Nuclear Physics

Chapter 25

3Li

lithium6.941

1

namesymbolatomic number (# of p+)

average atomic masselectrons in outer energy level

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

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

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

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)

Li73

mass number

atomic number(charge)

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

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).

10-15 m rnucleus= (1.2x10-15 m )A1/3 where A is the atomic number

Stable “big” atoms have more neutrons than protons, because the electric force acts over greater distances.

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

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

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

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

enp 01

10

11

proton neutron positron+

neutrino

A thallium 208 nucleus emits a - particle (an electron). Write the equation.

Tl20881 e01

20882 Pb

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