The Atom
Greek Model
• One group of Greek philosophers who lived in
the forth and fifth centuries B.C. believed that
matter is composed of tiny indestructible
particles which they called atoms. However,
this theory was never verified by experiments
and was discarded. More than 2000 years
later, in 1804, John Dalton, an English school
teacher, reintroduced the model or theory and
developed it to such an extent that it was able
to explain the laws of chemical change.
Dalton Model
• Dalton proposed that elements are composed
of identical, indivisible atoms in his “billiard
ball model” of the atom.
• He devised a system of atomic symbols.
• He correctly predicted the Law of Multiple
Proportions (if the same elements combine to
form different compounds, the ratio of the
elements in the compounds are in simple
multiples.
• The discovery of subatomic particles showed
that Dalton’s proposal that atoms were
indivisible was wrong.
J.J. Thomson 1900
• Thomson revised Dalton’s theory with his
raisin bun or plum pudding model of the
atom.
• Thomson proposed that an atom could be
considered a sphere of positive electricity in
which negative electrons are embedded like
raisins in a bun.
• Most of the mass of the atom in this model is
associated with the positive electricity.
Rutherford Model 1911-1912
• In 1911, Rutherford’s gold foil experiment lead
to another revision of the atomic model.
• Rutherford proposed that
(1) An atom has a nucleus in which its positive
charge (protons) and mass are concentrated.
(2) The vast majority of the atom’s volume
would be empty space occupied only by the
moving negatively charged electrons.
• Rutherford also postulated the existence of
uncharged particles (neutrons)
Problems with Rutherford’s Model
• (1) If electrons are not moving, then the
attraction of the negative electrons for the
positive nucleus should collapse the atom.
• (2) If the electrons are moving (to counteract
the pull of the nucleus), then the electrons
should radiate energy and in time spiral down
to the nucleus.
Bohr Model 1913
• Bohr studied the atomic spectra of the
elements and discovered the each spectrum
showed a series of lines of definite energies.
• Bohr proposed that electrons of specific
energy moved in circular orbits around the
central atomic nucleus and that electrons
could not exist between the orbits.
• Bohr’s model worked well for hydrogen, but it
did not work well for multi-electron atoms.
Quantum Mechanical Model 1920’s
• This is the most recent model of the atom.
• This model supports Bohr for the most part,
but suggests that electrons do not exist in
fixed orbits or a fixed definite path. Instead,
they electrons exist anywhere within an
electron cloud.
• Determining where an electron will be at any
given moment is very difficult and can only be
theorized using mathematical equations.
The Atom
• The model of the atom has evolved from
Dalton’s “billiard ball” model to a highly
complicated quantum mechanical model. It
was first believe that an atom could not be
broken down into smaller parts. We now
know that it can, mostly by nuclear reactions.
The three main sub-atomic particles are the
proton, electron, and neutron.
Charge and Mass of Subatomic
Particles
Particle
Charge
Mass (kg)
Mass (u)
Proton (p)
+1
1.672 x 10-27
1.00728
Neutron (n)
0
1.675 x 10-27
1.00783
Electron
-1
9.110 x 10-31
0.000055
• The proton and neutron are about equal in
mass. These two types of particles are found
together in the nucleus (center) of the atom.
This explains Rutherford’s gold foil
experiment.
• The electron is much smaller then the other
two particles. It is found in orbits or shells
surrounding the nucleus and does not
contribute significantly to the mass of the
atom.
Atomic Number
• Atomic number refers to the number of
protons in the nucleus of the atom. It is
represented by the symbol Z.
• Atomic number determines the identity of the
element. Every atom of a given element has
the same unique number of protons.
• In a neutral atom, atomic number is also equal
to the number of electrons surrounding the
nucleus.
Atomic Mass
• An atom’s mass is expressed in Atomic Mass
Units (u or a.m.u.)
• It was impossible for scientists to determine
the mass of individual atoms (they are too
small) so they assigned relative atomic masses
that agreed with the known composition of
compounds.
Atomic Mass continued
• A new unit was developed to mass atoms.
Carbon-12 (C-12) was chosen as the reference
standard. An atom of C-12 was arbitrarily
assigned a mass of 12 atomic mass units.
• The masses of all other atoms are compared
with the mass of this type of carbon atom.
According to this definition, an atomic mass
unit is defined as 1/12 the mass of a carbon12 atom.
Mass Number
• Mass number is the total number of protons
and neutrons in an atom.
• The symbol for mass number is A.
• For now, we will round the atomic mass to the
nearest whole number to get the mass
number of an element.
Elements and the Periodic Table
• All elements can be represented as symbols
that are organized in the periodic table.
• In addition to the symbols, the periodic table
provides us with the atomic number and the
mass number of an element.
Summary
• Atomic Number (Z) = # of protons
• Mass Number (A) = # of protons + # of neutrons
• Neutral Atom: # of protons = # of electrons