2. Introduction
• Substance are made of molecules or atoms
• Atom is smallest, indivisible particle having
independent existence
• John Dalton gave this concept.
3. What we will see this chapter?
• How concept of atom changed over time?
• How the different components of atom were
found?
• Various experiments that evolved Atomic
theory
• Isotopes
• Radioactivity
4. Chronology
Cathode Rays & Discovery of Electrons
X-Rays (1896)
Alpha, Beta, Gamma Rays
Idea of Proton
Thomsons Atomic Model
Rutherfords Atomic Model
Bohr model – electron arrangement
Neutron discovered – Chadwick (1932)
7. • The line diagram of electric discharge tube is
given
• Vacuum is created in this tube
• Two metal plates in tube are Cathode (-ve)
and Anode (+ve) terminals
• On joining with battery, Cathode rays are
produced from cathode
• They have negative charge
• They move towards the Anode
• These are actually the flow of Electrons
• J J Thomson and Crookes proved that Cathode
rays were in fact flow of Electrons
10. Hi, I’m Rontgen
I discovered X – Rays in
1896
They are also
called Rontgen
rays after me
11. • In cathode ray tube, cathode rays were
travelling in straight line
• They were made to strike the walls of tube
• New type of rays formed
• They travelled in straight line and were not
deviated in strong Electromagnetic field
• Could pass through opaque substances like
black paper
• They affected photographic plates
• He named them X -rays
12. • Cathode rays is beam of electrons
• X – rays are electromagnetic waves having
very short wavelength
• If electrons having very high velocity are
stopped by putting some type of resistance
(anti- cathode) the X –rays are produced
16. Uses of X- rays
• In medical science
• To detect fracture in bones
• To observe lung defect
• To diagnose cancer of oesophagus
• To diagnose physical disabilities
18. • In 1889, Ernst Rutherford found Alpha and
Beta rays from radioactive atom like Uranium
• Later, Willard discovered Gamma rays.
• Alpha rays – Positive electric charge
• Beta rays – Negative electric charge
• Gamma rays – do not possess electric charge
• Gamma rays – electromagnetic rays , have
highest penetration power
19.
20. Conclusions of Rutherford
• Atoms must have at least two types of
particles
• Some should have +ve charge, some –ve
• Number of particles having +ve charge should
be equal to number of particles of –ve charge
• Atom is electrically neutral
Neutrons
• +ve charge ---- protons
still not
• -ve charge ---- electrons discovered
21. Ernst Rutherford
Rutherfords model of atom showing Proton in
centre (nucleus) and electrons around it
So how does the atom look???
22. This question was attempted by three people
1) J. Thomson
2) Rutherford
3) Neil Bohr
24. • Atom is like a spherical ball
• Positive electric charge is uniformly spread on
its total volume
• Protons possessing +ve electric charge and
Electrons possessing –ve electric charge are
arranged at definite places
25. Limitations of Thomsons Model
• The positive and negative particles being
together would attract each other and
become chargeless
• It could not explain the chemical properties of
different elements
27. • The proton is 1836 times heavier than
electron
• How are they arranged?
• This is what Rutherford tried to answer.
• He made Alpha rays obtained from radioactive
Polonium (Po) fall on one side of a very thin
(0.004mm thick) gold foil
28.
29. • If the atom is like a sphere (Thomson model)
then all Alpha rays should return back after
striking foil.
• But,
• Rutherford observed that most rays passed
straight through and only very few got
deviated or got thrown back
• Proportion of Alpha rays returning and those
passing through was 1 : 1200
Mr. Thomson you are wrong!!!
30. Rutherford’s conclusions
• Centre of atom must be very small, heavy and
positively charged
• He named it Nucleus
• Around nucleus, electrons with negligible
weight and possessing negative charge must
be arranged
• The atom must be very hollow
• Atomic nucleus was 105 times smaller than
total area of atom
31. If atom size is this stadium then,
Nucleus is size of a Peanut
put at centre
32. To summarize…..
• Atom is hollow
• Centre – heavy, very small, positively charged
nucleus
• Electrons – negligible mass, negatively
charged are arranged around nucleus
How are electrons arranged, Mr.
Bohr???
34. • Proposed model in 1912
• Electrons moving around nucleus in atomic
levels at fixed distance from nucleus
• This level is called Energy level or Orbit
• Electrons continuously move in these orbits
• They do not lose energy, so orbits are
stationary orbits
• Energy of orbit nearest nucleus is least
• As u go away from nucleus, energy of orbit
increases
35. Arrangement of electrons around
nucleus
• i) 1st orbit (K orbit) – 2 electrons
• ii) 2nd orbit (L orbit) – 8 electrons
• iii) 3rd orbit (M orbit) – 18 electrons
• iv) 4th orbit (N orbit) – 32 electrons
• 2nd, 3rd and 4th orbit have sub-orbits called
Orbitals
• First electrons fill 1st orbit then go stepwise to 2nd,
3rd and 4th orbits
40. • Electrons arranged in different orbits having
increasing energy around the nucleus
• Electrons in outermost orbit are responsible
for emission spectra and chemical properties
• They are called Valence electrons
• Valency = number of electrons in valence orbit
42. • Rutherford had suggested in 1920 that the
element Helium (He) after Hydrogen (H) has 2
protons, so its mass should be double than
Hydrogen
• But it was later found that He mass was 4
times mass of Hydrogen
• Why was this so?
• There must be something else in the atom (i.e.
in nucleus) apart from proton and electron
43. • These particles would have mass almost equal
to that of protons (electrons have negligible
mass)
• These particles also would have no electric
charge
• They were named Neutrons (n)
• In 1932, Chadwick discovered Neutrons
44. Properties of neutrons
• Do not possess any electric charge
• It is neutral
• Mass almost equal to mass of proton
• 1838 times more mass than electron
46. Atomic mass
• Total mass of atom = mass of proton +
neutron + electron
• But electron have negligible mass compared
to proton and neutron
• So mass of atom = mass of proton + mass of
neutron
• A=p+n
49. • Some elements have more than one mass
• They are called Isotopes
• Now, A = p + n
• If the number of protons remain same (atomic
number) but number of neutrons vary than
the element will have different masses
• Atomic masses different – but chemical
properties similar (same element)
50. • Iso – same
• Topes – place
• Thus the atoms of elements whose position in
the periodic table is same are called Isotopes
55. Radioactivity
• The isotopes of elements having high atomic
masses possess the property of radioactivity
• Not all high mass elements are radioactive
• Eg. Lead is not radioactive
• As atomic number increases, number of
protons also increase
• If no. of neutrons also increase the atom
becomes unstable
56. • This unstable atom will emit alpha, beta and
gamma rays
• It is radioactive
• Normally,
• If the ratio of neutrons to protons exceeds 1.6,
the property of radioactivity is acquired
59. Use of radioactive isotopes
• Determining age of old trees
• Age of fossils
• Radiometric dating
• Medical treatments
• Industries
• Treatment of diseases like cancer, etc.
