Transcript Nuclear Physics

Nuclear Physics
Chp 30
The Atom
 A nucleus of equal mass positive protons
and neutral neutrons, surrounded by
almost massless, negative electrons
 Atomic number = # protons
 Atomic mass = # protons + neutrons
 Most atoms are neutral, so electrons =
protons
 The nucleus, although containing the most
mass, takes up very little space in the atom
Isotopes
 Different versions of an
element
 Only the atomic mass and #
neutrons change (changing
protons would change the
element)
 Atomic mass as given on the
periodic table is an average of
all possible isotopes (this is
why it’s a decimal)
 Use AZX to show isotopes
 A = atomic mass
 Z = atomic number
 X = element symbol
Strong Nuclear Force
 The force that holds an atomic nucleus
together
 Must be very strong to hold like charges
together (they normally repel each other)
 Even stronger than electricity
 Only works over a very short range though
 Energy must be added to take a nucleus
apart (need to overcome that force)
E = mc2
 Mass is a form of energy
 That means if mass changes, energy is
released or absorbed
 For atoms smaller than iron, they have less
mass when they combine than when
separate (fusion)
 For atoms larger than iron, they have less
mass when they separate than when they
are held together (fission)
 Iron is stable and undergoes neither fission
or fusion
Radioactive Decay
 Alpha
 A particle of 2 protons and 2 neutrons are emitted
 Most massive, but easiest to stop
 Beta
 A neutron turns into a proton and an electron, the electron is emitted
and the proton stays
 Fairly easy to stop because its charged
 Gamma
 Massless energy is released
 Hardest to stop and most dangerous
 Substances often undergo the release of many of these
particles in stages until a stable isotope is reached
Half Life
 The time required for half the
atoms in a radioactive
sample to decay
 The time it takes is unique
and constant for each
isotope
 If an isotope has a short half
life, it decays more quickly,
and therefore is more
dangerous
 Used to “date” objects
 Carbon – 14 has a half life of
5730 yrs (good for living things)
 Uranium – 238 has a half life of
4.5 billion years (good for
planets)
Nuclear Reactors
 Currently all based on fission of uranium - 235
 Needs a neutron to start the process, then 3 new
neutrons are created
 Each of these can start a new reaction (chain reaction)
 Reactions are kept in check by mixing in U-238, which
doesn’t react and having control rods, which can drop
down and absorb neutrons to stop the reaction
Reactors, Cont’d
 The heat created by U-235
fusion, heats water which
creates steam to turn
turbines and generate
electrical energy
 Creates tons more energy
than coal burning, but does
leave radioactive byproducts
that must be properly
disposed of