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Atomic Theory and
Atomic Structure
Courtesy of Mrs. Danza
Ch. 4 Intro to Matter Text
Democritus
440 BC Greek Philosopher
 Universe
was made of empty space and
tiny bits of stuff
 Called these tiny bits of stuff “atomos”
 If you keep cutting something in half,
eventually you would end up with a
particle that could not be cut.
 Atom- “not able to be divided”
Aristotle
Greek Philosopher
 Matter
is made of particles, which we call
atoms.
 An atom is the smallest particle into which
an element can be divided and still be the
same substance.
John Dalton
Late 1700’s- British Chemist and
Teacher
 Published



in 1803:
All substances are made of atoms. Atoms are
small particles that cannot be divided,
created, or destroyed.
Atoms of the same element are exactly alike,
and atom of different elements are different.
Atoms join with other atoms to make new
substances.
J.J. Thomson
1897- British Scientist
 There
are small particles inside the atom.
 Created the “plum-pudding” model.
(Looks like chocolate chip ice cream)
 Atom made of a positively charged
material with the negatively charged
electrons scattered through it.
Ernest Rutherford
1909- A Student of Thomson




“Gold Foil” Experiment
Atoms are made of mostly empty space.
A tiny part of an atom is made of dense matter.
1911- Revised Theory- New Model
 The center of the atom, the nucleus, is extremely
dense and has a positive charge.
Niels Bohr
1913- Danish Scientist
 Electrons
move around the nucleus in
paths or energy levels.
Erwin Schrodinger (Austrian
Physicist) and Werner Heisenberg
(German Physicist)
 Electrons
do NOT travel in specific paths.
 They travel in Electron Clouds (a region
around the nucleus where electrons are
found)
What is an atom made of?
What are Protons?
 Much
larger and heavier than electrons
 Protons have a positive charge (+)
 Located in the nucleus of the atom
 Atomic Mass Unit (amu)- SI Unit used to
express the mass of parts of atoms

1 proton = 1 amu
What are Neutrons?
 Large
and ______ like protons
 Neutrons have __________
 Located in the nucleus of the atom
 Mass of one neutron = ________
 Mass of proton is __________ to the mass
of a neutron
What are Electrons?
 Tiny,
very light particles
 Have a negative electrical charge (-)
 Move around the outside of the nucleus
 Mass is almost 0 amu
 Electron mass is so low, it is often not
even considered when recording the mass
of the whole atom
How do we describe atoms?
 Atomic
Number equals the number of
protons in the nucleus.
 In a neutral atom, the number of protons
equals the number of electrons.
 Atomic Mass Number is equal to the
number of protons plus neutrons.
What are Ions?
 An
atom that carries an electrical charge is
called an ion
 If the atom loses electrons, the atom
becomes positively charged (because the
number of positively charged protons will
be more the number of electrons)
 If an atom gains electrons, the atom
becomes negatively charged (more
negative charges than positive charges)
What are Ions?
•The number of
protons does not
change in an ion
•The number of
neutrons does not
change in an ions
•So, both the atomic
number and the
atomic mass remain
the same.
What are Isotopes?

An atom that has the same number of protons
(atomic number) as other atoms in the same
element BUT a DIFFERENT amount of
neutrons; therefore, it has different atomic
masses.



Some are unstable- the nucleus will change over
time, losing neutrons until it becomes stable
Some are considered Radioactive- as the nucleus
loses neutrons, it gives off high energy light particles
Identify Isotopes by mass number. (Sum of the
protons and neutrons.)
What are Isotopes?
What holds atoms together?
 Gravitational
Force- mass attraction
 Electromagnetic
 Strong
 Weak
Force- charge attraction
Force- keeps the nucleus together
Force- Radioactive Atoms Only.