1) Democritus first proposed the idea of atoms in 440 BC as indivisible particles that make up all matter. 2) In 1803, John Dalton developed the first atomic theory stating that atoms are tiny, solid spheres that cannot be divided further and that different types of atoms combine in simple whole number ratios to form compounds. 3) In the early 1900s, experiments by Thomson, Rutherford, and Chadwick led to discoveries of the electron, nucleus, and neutron, respectively, and established the modern nuclear model of the atom with a small, dense nucleus surrounded by orbiting electrons.

2. Atomic Theory Timeline
Here is a timeline of some of the major ideas.
Dalton Thomson Rutherford Chadwick
Bohr Modern

3. In 440 BC, Democritus, a Greek Philosopher
theorized that if you cut an object in half, then
cut that one in half, you would eventually end up
with a particle that could not be cut.
He called this an atom. From the Greek word
Atomos, meaning “not able to be divided.”

4.  Atoms are small hard particles made of a single
material that can’t be divided.
 Developed his ideas by thinking.
 Problem: no evidence and not as popular

5.  All substances are made of atoms
 Atoms cannot be created, divided or
destroyed
 Atoms of same element are exactly alike
 Atoms join together to make new substances
(John Dalton created the very first
Atomic theory by his observations from
experiment)
(He viewed atoms as tiny, smooth solid
balls that could not be broken into
smaller pieces)

6.  Thomson discovered that atoms were
not simple, solid spheres
 Atoms contained subatomic particles
 Very small, negatively charged particle.
 Called them electrons.
Hewasthefirstscientisttoshowthat the atom
wasmade ofevensmaller things.

9.  Also knew that atoms were
electrically neutral
 Must contain enough positive
charge to balance negative charge
of electrons
 Thompson proposed a model
where electrons were stuck into
a positively charged sphere
 Like chocolate chips in cookie dough

11. He explained that hydrogen can give rise only to a
single spectral lines.
 He couldn't explain the fine spectra

12. Robert Milikan (1909)
◦ Oil Drop Experiment
◦ Measured the electrical charge on the electron
◦ Mass can be calculated (Thomson determined
the e/m ratio)
◦ electron has a mass of 9.11 x 10-28 g
 So, at this point we know:
◦ Electrons are negatively charged
◦ Atoms are neutral
◦ The mass of an electron is very small
◦ Where is the mass of the atom?

13. Gold Foil Experiment

14. ◦ Alpha particles (+2 charge) beamed at thin gold
foil
◦ When alpha particles (+2) hit screen, the screen
lights up
◦ Most particles went through
◦ Some particles bounced to the sides and at 180º
◦ In process: discovered the proton
◦ Proton is positively charged

16.  Atom is mostly empty space
 Tiny part is highly dense matter that is
positively charged
 Named this dense part the nucleus
Based on his experimental results, Rutherford
made the following conclusions about the
structure of the atom:
 Like the pit in a
peach

17. 17
 Discovered the nucleus.
 Proposed the NUCLEAR MODEL of the atom
also called the PLANETARY ATOM
 Proposed the existence of the neutron
 Determined the size of the nucleus (1x10-15
m) and the atom (1 x 10-11 m for hydrogen
atom, larger for other atoms)
 But, Did not know where to place the
electrons

18. (Studied the way atoms react to
light)
 Modified Rutherford’s model
 Proposed that each electron has a certain
amount of energy
 Helped electron move around nucleus
 Electrons move around nucleus in region
called energy levels
 Energy levels surround nucleus in rings,
like layers of onion

19.  It Has been called planetary model
 Energy levels occupied by electrons are
like orbits of planets at different
distances from the sun (nucleus)

20.  Electrons move around nucleus in certain
paths
 These paths are energy levels
 Electrons can jump from one level to another.

21. • Spectrograph of high resolving showed
that lines are not single.
• Each spectral lines actually consisted
of several very close line packed
together.
• This is called fine structure of
spectral lines. Bohr theory could not
explain this fine structure.

22. Sommerfeld’s atom model
Sommerfeld introduced two main modification in
Bohr’s model:
(1)The path of an electron around the nucleus, in
general ,is an ellipse with the nucleus at one of the
foci.

23. N
elect
ron
r
Elliptical orbit for hydrogen
atom
Sommerfeld’s theory was able to give an
explanation of the fine structure of the
spectral line of hydrogen atom.But he could
not predict the correct number of spectral lines.

24.  Particle in nucleus with protons
◦ Mass about the same as a proton
◦ No charge
 Particle is called Neutron

25.  Model accepted today
 Electrons dart around in an energy
level
 Rapid, random motion creates a
“cloud” of negative charge around
nucleus
 Electron cloud gives atom its size
and shape

27.  In 1932, Chadwick discovered another
particle in the nucleus of an atom. This
new particle is called a neutron.
 Neutrons have no electrical charge.
 According to this theory,
“At the center of the atom is a tiny,
massive nucleus containing protons and
neutrons. Surrounding the nucleus is a
cloudlike region of moving electrons.”