3. J.J. THOMSON
Discovered electron 1897 – Cathode Ray
“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
5. In 1886, He was one of the first to observe the proton
Discovered the positive charged – Canal Ray
“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.
8. Nucleus Theory 1910
“alpha particle gold-foil”
Rutherford’s model states “All of
an atom’s positive charge is
concentrated in its nucleus.”
An atom’s mass is mostly in the
He coined the word “Proton” for
the smallest unit of positive
charge in the nucleus.
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.
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
13. Protons are the positively charged
particles found inside the nucleus of an
Every element has a unique atomic #.
This # is the amount of protons inside that
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
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.
19. Atoms that have the same number of
protons, but have different numbers of
Hydrogen (Protium) Hydrogen (Deuterium) Hydrogen (Tritium)
ELEMENT charge A Z P N e
1. Boron 0
2. Nitrogen 0
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
The parts are the electrons (-), protons (+), and neutrons
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
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.