there is a centre of heavy positive charge in an atom which causes repulsion as shown in the figure. The heavy part of the atom is called its nucleus.

3.                   All the space outside the nucleus is occupied by the fast revolving electron. The massive a-particles are not deflected by these electrons.

4.       The centrifugal force which is produced due to rotation of electrons balances the electrostatic force of attraction between the nucleus and electron. This prevents the electrons from falling into nucleus.

1.        Major portion of the atom is empty.

2.       The whole mass of the atom is concentrated in the center of atom called nucleus.

3.        The positively charged particles are present in the nucleus of atom.

4.        The electrons revolve around the nucleus in different circular orbits.

5.       Size of nucleus is very small as compare to the size of atom.

1.        According to classical physics, electrons being charged particle should emit energy continuously. The orbit of electron should become smaller and smaller until it falls into the nucleus. But it never falls because if it falls at all, the atomic structure would collapse.

2.       If the electron emits energy continuously, atom should form continuous spectrum. Actually atoms give line spectrum.


In 1913 Neil Bohr presented his atomic model, he introduced circular orbit concept on basis of Plank’s Quantum theory.

Main postulates of Bohr’s atomic theory are:
1. Electrons revolve around the nucleus in circular path, which are known as “ORBITS” or “ENERGY LEVEL.
2. Energy of an electron in one of its allowed orbits is fixed.As long as an electron remains in one of its allowed orbit, it can not absorb or radiate energy.
3. If an electron jumps from lower energy level to a higher energy level, it absorbs a definite amount of energy.
4. If an electron jumps form higher energy level to a lower energy level, it radiates a definite amount of energy.
5. Energy released or absorbed by an electron is equal to the difference of energy of two energy levels.
Let an electron jumps from a higher energy level E2 to a lower energy level E1.The energy is emitted in the form of light .Amount of energy released is given by:
∆E = E2 – E1
E2 – E1= hν

h = Planck’s constant ( 6.6256 x 10-34 J.s)

ν= Frequency of radiant light

6. Spectrum of light emitted from an electron is a “LINE SPECTRUM”.

7. Angular momentum of an electron is given by:
m v r =                             Where n =1, 2, 3, ………

m = mass of electron    V = velocity of electron


The word isotope was first used by Soddy. It is a Greek word “isos” means same and “tope” means place.

Isotopes are atoms of an element whose nuclei have the same atomic number but different mass number. This is because atoms of an element can differ in the number of neutrons.

Isotopes are chemically alike and differ in their physical properties.

Isotopes of Hydrogen

Hydrogen has three isotopes.  1.Hydrogen –1 ( Protium) Ordinary hydrogen is knows as protium .

  • It has one electron ,one proton but it has no neutron.
  • Mass number : 1
  • Symbol : 1H1
  • Percentage in natural hydrogen:99%                     

2.Hydrogen – 2 ( deuterium) Deuterium

  • It has one electron one proton and one neutron.
  • Mass number: 2
  • Symbol: 1H2or D  Percentage In Natural Hydrogen: 0015%.
  • Heavy water(D2O) consists of deuterium isotope of hydrogen.
  1. Hydrogen –3 (Tritium)
  • It has one electron, one proton and two neutrons.
  • Mass number : 3
  • Symbol : 1H3
  1. It is a radioactive isotope. Not occurring naturally
  2. It emits radioactive rays.
  3. It is present in traces.

Comparision of ordinary water and heavy water.

Property Ordinary water Heavy water
Melting Point 0.000C 3.810C
Bioling point 1000C 101.20C
Density at 250C 0.99701 g/cm3 1.1044 g/cm3


Isotopes of Carbon

Carbon has three isotopes. Carbon–12, carbon–13 and carbon –14. Almost all the carbon is carbon–12. Its symbol is It has six neutrons and six protons. Carbon–13 has symbol It has seven neutrons and six protons. Carbon–14 has eight neutrons and six protons. Its symbol is  Different forms of carbon are black or greyish black solids except diamond. They are odourless and tasteless. They have high melting and boiling points and are insoluble in water.

Isotopes of Carbon

Natural abundance of isotopes of carbon is as follows






Isotopes of Chlorine

There are two natural isotopes of chlorine,

chlorine–35 and chlorine–37.

  • An atom of chlorine–35 has 17 protons and 18 neutrons.
  • An atom of chlorine–37 has 17 protons and 20 neutrons. Chlorine–35 occurs in nature about 75% and chlorine–37 about 25%.
  • Chlorine is a greyish yellow gas with sharp pungent irritating smell. It is fairly soluble in water.

Natural abundance  75.77%                           24.23%




Isotopes of Uranium

ranium has three isotopes with mass number 234, 235 and 238 respectively.

  • Theisotope is used in nuclear reactors and atomic bombs
  • the isotope lacks the properties necessary for these applications. is rare. Natural abundance of Uranium isotopes is as follows:

Fill in the blanks?

has ___ protons, ___ electrons and ___ neutrons

has ___ protons, ___ electrons and ___ neutrons

has ___ protons, ___ electrons and ___ neutrons

Uranium Decay When uranium–238 decays into thorium–234, it emits alpha particle. An alpha particle is doubly positively charged helium nucleus.

The fission of uranium–235 yields smaller nuclei, neutron and energy. The nuclear energy released by the fission of one kilogram of uranium–235 is equivalent to chemical energy produced by burning more than 17000 kg of coal.







