Transcript viewgraph set # 4 the atom and the nucleus

VIEWGRAPH SET # 4
THE ATOM AND THE
NUCLEUS
Atomic Mass Unit
= 1.66x10-27kg
Mass of one Carbon
Atom
=930 MeV/c2
ISOTOPES -> same Z different A
BOHR THEORY
Rules:
1. The atom has a number of stepped orbits that electrons
that rotate at constant tangential velocity do not radiate
(Stationary states or allowed orbits)
2. Electrons can jump from one allowed orbit to another
with the emission or absorption of a photon with energy
equal to their energy difference
All matter is an
assembly of Atoms
Stationary orbits
of Hydrogen
Emission of photon.
Electron moves from
higher to lower
allowed orbit
Absorption of
photon.Electron
moves from lower to
higher allowed orbit
Mathematics of Bohr atom
• Allowed or stationary Orbits for Hydrogen
rn= n2ao
ao= .5 x 10-9 meters
En= - Eo/n2
Eo= 13.6 eV
n=1,2,3,… integer
• Allowed or stationary orbits for element with atomic
number Z
rn= n2ao/Z
ao= .5 x 10-9 meters
En= - Z2(Eo/n2 )
Eo= 13.6 eV
n=1,2,3,… integer
• Explain basic spectra of all atoms
Rare Earths
Actinide
SHELL STRUCTURE OF ATOM
• Periodicity in physical and chemical properties
of atoms (color, hardness, state –gas,liquid,
density, heat and electric conduction, ..)
• Magic numbers 2,8,8,18,18,32
• Electrons are Fermions – spin 1/2
• Pauli’s exclusion principle – Two Fermions
cannot occupy the same state (cannot have the
same address)
• Chemical properties of atoms controlled by the
number of electrons in outer shell (valence
electrons)
6
n
6
10
14
2
5
6
10
2
4
6
10
2
3
6
2
En
2
6
2
1
2
Left is number n
of energy state
Right is the maximum
Number of electrons
allowed on each level.
They fill sequentially
as we move from low Z,
e.g Z=1 Hydrogen to
higher Z. For example,
Ca with Z=20 has filled
The n=1,2,3 levels with18
electrons and has
2 electrons on the first
Level of n=4. It has similar
Properties with the elements
that have 2 electrons on the
First level of n=2 (Be), n=3
(Mg), n=5 (Sr), and n=6 (Ba)
See periodic table group 2.
Radioactivity alpha decay
Ra(226,88)->Rn(222,86)+He(4,2)
U(238,92)->Th(234,90)+He(4,2)
Beta decay
C(14,6)->N(14,7)+e-+n
Binding Energy:
A nucleus is dismantled by removing a nucleon at a time and
the amount of work done in the process is measured.
Next if we next reassemble the nucleons in the form of the original
nucleus, an amount of energy equal to the work done would be
released.
This is the called the binding energy of the nucleus. It indicates how
tightly bound is.
Key quantity is the binding energy per nucleon. It is the binding
energy divided by the number of nucleons.
Curve of the binding energy
DOUBLING TIMES
Doubling time
1
2
4
10
25
50
80
Critical Mass
Growth factor
21=2
22=4
24=16
210=1024
3.3x107
1.1x1015
1.2x1024
Energy per U235 fission 235 MeV