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PS Chapter 4 Notes
Section 1
Introduction to Atoms
The properties of matter that you can
observe result from the properties of
tiny objects that you cannot see.
Development of Atomic
Models
Democritus – Greek philosopher
Atomos = “uncuttable”
An atom is the smallest particle of an
element
 Dalton’s Atomic Theory –
Atoms have certain characteristics.
i. All elements are composed of
atoms that cannot be divided
ii. All atoms of the same element
are exactly alike and have the
same mass. Atoms of different
elements are different and have
different masses.
iii. An atom of one element cannot
be changed into an atom of a
different element. They can not
be created of destroyed in any
chemical change, only
rearranged.
iv. Every compound is composed
of atoms of different elements,
combined in a specific
ratio.
 Thompson Model – found that
atoms contain negatively charged
particles. Thought they must have
some sort of positive charge.
Negatively charged particles later
became known as ELECTRONS.
 Rutherford and the Nucleus –
through experimentation he
discovered the atom’s positive
charge must be clustered in a tiny
region in its center, called the
nucleus. Found out that electrons
had almost no mass so nearly all of
the atom’s mass is located in the
nucleus. He named the positively
charged particles in the nucleus
PROTONS.
 Bohr’s Model – showed that
electrons could have only specific
amounts of energy, leading them
to move in certain orbits. Like
plants around the sun.
 A Cloud of Electrons – The
model changed again in the
1920’s. Electrons do not orbit the
nucleus. They can be anywhere in
a cloudlike region around the
nucleus. An electrons movement
is related to its energy level.
The Modern Atomic Model
James Chadwick discovered a new
particle in the nucleus of atoms –
NEUTORNS (electrically neutral).
They have nearly the same mass as a
proton.
An atom’s nucleus contains protons
and neutrons surrounded by a
cloudlike region of moving
electrons.
Particle charges –
 # of protons = # of electrons
(balance to make the atom
neutral
 # of protons do NOT =
protons (don’t effect the charge
of an atom because they have
no charge
It take 2000 electrons to = the mass of
just one proton
Protons and neutrons almost = in
mass.
amu is the atomic mass unit. A
proton or neutron = 1 amu