Atoms: The Building
Blocks of Matter
• The particle theory of matter was
supported as early as 400 BC by the
Greeks (Democritus)
– He called these particles atoms (Greek
for indivisible)
• Aristotle followed Democritus and
felt that matter was continuous
Foundations of Atomic
Theory
• Several theories proposed in the late
1700’s and early 1800’s
• Law of conservation of mass - mass
is neither created nor destroyed
during ordinary chemical reactions
• Law of definite proportions – a
chemical compound contains the same
elements in exactly the same
proportions by mass regardless of
the size of the sample or source of
the compound
• Law of multiple proportions – If two
or more compounds are composed of
the same two elements, then the
ratio of the masses of the elements
is a ratio of small whole numbers
Dalton’s Atomic Theory
• All matter is composed of extremely
small particles called atoms
• Atoms of a given element are
identical in size, mass, and other
properties; atoms of different
elements differ in size, mass, and
other properties
• Atoms cannot be subdivided, created,
or destroyed
• Atoms of different elements combine
in simple whole-number ratios to
form chemical compounds
• In chemical reactions, atoms are
combined, separated, or rearranged
Modern Atomic Theory
• Today we know that atoms
themselves are divisible
• We also know that individual atoms
can have different masses (isotopes)
• In general however, Dalton’s original
atomic theory still holds
The Structure of the
Atom
• The atom is defined as the smallest
particle of an element that retains
the chemical properties of that
element
• Atoms consist of the nucleus (core –
protons and neutrons) and the
electrons traveling around the
nucleus
The Structure of the
Atom
• The first subatomic particle discovered
was the electron (mid 1800’s)
• The discovery involved the use of a
cathode ray tube
Gases at
atmospheric pressure
don’t conduct
electricity well
• The current passed from the cathode to
the anode
– The rays created shadows and could turn a
paddle wheel
– They deflected as though they were negative
– Called these cathode rays
• http://www.youtube.com/watch?v=X
U8nMKkzbT8
• JJ Thomson was able to use these
cathode ray tubes to determine the
charge to mass ratio of the particles
(electrons) in 1897
• Robert Millikan performed an oil drop
experiment that allowed him to
calculate the charge on a single
electron
• http://www.youtube.com/watch?v=X
MfYHag7Liw
• With this information, he was able to
calculate the charge and mass of an
electron
• Since atoms are neutral and the
presence of electrons was confirmed,
atoms were assumed to have some
sort of positive charge
• Thomson proposed
the plum pudding
model
• Ernest Rutherford
performed the
gold foil
experiment and
disproved the plum
pudding model
--->
--->
• http://www.youtube.com/watch?v=5p
Zj0u_XMbc
• Atoms are very small (40-270 pm)
– Nuclei are 0.001 pm
– Nuclei are very dense (2 x 108 metrictons/cm3)
Counting Atoms
• The atomic number (Z) of an element
is the number of protons in the
nucleus of each atom of that element
– The identity of the element is based on
this number
• Isotopes are atoms of the same element
(same number of protons) with different
numbers of neutrons
• The mass number is the number of protons
and neutrons in the nucleus
• Nuclide is the general term for any isotope
of any element
• Nuclides can be referenced with a hyphen
notation or a nuclear symbol
• One atomic mass unit is defined as 1/12th
of the mass of the carbon-12 atom
– The mass is therefore approximately the
mass of a proton or a neutron
• The mass of an electron is 0.0005486
amu, a proton is 1.007276 amu, and a
neutron is 1.008665 amu’s.
• Although isotopes have different masses,
they do not differ significantly in their
chemical behavior
• Average atomic mass is the weighted
average of the atomic masses of the
naturally occurring isotopes of an element
Ions
Formed when an atom gains or loses an electron
a. Charge = # of protons - # of electrons
Ex) Mg +2 = lost 2 electrons
# of protons: 12 # of electrons: 10 Charge: +2
Ex) N-3 = gained 3 electrons
# of protons: 7 # of electrons: 10 Charge: -3
Molar Mass and
Avogadro’s number can
be used for conversions
Nuclear Forces
• The number of
protons is the
atomic number (Z)
• The number of
protons and neutrons
is the mass number
(A)
General Symbol
• The protons and neutrons of a
nucleus are called the nucleons
• A nuclide is the general term applied
to a specific nucleus with a given
number of protons and neutrons
– Can be shown with a symbol (previous
slide)
– Can also be shown with name (radium –
228)
• Isotopes are nuclides with the same
number of protons, but different numbers
of neutrons
Radioactive Decay
• Radioactivity is the spontaneous
change of an unstable nucleus to
form a more stable one
• The release of particles and energy
from this process is called
radioactivity
• If a nucleus has too many neutrons, it can
decay by turning a neutron into a proton and
emitting a beta particle
– Occurs if the N/Z number is too large
• Electron capture is the reverse of this
process
• Some nuclei that have too many protons can
become more stable by emitting positrons
(the antiparticle of an electron)
• These will collide with an electron and
annihilate each other to release energy
• Very large atoms will emit alpha
radiation
• An alpha particle consists of two
protons and two neutrons
• Many decay processes leave the
nucleus in an unstable state in which
it releases gamma radiation
• Nuclear equations must be balanced
– The superscripts and subscripts must be equal on
both side of the arrow
Nuclear Fission
• Nuclear fission occurs when a very heavy
nucleus splits into two smaller nuclei
• When Uranium-235 is bombarded with a
neutron, it undergoes fission
• The three released neutrons can
strike other nuclei and cause them to
decay
• If there is a critical mass, the
reaction will continue
• This continuation is called a chain
reaction
• The fission of 1 gram of uranium
generates as much energy as the
combustion of 2700 kg of coal
• This process is used in a nuclear reactor
• The radioactive material decays to
release heat, which creates steam, that
runs a steam turbine to produce energy
• The moderator slows the neutrons
down, so that they can be captured
• Control rods absorb neutrons to stop
the chain reaction
• There is a subcritical mass of U-235
in nuclear reactors
– Cannot explode
– Can overheat and “meltdown”-many
safety mechanisms to prevent this
• In Chernobyl (1986) technicians
briefly removed the control rods
during a safety test
• Nuclear fusion occurs when small nuclei
combine to form a larger atom
• Stars perform fusion is their cores
• Scientists are attempting to create a
fusion reaction for energy generation
– So far, we are only at a break even point
with energy generation/consumption
• Currently done in a strong magnetic
field
• Currently 100 nuclear reactors
generate 20% of the energy needs in
the United States
• Nuclear waste disposal is an issue
Half Life
• The rate of radioactive decay is measured in
half lives
• Half life is the time it takes for half a
radioactive sample to decay
• Can be used to date materials
– Carbon-12/Carbon-14
• The more unstable the nucleus, the
shorter it’s half life
• A body was found in the Alps in 1991
– Was dated to 3500-3000 BC using C-14
dating
• C-14 dating is only good for living
things in the last 50,000 years
• K-40 can be used for geologic dating
– Half life of 1.28 billion years