unit 8 - chpt 19 - nuclear chemistry radioactive decay nuclear transformations stability of nucleus...

Unit 8 - Chpt 19 - Nuclear Chemistry

• Radioactive Decay• Nuclear Transformations• Stability of nucleus• Uses of radiation• Fission and Fusion• Effects of radiation

• HW set1: Chpt 19 - pg. 901-905 # 12, 14, 16, 20, 22, 26, 29, 34, 40, 42 - Due Tues. Apr 13

Radioactivity review

• Atomic Number (Z) – number of protons

• Mass Number (A) – sum of protons and neutrons

XAZ

Radioactive Decay - Nucleus undergoes decomposition to form a different nucleus.

Nuclear Stability - review• Nuclides with 84 or more protons are

unstable.• Light nuclides are stable when Z equals A – Z (neutron/proton ratio is 1).

• For heavier elements the neutron/proton ratio required for stability is greater than 1 and increases with Z.

• Certain combinations of protons and neutrons seem to confer special stability. Even numbers of protons and neutrons are more

often stable than those with odd numbers.

Nuclear Stability - review

• Certain specific numbers of protons or neutrons produce especially stable nuclides.

• 2, 8, 20, 28, 50, 82, and 126

Decay Processes

Decay Series (Series of Alpha and Beta Decays)

Concept checkWhich decay process are the following?

electron capture

positron

alpha particle

beta particle

68 0 6831 1 30a) Ga + e Zn− →

62 0 6229 1 28b) Cu e + Ni+→

212 4 20887 2 85c) Fr He + At→

129 0 12951 1 52d) Sb e + Te−→

If the atomic number is not provided, you need to look it up in the periodic table i.e. 129Sb find Sb put in 51

Rate of decay

Rate = kN• The rate of decay is proportional to

the number of nuclides. This represents a first-order process.

• Half-life - Time required for the number of nuclides to reach half the original value.

( )1/ 2 =

ln 2 0.693 = t

k k

Rate problem

We don’t have to have an exact number of half-lives anymore, now that we know the rate is 1st order.

A first order reaction is 35% complete at the end of 55 minutes. What is the value of k?HINT: go back to kinetics for 1st order

Nuclear Transformation

• The change of one element into another.

• Math is same as radioactive decay can solve for what may be missing

27 4 30 113 2 15 0Al + He P + n→

249 18 263 198 8 106 0Cf + O Sg + 4 n→

How to make collisions - new elements

Schematic diagram of a cyclotron

Carbon-14 dating

• Used to date wood and cloth artifacts.

• Based on carbon–14 to carbon–12 ratio.

• Half-life 5730years

Radiotracer Applications• Radioactive nuclides that are

introduced into organisms in food or drugs and whose pathways can be traced by monitoring their radioactivity.

Energy Stability of Nucleus

• When a system gains or loses energy it also gains or loses a quantity of mass.

E = mc2

m = mass defectE = change in energy

• If E is negative (exothermic), mass is lost from the system.

Mass Defect

• Calculating the mass defect for : Since atomic masses include the masses of the

electrons, we must account for the electron mass.

• nucleus is “synthesized” from 2 protons and two neutrons.

4 42 24.0026 = mass of He atom = mass of He nucleus + 2 em

1 11 11.0078 = mass of H atom = mass of H nucleus + em

42He

( ) ( ) ( )[ ] 2 1.0078 + 2 1.0087 = 4.0026 2 e e− −Δ −m m m

= 0.0304 amu −m

42He

Missing mass in binding energy

Binding Energy• The energy required

to decompose the nucleus into its components.

• Iron-56 is the most stable nucleus and has a binding energy of 8.97 MeV per nucleon.

• 931.5 MeV per amu

Fission or Fusion

• Fusion – Combining two light nuclei to form a heavier, more stable nucleus.

• Fission – Splitting a heavy nucleus into two nuclei with smaller mass numbers. (A self-sustaining fission process is called a chain reaction.)

1 235 142 91 10 92 56 36 0n + U Ba + Kr + 3 n→

Fission or Fusion?