Quiz Time!
You have 5 minutes to go
over the following:
 Parts of an atom
 Calculating
protons/neutrons/electro
ns
 Drawing Bohr models!
Austin High School Chemistry
2015-2016
The idea that all matter is made up
of atoms. It is a very old idea
dating back to the ancient Greeks.
Over time, scientists have come up
with various models for the atom
based on their observations.
These atomic models have been
altered and revise as new
scientific evidence is discovered.
Democritus was an ancient Greek
philosopher who lived from 460 370 B.C.
He proposed:
Matter is formed of small pieces
that could not be cut into smaller
parts called “ATOMS” meaning uncut-table.
 Atoms are small hard particles.
 Made of a single material that’s
formed into different shapes and
sizes.

• 2500 years later a British
chemist and schoolteacher
brings back Democritus’s idea
of the atom
• He performed many
experiments to study how
elements join together to form
new substances
Atoms are tiny invisible particles.
Atoms of one element are all the same.
Atoms of different elements are
different.
4. Compounds form by combing atoms.
1.
2.
3.
• Discovered
negatively charged
particles with the cathode ray
tube (electrons).
• Measured the charge to mass
ratio of the electron
•Knew there had to be other
particles in atoms (because of
the mass).
Passing an electric current through the cathode makes
a beam appear to move from the negative to the
positive end. Electrons are negatively charged and are
attracted to a positive magnetic source.

Atoms are made mostly out of
(+) charged materials, like
dough in a bun. The (-) charged
electrons are found inside the
(+) dough.
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
Rutherford's 'gold foil' experiment using
positively charged alpha particles:
 Most alpha particles passed through the gold foil
suggesting that an atom is largely empty space.
 Some alpha particles were deflected significantly
suggesting that the positive charge of an atom must
be concentrated in a very small sphere.
 A very small number of alpha particles actually
bounced back.
Alpha particles are deflected if they get close
enough to the nucleus
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
Proposed that electrons move
around the nucleus in specific layers
or shells.

Every atom has a specific number of
electrons shell.

An electron can absorb energy and
move to a higher energy orbit of
larger radius. (excited electrons)

An excited electron can fall back to
its original orbit by emitting energy
as radiation.
13

Chadwick discovered the
neutron which is a particle
with no charge, found in the
nucleus.
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