The document summarizes the models and contributions of several important scientists throughout history towards developing the modern understanding of atoms. It describes Democritus' early atomic theory of indivisible atoms in different shapes and sizes. John Dalton improved upon this with his billiard ball model of atoms and proposed atoms of elements are identical. J.J. Thomson's plum pudding model viewed atoms as positively charged material with electrons scattered about. Rutherford's gold foil experiment showed atoms have a small, dense nucleus. Niels Bohr incorporated this into his model of electrons orbiting the nucleus in defined energy levels. Werner Heisenberg introduced the concept of electron clouds rather than defined orbits. James Chadwick discovered the neutron in 1932,

2. **** The model of the atoms changing throughout
history
**** The contributions of many men towards the
discovery of the atom
**** Each philosophers, scientists, etc. theory on the
atom, and their model
Ex: - Democritus
- Aristotle
- Bohr

4. He found that
matter is made of
atoms. He also
found that matter
will always exist.
He also found 4
elements, fire, wa
ter, earth, and air.
With all this, he
came up with the
theory of motion.

5. • This is the Greek philosopher
Democritus who began the search
for a description of matter more
than 2400 years ago.
• Proposed an Atomic Theory which
states that all atoms are
small, hard, indivisible and
indestructible particles made of a
single material formed into
different shapes and sizes.
• Found that atoms are not all the
same, they are eternal, and always
moving
• Aristotle did not support his
atomic theory
This was Democritus’ idea of an
atom which was forgotten for over
2,000 years

6. Named oxygen and hydrogen
He invented the first periodic
table, which had 33 elements.
Known as the “Father of
Modern Chemistry”
He also proposed the Law of
Conversation of Mass which
represents the beginning of
modern chemistry, which
states, in a chemical
reaction, matter is neither
created nor destroyed

7.  A brilliant youth that he
became an English school
teacher when barely 12 years
old
 Atomic theory of matter based
on his experimental
observations. :
*All matter is made of
atoms. Atoms are indivisible and
indestructible.
*All atoms of a given
element are identical in mass
and properties.
* Compounds are formed
by a combination of two or more
different kinds of atoms.
* A chemical reaction is a
rearrangement of atoms.
* Atoms can be neither
This is Dalton’s model of an atom,
created nor destroyed.
is once again compared to the
 This theory became one of
billiard ball, and similar to
the foundations of modern
Democritus’ model.
chemistry

8. • Proved that an atom can be divided
into smaller parts.
• In 1897, proposed the Plum Pudding
Model. Atoms were made from a
positively charged substance with
negatively charged electrons
scattered about, like raisins in a
pudding.

9.  Performed the Gold Foil
Experiment and
suggested the following
characteristics of the
atom:
**It consists of a small
core, or nucleus, that
contains most of the mass of
the atom
**This nucleus is made
up of particles called
protons, which have a
positive charge
**The protons are
surrounded by negatively
charged electrons, but most
of the atom is actually empty
space
 “Father of Nuclear
Physics”
 Was on the New Zealand
1. Some “bullets” went
straight through the gold foil
without changing its course
2. Some did bounce away
from the gold sheet as if
they had hit something
solid
3. Atoms were not a pudding
filled with a positively
charged material.
4. Rutherford concluded that
an atom had a
small, dense, positively
charged center that
repelled his positively

10. Proposed the Bohr Model, which suggests that electrons travel
around the nucleus of an atom in paths.
The electrons can jump from a path in one level to a path in
another level (depending on their energy level)

11. Protons: 36
Neutrons: 48
Let’s draw
a Bohr
Model!

12. He further explained the
nature of electrons in an
atom by stating that the
exact location of an
electron cannot be stated;
therefore, it is more
accurate to view the
electrons in regions called
electron clouds; electron
clouds are places where
the electrons are likely to
be found

13. Realized that the atomic mass of
most elements was double the
number of protons which resulted in
discovery of the neutron in 1932
Worked with Ernest Rutherford

14. - Electrons do not have a defined
path, they are in an electron cloud
- Small positively charged nucleus
surrounded by a large region
filled with electrons. (Wave
Mechanics)