Chapter 2 Atoms, Molecules and Ions 6 Slides.pdf - 1/7/2019...

1/7/2019
1
Chapter 2
Atoms, Molecules and Ions
© Eric L. Johnson, 2019
Dalton’s First Postulate
1)Each element
is composed
of extremely
small particles
called atoms.
2
Dalton’s Second Postulate
2) All atoms of a
given element are
identical, but the
atoms of one
element are
different from the
atoms of all other
elements.
3
Dalton’s Third Postulate
3) Atoms of one
element cannot be
changed into
atoms of a
different element
by chemical
reactions; atoms
are neither created
nor destroyed in
chemical reactions.
4
Law of Conservation of Mass
• The total mass of
materials present after
a chemical reaction is
the same as the total
mass present before the
reaction.
5
Dalton’s Fourth Postulate
4) Compounds are
formed when
atoms of more
than one element
combine; a given
compound always
has the same
relative number
and kind of
atoms.
6

1/7/2019
2
The Electron
• Streams of negatively charged particles were found to emanate
from cathode tubes, causing fluorescence.
• J. J. Thomson is credited with their discovery (1897).
7
The Atom, circa 1900
• The prevailing theory
was that of the “plum
pudding” model, put
forward by Thomson.
• It featured a positive
sphere of matter with
negative electrons
imbedded in it.
8
Discovery of the Nucleus
Ernest Rutherford
shot  particles at
a thin sheet of
gold foil and
observed the
pattern of scatter
of the particles.
9
The Nuclear Atom
• Rutherford postulated a
very small, dense nucleus
with the electrons around
the outside of the atom.
• Most of the volume of the
atom is empty space.
• Other subatomic particles
(protons and neutrons)
were discovered.