Transcript Chapter 5 Notes Atomic Theory As Seen Through History Democritus—The First Model • Democritus of Abdera (~400 B.C.) is the first person known.

Chapter 5 Notes
Atomic Theory As Seen Through History
Democritus—The First Model
• Democritus of Abdera (~400 B.C.) is
the first person known to come up
with the theory of the existence of
particles called atoms.
• He said atoms were
indivisible and indestructible
particles that made up
everything
• His theories were unable
to be tested at the time. Statue of Democritus
John Dalton-the atom
•Englishman John Dalton
is generally given credit
for the first experiments
to prove the nature of
atoms
•The experiments he did
resulted in Dalton’s
atomic theory.
John.Dalton,1766-1844
Dalton’s atomic theory
• All elements are composed of tiny indivisible
particles called atoms.
• Atoms of the same element are identical. The
atoms of any one element are different than
those of any other element.
• Atoms of different elements can physically
mix together or chemically combine in
simple, whole number ratios.
• Chemical reactions occur when atoms are
separated, joined or rearranged, but atoms of
one element are never changed into another
element as a result of chemical reaction.
After Dalton
•Much of Dalton’s theory still
stands, but some has been
changed.
•One change is that atoms are
now known to be divisible.
•We will now learn about the
three subatomic particles the
atom can be broken down into.
J. J. Thompson-Electrons
• English physicist J.J. Thompson
first discovered the electron in
1897.
• His experiments with cathode ray
tubes led to the theory that
particles much smaller than atoms
existed, and that their charge was J.J.Thompson,
negative.
1856-1940
• Today the electron carries exactly
one unit of negative charge, and
its mass is 1/1840th of the mass of a
hydrogen atom.
Goldstein and chadwick
• After the discovery of the electron,
the race was on to find other particles;
they knew they were there because
the atom is electrically neutral.
• E. Goldstein is credited for discovery
of the positively charged proton,
which has a much greater mass than
James Chadwick
electrons.
1891-1974
• James Chadwick found yet another
subatomic particle—the neutron. It
has no charge, but the same mass as a
proton.
Rutherford—The nucleus
• At this point in history, the prevalent
thought was that the atom was mostly a
positive substance with negative electrons
throughout.
• Rutherford sought to prove this by shooting
particles through gold foil onto photo
paper.
• He expected to see the particles go
through with slight deflection.
• What he saw was that some particles
went straight through, but some
bounced back!
rutherforD, cont’D.
• This gave rise to the theory that
atoms mostly consist of empty
space, with the majority of the mass
in the center of the atom—called
the nucleus.
• The protons and neutrons are
located in the nucleus with the
electrons orbiting around it.
Synopsis
• The atom is the smallest part of an element
that retains its properties.
• It is made of mostly empty space,with the
majority of the mass concentrated in the
middle (the nucleus).
• The nucleus contains the positively
charged protons and the chargeless
neutrons.
• The electrons are situated outside of the
nucleus in the empty space.
Atomic number, mass number,
atomic mass & isotopes
Atomic Number
• The atomic number of an element tells you the number
of protons in the nucleus.
• Because atoms are electrically neutral, the number of
protons will be the same as the number of electrons.
• Each element has a different atomic number.
Atomic
number
Answer the following
•What element has atomic number
32?
•How many protons does helium
have?
•What element has 79 protons?
•What is uranium’s atomic number?
•How many electrons does calcium
have?
Mass number
• Most of the mass of an atom is in the
nucleus; so much that the electrons can be
neglected.
• The mass number is the number of protons
and neutrons in the nucleus.
• If you know the mass number and the
atomic number, you can determine the
composition of an atom.
# of neutrons = mass number – atomic
number
mass number, cont’D.
• Composition can be written in shorthand by
putting the symbol for the element with the
mass number and atomic number to the left.
12
C
6
• Or, the mass number and chemical
element can designate atoms, such as in:
Carbon-12
Answer the following
•How many neutrons are in
potassium-39?
•How many protons are in lead-208?
•If an element has 22 protons and 26
neutrons, what is its mass number?
•What element is in the previous
question?
201
•How many neutrons are in 80 Hg ?
Isotopes
• Atoms of the same element always have
the same number of protons, but can have
different numbers of neutrons.
• An atom with the same number of protons
but different number of neutrons are
called isotopes.
• Isotopes are chemically alike, because it is
the protons which are responsible for the
chemical behavior.
Answer the following
•What is the difference between
iodine-127 and iodine-125?
•Is an element with 25 protons and
30 neutrons an isotope of one
with 26 protons and 30 neutrons?
Why or why not?
Atomic mass
• Because there can be several isotopes of an
element, there is another number, the
atomic mass, that is the weighted average
mass of all an element’s isotopes.
• Carbon-12 was used as a reference to
measure mass of atoms, and the carbon-12
atom was said to have a mass of exactly 12
amu’s.
Atomic mass
•The End