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Atomic and Nuclear Physics
Atomic and Nuclear Physics
• Fundamental Particles
– Democritus {Greece 460 B.C.} is credited with the
idea that matter is made up of fundamental
building blocks. He introduced the word atom to
describe these building blocks.
– In the 1930’s, the atom was believed to be
constructed of neutrons, protons, and electrons.
These were thought to have no smaller fractional
parts and were termed elementary particles.
Fundamental Particles
Atomic and Nuclear Physics
• Standard Model
– Recognizes 12 fundamental matter particles and 4
force carrier particles (photon, gluon, W & Z boson) .
• Fundamental Particles
• Quarks
Leptons
U
up
C
charm
T
top
νe
electron
neutrino
νu
muon
neutrino
νt
tau
neutrino
D
down
S
strange
B
bottom
e
electron
μ
muon
τ
tau
Fundamental Particles
Leptons cont…
3. Tau (τ) {+/-} Extremely massive charged particle
1. ~ 17 x size of µ
2. Unstable , decays a number of ways to produce
3. For example
1. τ => e + ν
2. τ => µ + ν
4. Neutrinos (νe, νμ, ντ)- One type for each lepton
• No charge
• No mass
• Never (recently ?) detected.
• Over 100 other “elementary particles” were
discovered since 1930. These particles are all
made from quarks and/or leptons.
• Today quarks, leptons, and their associated
anti-particles are candidates for being the
fundamental building blocks, from which all
thing are made.
• The current theory of matter is known as the
STANDARD MODEL.
• Both leptons and quarks are small < 10-19
meters in radius.
• Leptons- 6 particles and their anti-particles
1. Electron/Positron {e-/e+} least massive of all
charged particles. Absolutely stable, do not
decay.
1. Annihilation: electron + positron = > ϒ
2. Pair production: ϒ => electron + positron
2. Muon (µ) {+/-} Massive charged particle
1. ~200 times mass of electron
2. Unstable decays rapidly to W-boson + neutrino
Fundamental Particles
• Hadrons- particles made from quarks and
bound together by strong interactions.
• Particles include
– Protons, anti-protons, neutrons, mesons, plus
pions, k-mesons, lambda, omega and others..
– Protons only stable particle
p = two up + one down quark
– Neutrons not stable outside of nucleus.
n = two down + one up quark
– Mesons No stable mesons. Composed one quark +
one anti-quark.
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Fundamental Forces
Fundamental Forces
1. Electromagnetic interactions responsible for
electric and magnetic forces.
2. Strong interactions responsible for forces
between quarks & gluons and nuclear binding.
3. Weak interactions responsible for the instability
of all but the least massive fundamental
particles.
4. Gravitational interactions responsible for forces
between any two objects due to their masses.
PARTICLES OF INTEREST
Particle
Rest Mass {kg}
charge
Electron
9.109 x 10-31
- e = 1.609 x 10-19 coulombs
Positron
9.109 x 10-31
+e
Proton
1.67265 x 10-27
+e
Neutron *
1.67495 x 10-27
none
Photon **
none
none
Neutrino***
none
none
* Neutron is not stable unless bound to an atomic nucleus.
** Particles associated with electromagnetic waves that travels at the speed of light.
***Appears during decay of certain nuclei.
Atomic and Nuclear Structure
• Fifth force?
In the standard model at least one
additional force is needed to explain how all
fundamental particle masses are generated. The
current and yet unproven theory postulates one
force {Higgs Force} and one additional particle
type {Higgs Particle}.
Atomic and Nuclear Structure
A. Atomic Number (Z) – number of protons in nucleus
B. Neutron Number (N) – number of neutrons in
nucleus
C. Atomic Mass number (A) – number of nucleons
A = Z +N
D. Nuclides – Various species of atoms where nuclei
contain particular number of protons and neutrons.
Symbolized:
()
Atomic and Nuclear Structure
E. Isotopes - Atoms containing the same number of
protons but a different number of neutrons
is an isotope of O with 9 neutrons.
is an isotope of O with 10 neutrons.
, , , , unstable
{Z=8, N = 5, 6, 7, 11, 12} are
F. Isotopes are found in a different abundances
in nature.
99.76% of natural oxygen atoms are
isotope 16O
0.037% are isotope 17O
0.204% are isotope 18O
where 99.76% implies Atom Percent (a/o)
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Atomic and Nuclear Structure
G. Atom percent (a/o) - the percentage of atoms of
a given element that are a particular isotope.
