J.J. Thomson discovered the electron in 1897 through his cathode ray experiment and proposed the "plum pudding" model of the atom in 1904. Later experiments provided evidence that atoms are made of even smaller subatomic particles. In the 1910s, Rutherford discovered the nucleus through his gold foil experiment and proposed a nuclear model of the atom. In 1932, Chadwick discovered the neutron through experiments bombarding beryllium with alpha particles. Atoms are now understood to have a small, dense nucleus containing protons and neutrons, surrounded by electrons in orbit.

1. STRUCTURE OF

2. EVIDENCE OF
SUB ATOMIC PARTICLES

3. J.J. THOMSON
1856-1940
 Discovered electron 1897 – Cathode Ray
Experiment
 “Plum Pudding” model 1904
 Electrons in a soup of positive charges
 Discovered isotopes 1913.
 Thomson’s experiments provided the first
evidence that atoms are made of even
smaller particles.

5.  In 1886, He was one of the first to observe the proton
(then unnamed).
 Discovered the positive charged – Canal Ray
Experiment
 “Raising Pudding” model
 During his work with discharge tubes, he observed
rays travelling from cathode to anode (electrons), and
also noticed the presence of waves travelling in the
opposite direction. These waves, called anode rays and
having the lowest e/m ratio, were found to be
characteristic of the gas in the tube.

6. CANAL OR ANODE RAY
EXPERIMENT

7. DISCOVERY OF THE
NUCLEUS

8.  Nucleus Theory 1910
 “alpha particle gold-foil”
experiment.
 Rutherford’s model states “All of
an atom’s positive charge is
concentrated in its nucleus.”
 An atom’s mass is mostly in the
nucleus.
 He coined the word “Proton” for
the smallest unit of positive
charge in the nucleus.

9. Alpha Particle Experiment

10.  In 1932, James Chadwick, an English physicist who had
worked with Rutherford, detected neutrons and
measured their mass in an invisible game of billiards.
 He bombarded beryllium atoms with alpha particles. An
unknown radiation was produced. Chadwick interpreted
this radiation as being composed of particles with a
neutral electrical charge and the approximate mass of a
proton. This particle became known as the neutron.
With the discovery of the neutron, an adequate model of
the atom became available to chemists.

11. SUBATOMIC
PARTICLES

12.  Every Atom has a core called a nucleus.
 The Nucleus is largest part of an atom
(about 99.9% of it’s mass)
 Within the nucleus are very small particles
called protons and neutrons
 Electrons are in an orbit around the
nucleus

13.  Protons are the positively charged
particles found inside the nucleus of an
atom
 Every element has a unique atomic #.
This # is the amount of protons inside that
elements nucleus.
Example:
Oxygen has an atomic # of 8. How many
protons does it have?

14.  Negatively charged particles found orbiting
around the nucleus
 Electrons are very small; only about
1/2000th the mass of a proton or neutron
 Electrons can move between atoms which
causes bonds; this is how new compounds
are formed.

15.  Neutrons are the other particle found
inside the nucleus of an atom
 Neutrons have no charge
 Atoms of a same element may not carry
the same number of neutrons.

18. SHORTHAND NOTATION
Element A Z P N e Charge
Lithium 7 3 3 4 2 +1

19.  Atoms that have the same number of
protons, but have different numbers of
neutrons.
Examples:
+
-
+
-
+
-
Hydrogen (Protium) Hydrogen (Deuterium) Hydrogen (Tritium)

21. SEATWORK
ELEMENT charge A Z P N e
1. Boron 0
2. Nitrogen 0
3.Fluorine -1
4. Neon 0
5. Magnesium +2
6. Aluminum +3
7. Silicon 0
8. Sulfur 0
9. Potassium +2
10. Carbon -2

22. SCIENCE IDEAS
 Atom is regarded as the smallest, basic unit of matter.
Even if it is the basic unit of matter, it is still composed of
parts.
 The parts are the electrons (-), protons (+), and neutrons
(0).
 An atom has equal number of protons and electrons. This
makes the atom neutral.
 Protons and neutrons are relatively heavier than
electrons. They compose the nucleus and collectively
called as nucleons. The mass of an atom is mainly
determined by the mass of the nucleus.

23. SCIENCE IDEAS:
 Several models have tried to show how the subatomic particles are
arranged in an atom. Thus far, the accepted model places the protons
and neutrons in the center of the atom; or the so-called, nucleus. Moving
rapidly around the nucleus are the electrons.
 Atomic number, or the number of protons of an atom distinguishes an
element from the others. This number is the same for all atoms of a
particular element.
 While the number of protons is fixed for an atom of an element, the
number of neutrons may vary. Atoms having the same number of
protons but different number of neutrons are referred as isotopes. The
isotopes are identified through their mass number which is the sum of
the number of protons and the number of neutrons in an atom.
 Information on the subatomic composition of an element may be
represented through shorthand notations.

24. Prepared by:
JIMNAIRA U. ABANTO