nuclear physics chapter 25. 3 li lithium 6.941 1 name symbol atomic number (# of p + ) average...
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Nuclear Physics
Chapter 25
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3Li
lithium6.941
1
namesymbolatomic number (# of p+)
average atomic masselectrons in outer energy level
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Protons
positive charge
made of two up quarks and a down quark
Electric charge +2/3 e Electric charge -1/3 e
in the nucleus…
Neutrons
made of two down quarks and an up quark
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Li 7
= proton
= neutron
number of nucleons(mass number)
Li 6an isotope of Li
mass number talks about 1 atomatomic mass is an average of all atoms
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Mass is usually expressed in amu or u
1 amu = 1/12th the mass of a carbon 12 atom
1 amu = 1.66 x 10-27 kg = mass of p+ or n0
an n0 is actually about 5 x 10-4 u more massivean e- has a mass of about 5 x 10-4 u…hmmm
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Mass can also be expressed in MeV/c2
rest energy (E0) = MeV
20
c
Em2
0 mcE
1 proton = 938.3 MeV/c2
Mega (1 x 106) electron volt (1.60 x 10-19 J)
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Li73
mass number
atomic number(charge)
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The electric force between protons is repulsive
)(2
21
r
qqkF celectric
In a nucleus, there must be a force stronger than the electric force holding the nucleus together
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The Strong Force is the force that holds nucleons together.
It is independent of charge, it binds neutrons and protons.
It only works over very small distances (nuclear).
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10-15 m rnucleus= (1.2x10-15 m )A1/3 where A is the atomic number
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Stable “big” atoms have more neutrons than protons, because the electric force acts over greater distances.
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Nucleons bound together in a nucleus have less mass (energy) than unbound nucleons
Ebind = mc2
binding energy is that energy that holds the nucleus together
mass defectthe difference in mass
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Find the total binding energy of Al 27 if it has a mass of 26.981534 u. H 1 has a mass of 1.007825 u and the mass of a n0 is 1.008665 u. There are 931.50 MeV/u.
Al is atomic # 13, so it has 13(1.007825 u) of p+ and e-
Al has 14 n0 = 14(1.008665 u)
m = (13.101725 + 14.12131) u – 26.981534 u = .241501 ux 931.50 MeV/u = 225 MeV
= 13.101725 u.
= 14.12131 u
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Nuclear Decay
Alpha Decay: an particle (He nucleus) escapes the strong force by quantum tunneling. The nucleus loses 2 p+ and 2 n0.
Pb21282 He4
220880Hg
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Beta decay: the weak force causes an up quark to change into a down quark, or vice versa. This causes a proton to change into a neutron or vice versa.
epn 01
11
10
neutron proton electron-
antineutrino
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enp 01
10
11
proton neutron positron+
neutrino
A thallium 208 nucleus emits a - particle (an electron). Write the equation.
Tl20881 e01
20882 Pb
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Gamma Decay: a nucleon can be in an excited state, needing to get rid of energy to reach a stable energy level. The nucleon emits a photon, but the structure of the nucleus stays the same.
CC 146
146