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1. MODELS OF THE ATOM
represents what the
structure of an atom could
look like, based on what we
know about how atoms
behave.

3. The idea of atoms was originated by the Greek
philosopher, Democritus in the fifth century BC. The
Greek word atomos (atom) means indivisible because
they believed that atoms could not be broken into
smaller pieces.
Democritus atomic model

4. Dalton’s Atomic model
He pictured the atom as a tiny, indestructible sphere
with mass.
When Dalton proposed his model, electrons and the
nucleus were unknown yet.

5. John Dalton was an English chemist and teacher who
used experimental evidence to form the atomic theory
of matter:
1. All elements are composed (made up) of atoms. It is
impossible to divide or destroy an atom.
2. All atoms of the same element are alike. (One atom of
oxygen is like another atom of oxygen.)
3. Atoms of different elements are different. (An atom of
oxygen is different from an atom of hydrogen)
4. Atoms of different elements combine to form a
compound. These atoms have to be in definite whole
number ratios. For example, water is a compound
made up of 2 atoms of hydrogen and one atom of
oxygen. (A ratio of 2:1) Three atoms of hydrogen and 2
atoms of oxygen cannot combine to make water.

6. Thomson’s Atomic Model
He revised Dalton’s atomic model. He said that the
negatively charged electrons were embedded in a positively
charged sphere similar to the way raisins are embedded in
bread. His atomic model is also known as the “raisin bread
model” of the atom.
The negative charge of the electrons and the positive
charge of the sphere cancel each other out to form a
neutral atom.

7. A group of scientists composed of Ernest Rutherford, Johannes
"Hans" Wilhelm Geiger and Ernest Marsden tested Thomson’s
model by bombarding a very thin sheet of gold foil with
positively-charged alpha particles. Their experiment is referred to
as the alpha particle scattering experiment or gold foil
experiment.
His model is also known as the nuclear model of the atom.

9. Explanation:
 He pictured an atom as mostly empty space to explain
why most alpha particles pass through undeflected.
 There is a tiny, positive central core called the nucleus
where the mass of the atom is concentrated.
 When an alpha particle occasionally runs into the
nucleus, it is repelled. The alpha bounces back if a
head-on collision occurs. Rutherford was reported to
have exclaimed, “It was as if you fired a 15-inch shell at
a sheet of tissue paper and it came back to hit you.”
 The electrons in the atom revolve around the nucleus.
They are so light that they have no appreciable effect
on the path of the alpha particle.

10. To sum up the discussion:
 You have learned the three sub-atomic particles:
the protons, electrons and neutron — and how
they are arranged in the currently accepted model
of the atom. Among these subatomic particles, it is
the number of protons that identify the atoms of
an element. All atoms of an element contain the
same number of protons in their nuclei. This
number is the element’s atomic number. Every
element has a unique atomic number. Thus,
the atomic number is the identity number of
an element.

11.  The atomic number of the element is the
number of protons in the nucleus. Since an
atom is electrically neutral, then the
number of protons are equal to the number
of electrons.
 The mass of an atom is concentrated in the
nucleus. Therefore the mass number is
equal to the sum of the protons and
neutrons.
 Agreement: Bring a periodic table next
meeting.