Ex… How many atoms of deuterium ( 2H) are in 6.6
x 1024 atoms of H ?
From text Table II.2 -> 2H = 0.015 a/o
2H atoms = 6.6 x 1024 atoms x 1.5 x 10-4
= 9.9 x 1020
Atomic and Molecular Weight
C. Atomic Weight (relative atomic mass) (M) –
mass of a neutral atom relative to the mass of
neutral 12C atom where the atomic weight of 12C
is taken to be 12.
It is the average of the atomic masses of all the
atoms of a chemical element found in a particular
sample, weighted by isotopic abundance
Atomic weights (relative atomic mass) are ratios
and therefore dimensionless physical quantities and
UNITLESS numbers.
Atomic and Molecular Weight
Atomic and Molecular Weight
A. Atomic Mass The mass of a specific isotope of
an element. It is the total mass of protons,
neutrons, and electrons in a single atom. It is
sometimes expressed in atomic mass units
(amu).
B. Unified atomic mass unit (u) In 1961 the amu
reference unit was changes from O16 to C12 and
the symbol u replaced the amu.
1u = m(12C)/12
Atomic and Molecular Weight
Example:
Isotope
=
Atom percent a/o
M atomic weight
O -16
99.759
15.99492
0-17
0.037
16.99913
0-18
0.204
17.99916
1
{ 99.759 15.99492 + 0.037 16.99913
100
+ 0.204 17.99916 }
= 15.99938
Atomic and Molecular Weight
D. The scientific unit for dealing with atoms is the mole. It
is the number of atoms in 12 grams of C-12.
Ex… Compute the mass of one atom of C-12
NA is Avagradro’s number, which are the number of atoms
found in 12 grams of C-12.
NA atoms grouped together are called a mole.
m(C-12) = 1.234 56.-/-./0 = 1.992688109g
+/-./0
Ex… Compute the number of grams in 1u.
The atomic weight (relative atomic mass) is not the mass
of one atom. It is the mass of NA = 6.02 x 1023 atoms.
When we say “ The atomic weight of Pb = 207.2.”
We mean that 1 mole or 6.02 x 1023 atoms of Pb weights
207.2 grams.
1u = (12: )= ; = ; = 1.66x109g
<
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Atomic and Molecular Weight
E. Molecular Weight – mass of a molecule
relative to the mass of neutral 12C atom
Ex.. Find molecular weight of O2 gas.
2 x M(O) = 2x15.99938 = 31.99876
Atom Density (N)
• Atom density is number of atoms in 1 cm3
ρ>@
>=
• ρ – density of material in grams/cm3
• M – gram atomic weight in grams/mole
• NA – Avagodros number atoms/mole
Ex… Sodium has a density of 0.97 g/ cm3, what is
its atom density?
>=
Weight Percent (w/o)
• One method of describing the composition of
a mixture. The i component of a mixture has
w/o
w/o =
CD
CEFEGH
x100%
-LM
x100
M NKLO
w/o(X) = -L
where M is the atomic weight.
Computing Atomic weight of mixture
CD R;<
and >
LD
R;<
CD R;<
N = L =∑ L
D
CD
=∑
L
LD
• Then
• So
Ex… For water of unit density compute:
(a) the number of P molecules per (b) atom density of H and O
(c) atom density of P
Solution: M(H2O) = 2 x M(H) + 1 x M(O) = 2 x 1 + 16 = 18
a) N(H2O) =
• In a chemical compound A- JK element Ahas
• Since >Q =
0.97 6.022A10
= 2.54810 B/
22.990
= ∑ >Q
21.234
= 3.3 x 10 molecules/cm3
b) N(H) = 2 x N(H20) = 6.6x 10 atoms/cm3 and
N(O) = N(H2O) = 3.3 x 10 atoms/cm3 since 2 atoms
of H and one atom of O per molecule.
c) The relative abundance of P is 0.015% thus
N( P) = 1.5 x 10-4 x 6.6x 10 = 1 x 10atoms/cm3
Ex.. What is the weight percent w/o of
uranium in the fuel?
Solution:
M(UO2) = 237.1 + 2 x 16 = 269.1
.
Ex.. Pellets of uranium oxide (UO2) contains 30
w/o U235. Find the atomic weight of the
uranium.
L
C
w/o(U) = 1. x100% = 88.1%
= ∑ LD = ( + ) = .
D
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