1) Michael Faraday discovered cathode rays (now known as electrons) in 1838 through experiments with glass tubes containing rarefied air and electrodes. 2) The experiment showed a strange luminous arc near the anode and a dark space near the cathode, marking the beginning of research into cathode rays. 3) Other scientists like J.J. Thomson, R.A. Millikan, and J. Chadwick later discovered the subatomic particles proton, electron, and neutron through discharge tube and oil drop experiments.

1. Electron
Proton
Neutron

3. MICHAEL FARADAY
In 1838, Michael Faraday passed current through the
glass tube filled with rarefied air. Conducting the
experiment he noticed a strange light arc with its
beginning at the anode (the positive electrode) and
its end almost at the cathode (the negative
electrode). The only place where there was no
luminescence was just in front of the cathode. The
place is called "cathode dark space", "Faraday dark
space", or "Crookes dark space". That was the
beginning of the long and "turbulent" time of
researches on that luminescence. And the
luminescence is called "cathode rays".

5. In this experiment, Faraday showed
that the mass of elements was
proportional to the quantity of
electricity that passed through
them.
2. George J. Stoney- he was a
scientist who first suggested the
term “ Electron” to refer to a
negatively charged particle.

6.  3. Joseph John Thomson-was credited for
his discovery of the first subatomic particle
through his work with the discharge tube.

8.  4. Robert A. Millikan- independently
measured the electrons charge through his
OIL-DROP Experiment. He successfully
attempted to detect and measure the
effect of an individual subatomic particle.

11. Using an atomizer, Millikan sprayed
tiny droplets of oil which passed
between two charged plates. Given a
negative charge(-) by the electrons
released from gas particles by x-rays,
the oil particles were attracted toward
the positive plate. He observed the
speed of the droplets as they moved
toward the positive plates. The
smallest possible charge on a droplet
was taken as the charge of an
individual atom.

12. The quantity is considered to be the basic unit
charge and is given a value of one minus (1-)

13. When the electron was
discovered , the scientists
searched for the positively
charged particles.
1.Eugene Goldstein-

14.  Goldstein observed rays passing
through the hole of a cathode in the
cathode tube. These rays are made up
of positive particles. Their
characteristics depend on the gas
inside the cathode tube.The lightest
particle was obtained when Hydrogen
gas was used.

15. 1. Sir James Chadwick- discovered
 the neutron in 1932.
 He found out that
 the particle was
 electrically neutral
an and its mass
was
 approximately the
 same as that of the
 proton.

17. Proton
Symbol p+
Position in atom Inside the nucleus
Relative charge 1+
Relative mass 1
Actual mass ( g)
1.673x10-24

18. Neutron
Symbol n
Position in atom Inside the nucleus
Relative charge 0
Relative mass 1
Actual mass
1.675x10 -24

19. Electron
−
Symbol e
Position in atom moving around the nucleus
Relative charge -1
1
Relative mass ≈0
1840
Actual mass 9.109x10-28

20. Atomic structure
e - - - - - - - - electron
e
n (s)
p (s) - - - - - - - - - - - - - - - nucleus
e
e
e

21. Atomic content of an atom
11e −
no. of protons = 11
no. of electrons =11
12n
11p
no. of neutrons = 12
23
Symbol 11 Na

22. 12
Atomic Content of 6 C
 no. of protons = 6
 no. of neutrons = 12 - 6
=6
 no. of electrons = 6
 (An atom is neutral in charge)

23. 14
Atomic Content of :7 N
 no. of protons = 7
 no. of electrons = 7
 no. of neutrons = 14 – 7
=7

24. 14
Atomic Content of 6 C
 no. of protons = 6
 no. of neutrons = 14 – 6
=8
 no. of electrons = 6

25. mass number
14 12
6 C 6 C
atomic number
Mass number = no. of p + no. of n
atomic number = no. of protons

26. Isotopes
12
14
6 C 6 C
Name of element : carbon carbon
No. of protons : 6 6
No. of neutrons : 8 6
No. of electrons : 6 6
Isotopes are atoms of same elements which have the same
atomic number but different mass number

27. Isotopes
14 12
6 C 6 C
Burn in air Carbon dioxide Carbon dioxide
Mass 14 units 12 units
Radioactivity Give radiation Does not give
radiation (stable)

28. Extension : Isotopes ( For reference only )
8n
6p
7n
7p + e +γ
−
0
1
14 14
6 C nucleus 7 N nucleus
A neutron in a nucleus of 14
C breaks into 1 p + e −.
1
Electrons and energy are emitted to the surroundings
Carbon -14 dating

29. Properties of isotopes
 Same chemical properties
 Different physical properties
 e.g. mass
b.p. and m.p.
radioactivity

30. Actual mass VS relative mass
12
Mass of 1
6 C atom = 1.993 ×10 −26 kg Or 12
Which one is more convenience?
Relative mass
mass of 1 12C
6
Standard reference : 1 is equivalent to
12
= 1.661 × 10 − 27 kg

31. Simplified rule : We regard
Relative mass of 1p = 1
Relative mass of 1n = 1
Relative mass of 1e = very small
=0

32. Relative mass of isotopes
28 29 30
14 Si 14 Si 14 Si
Relative mass 28 29 30
% abundance 92% 4.7% 3.1%
Do the three isotopes have the same chemical properties?
Yes

33. Relative atomic mass of isotopes
It is difficult and not necessary
to separate the isotopes in most
of the reactions. Average
relative mass of silicon is used to
denote the mass of a silicon
atom.

34. Relative atomic mass of silicon ( P.5 e.g.1 )
28 29 30
14 Si 14 Si 14 Si
Relative mass 28 29 30
% abundance 92% 4.7% 3.1%
( Relative ) Atomic Mass of silicon
= 28 x 92% + 29 x 4.7% + 30 x 3.1%
= 28.053
= 28.05

35. Ex1. Relative atomic mass of Chlorine
35 37
17 Cl 17 Cl
Relative mass 35 37
% abundance 75.5% 24.5%
Atomic mass of Chlorine = 35 x 75.5% + 37 x 24.5%
= 35.5