Uses of isotopes

Stable and radioactive isotopes have many applications in science and medicines. Some of these are as follows:

  • Radioactive iodine -131 is used as a tracer in diagnosing thyroid problem.
  • Na-24 is used to trace the flow of blood and detect possible constrictions or obstructions in the circulatory system.
  • Iodine-123 is used to image the brain.
  • Cobalt-60 is commonly used to irradiate cancer cells in the hope of killing or shrinking the tumors.
  • Carbon-14 is used to trace the path of carbon in photosynthesis.
  • Radioactive isotopes are used to determine the molecular structure e.g. sulphur-35 has been used in the structure determination of thiosulphate, S2O3-2
  • Radioactive isotopes are also used to study the mechanism of chemical reactions.
  • Radioactive isotopes are used to date rocks, soils, archaeological objects, and mummies.

Electronic configuration:The distribution of electrons in various subshells is called electronic configuration.

Shells: According to the Bohr theory electrons in an atom revolve around nucleus in one of the circular paths called shells or orbits or energy levels.

Each shell is described by an n value and n can have values 1, 2,3……………………….

When                     n=1         it is K shell

n=2         it is L shell

n=3         it is M shell

n=4         it is N shell

Subshells: A shell or energy level is subdivided into subshells aor sub energy levels.n value of shell is placed before the symbol of subshell.

For instance          n=1 for K Shell  and will be represented as 1s

n=2 for L Shell  and will be represented as 2s, 2p

n=3 for M Shell  and will be represented as 3s,3p,3d

n=4 for N Shell  and will be represented as 4s,4p,4d,4f

Principle:”In filling of the subshells , electrons always enter in lower energy orbit firs”. This principle s known by Auf Bau Principle.

Order of filling of subshells


Give short answers

  1. Distinguish between shell and sub-shell
Shell Sub-shell
electrons in an atom revolve around nucleus in one of the circular paths called shells. A shell is subdivided into subshells
These are labeled as K,L,M,…. These are labeled as s,pd,f
Electrons are distributed as per formula 2n2 and n is for number of shell

K=2, L=8, M=18 ………

They contain electrons as

S=2, p=6,d=10,f=14

And written with their parent shell No. as

1s<2s<2p<3s<3p<4s<3d —–


  1. ii) An atom is electrically neutral, why?

iii)    How many sub-shells are there in N shell.

  1. iv) Give notation for sub-shells of M shell.
  2. v) List the sub-shells of M Shell in order of increasing energy
  3. vi) Can you identify an atom without knowing number of neutrons in it.

Q.3:        The electronic configurations listed are incorrect. Explain what mistake has         been       made in each and write correct electronic configurations.

x = 1s22s22p43p2

y = 1s22s12p1

z = 1s22s22p53s1

Q.4:        Which orbital in each of the following pairs is lower in energy?

  1. 2s, 2p
  2. 3p, 2p
  3. 3s, 4s

Q.5:        Draw Bohr’s Model for the following atoms indicating the location for electron, protons and neutrons:

  1. Potassium (Atomic No 19, Mass No. 39)
  2. Silicon (Atomic No. 14 Mass No. 28 )
  3. Argon (Atomic No. 18 Mass No. 39 )

Q.6:        Write electronic configuration for the following elements:


Q.7:        Describe the contribution that Rutherford made to the development of the atomic theory.


Q.8:        Explain how Bohr’s atomic theory differed from Rutherford’s atomic theory.

It was based upon classical theory It is based upon quantum theory
Electron revolve around the nucleus Electron revolve around in fixed energy orbits
No idea about angular momentum described Orbits have angular momentum
Atoms produce a continuous spectrum Atoms produce line spectrum
Atom may collapse Atoms should exist

Q.9:        Describe the presence of subshells in a shell.

Q.10:      State the importance and uses of isotopes in various fields of life.

Q.11:      The atomic number of an element is 23 and its shells or orbits or atomic mass number is 56.

  1. How many protons and electrons does an atom of this element have?
  2. How many neutrons does this atom have?

Q.12:      The atomic symbol of aluminum is written as. What information do you get from it?

Q.15:   M-24 is a radioactive isotope used to diagnose restricted blood circulation, for example in legs. How many electrons, protons, and neutrons are there in this isotope.   The valence shell electronic configuration of M is 3s1.


Q.16:   Two isotopes of chlorine are and. How do these isotopes differ? How are they alike?


Q.17:   How many electrons can be placed in all of the sub-Shells in the n=2 shell?


Q.18:   Mass number of an atom indicates the total number of protons and neutrons in the nucleus. Can you identify an atom without any neutron?


Q.19:   The table shows the nuclei of five different atoms.


Name of atom Number of Protons Number of neutrons
A 5 6
B 6 6
C 6 7
D 7 7
E 8 8


a.       Which atom has highest mass number?

b.      Which two atoms are isotopes?

c.       Which atom has least number of electrons?

d.      Which atom will have electronic configuration 1s22s22p3.

e.       Which of the atom contains the most number of electrons?


Q.20:   Naturally occurring nitrogen has two isotopes N-14 and N-15 which isotope has a greater number of electrons.


Ans: Atomic no. 13

Mass No. 27

No. of Protons=13

No. of electrons=13

No. of neutrons= 14