Chemistry_1A_Lecture_04

Page 1

• All images in this file reproduced from:

  • Blackman, Bottle, Schmid, Mocerino, and Wille, Chemistry, 2012

  • John Wiley & Sons

  • ISBN: 9780470810866

Page 2

• Nuclear Chemistry Lecture Topics:

  1. Lecture 3: Radioactivity and nuclear decay; Half-life and carbon dating

  2. Lecture 4: Nuclear stability; Predicting radioactive modes of decay

  3. Lecture 5: Consequences and uses of radioactivity

Page 3

• Radioactivity: Divided into two types of atomic nuclei:

  1. Stable

  2. Radioactive

• Stability Factors:

  • Competition between forces:

    • Electrostatic (Coulomb) repulsion: Protons push apart over long range.

    • Strong nuclear force: A short-range attraction between nucleons (protons and neutrons).

• All known stable nuclides fall inside the Zone of Stability.

• Unstable nuclei undergo radioactive decay.

Page 4

• Influence of Forces on Decay:

  1. Too few neutrons → Electrostatic repulsion dominates strong attraction.

  2. Larger nuclei → Long-range repulsion accumulates, overwhelming attraction.

  • Nuclides with A > 208 (e.g., Uranium) are unstable.

  3. Too many neutrons → Unstable nuclei.

  • Explanation via Nuclear Shell Model (discussed subsequently).

Page 5

• Energy Levels in the Nucleus:

  • Similar energy levels and shells exist for protons/neutrons like electrons in atoms.

  • Example:

    • Energy level diagram of 12C (6 protons, 6 neutrons) demonstrates stability due to 2 complete shells (analogy: noble gas configuration).

    • More neutrons result in lower stability due to incomplete shells.

Page 6

• Radioactive Decay Mechanisms:

  • Nuclei undergo spontaneous decay; mass number and charge are conserved.

  1. α Decay:

     • Loss of a helium nucleus (4, 2+) represented as 2+.

  2. β Decay:

     • Ejects an electron (β−); transforms a neutron into a proton.

  3. Positron (β+) Decay:

     • Ejects a positron; transforms a proton into a neutron.

     • Positrons often collide with electrons post-emission.

Page 7

• Continued Radioactive Decay Mechanisms:4. Electron Capture:

  • An electron merges with a proton forming a neutron, releasing x-rays from lower energy states.

  5. Neutron Emission:

     • Direct emission of a neutron, changing mass number (M) but not charge (Z).

  6. Gamma (γ) Emission:

     • High-frequency radiation accompanying other decay types; does not alter M or Z.

Page 8

• What Happens in the Nucleus During Decay:

  • α Decay: Loss of (2 protons, 2 neutrons) N↓, Z↓

  • β Decay: (N↓, Z↑) – neutron to proton conversion

  • Positron (β+) Emission: (N↑, Z↓) – proton to neutron conversion

  • Electron Capture: (N↑, Z↓) – a proton captures an electron

Page 9

• Radioactive Decay Sequences:

  • Represents decay from 238U as a graph of atomic number vs. neutron number:

    • α Decay: Decrease of two protons (Z) and neutrons (N).

    • β Decay: Decrease by one neutron, increase by one proton.

  • Isotopes maintain the same atomic number (Z) but differ in neutron number (N).

Page 10

• Nuclear Stability Factors:

  • Observing stable nuclides reveals two main determinants...

    1. Size of the nucleus

    2. Composition of the nucleus (neutrons:protons or the N:Z ratio)

Page 11

• Size of Nucleus:

  • No stable nuclei heavier than 208Pb observed.

Page 12

• N:Z Ratio:

  • 208Pb has N/Z ratio = 1.54.

  • All stable nuclides fall within the Zone of Stability:

    • Zone has N/Z ratio close to 1 but slopes towards more neutrons as nuclei get larger.

  • Rule for Nuclear Stability: Unstable isotopes decay towards the zone of stability, specifically below 209Bi.

Page 13

• N:Z Ratio Example:

  • β− decay, neutron emission – too high N/Z ratio

  • α decay – too high mass

  • β+ decay or electron capture – too low N/Z

  • Plan: Adjust N and/or Z based on stability zone criteria.

Page 14

• Learning Outcomes:

  • After this lecture, students should be able to:

    • Explain factors affecting nuclear stability.

    • Balance nuclear equations.

    • Predict modes of radioactive decay for unstable nuclei.

    • Further Reading: Blackman 27.1