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Unit 5 The Structure of Matter
Chapter 16 The Atom
16A – The Atomic Model
• Objectives:
– Compare and contrast the modern atomic
models to the atom as understood by ancient
Greek philosophers
– Summarize the current understanding of the
– Describe the major discoveries leading to the
modern atomic model
– Show how each atomic model was more
workable than its predecessor
– Associate the key scientist with each advance in
the atomic model
• Assignments: Timeline of the Scientists,
Worksheet, and Section Review page 388
• What is the universe made of?
– Let’s Read…
– Page 379
• Models
– Physical, conceptual, mathematical
– Never absolute, can be overturned at
any time based on current
understanding and technology
Ancient Atomic Models
• Discussion of the structure of matter can be recorded back to
the Hindus more than 2600 years old; however, Democritus
is given credit for the first term of the atom.
– Atomos
• Atom
• Indivisible
– He stated that everything is made of atoms, even thoughts and
the soul
– Atoms were the only things that exist, always existed, and
would always exits
– Consistent with Greek philosophers that believed reason was
the path to all knowledge he did not perform any scientific
experiments to backup his ideas
– Denied the possibility of supernatural influence
– Ideas were so radical that even his fellow philosophers rejected
his theory
• Aristotle
– All matter was continuous, without tiny distinct particles
– This influenced scientists for thousands of years
John Dalton: The Indivisible Atom
• 1790’s
• Joseph Louis Proust
– French chemist and botanist, an early
– Discovered up with the law of definite
– The masses of chemical substances
combine in definite, repeatable ratios when
forming compounds
• John Dalton
– English school teacher used this law for the
basis of his theory
John Dalton: The Indivisible Atom
• Dalton’s Core-Envelope Model
– An atom cannot be created or destroyed in
chemical reactions
– An atom cannot be subdivided into smaller
– Elements are made of only atoms
– The atoms of an element are all alike
– The atoms of one element are different
from the atoms of all other elements,
especially their masses
• Much of Dalton’s theory held true for
JJ Thomson: The Divisible Atom
English physicist
Discovered the electron and isotopes of elements
Invented the mass spectrometer
Won the Nobel Prize for the discovery of the
electron (corpuscles)
• Known for his Plum-Pudding Model
– Pictured the atom as a positively charged mass with
negatively charged electrons embedded in it like plums
in a plum pudding (British dessert)
– The attraction of the opposite charges held things in
• Electrons don’t vary in mass or charge but are the
same in every element
• Electrons only were responsible for a fraction of the
total mass of the atom, so scientist still wondered
what else composed the rest of the atom
Ernest Rutherford: The Nuclear Atom
• English physicist, Discovered the nucleus, Father of
Nuclear Physics, Won the Nobel Prize in Chemistry in
• Known for his Gold Foil Experiment which resulted in
the Nuclear Model
– Lots of empty space in the atom
– A very dense, tiny, positively charged region in each
atom, called it the nucleus
– Positive particles, protons, first discovered by Eugene
– Electrons whirled around the nucleus at high speeds so
they were not drawn in
– The nucleus’ mass was more than the protons which led
to the discovery of the neutrons by James Chadwick
• Did not explain the motions or locations of electrons,
how far away from the nucleus were they, how were
they arranged….
Niels Bohr: Electron Energy Levels
• Danish physicist, helped develop the first atomic
bobs in WWII, won the Nobel Prize in Physics
• Knew that certain elements emit specific colors
of light when heated to incandescence
• Passed the emissions through a prism he was
able to identify elements by their line spectra
• He suspects these emissions were caused by the
electrons in the atoms
• Electrons can jump from one energy level to the
next and release/absorb energy
• The energy between orbits were fixed,
• His model is often called the Planetary Model
Bohr Model Diagram
The Quantum Atom
• Discoveries showed that the electron
acts like both a wave and a particle
• Physicists refined Bohr’s model
• Instead of having a defined area of
where an electron would be found,
they talked about the probability of
finding an electron in specific regions
around the nucleus
• The Quantum Model
– Page 388