Chemistry: Atoms to AP Chemistry · Module 9 of 15
Nuclear Chemistry & Electrochemistry
How unstable nuclei decay, how to predict what's left over time, and how redox reactions generate electricity.
Radioactivity & Types of Decay
- Unstable nuclei release energy or particles to become more stable. Alpha decay emits a helium nucleus (⁴₂He: 2 protons + 2 neutrons), decreasing mass number by 4 and atomic number by 2.
- Beta decay emits an electron as a neutron converts to a proton, increasing atomic number by 1 with no change in mass number; gamma decay emits high-energy photons, changing neither mass number nor atomic number.
Uranium-238 (atomic number 92) undergoes alpha decay. What nuclide results?
Solution. Alpha decay removes 2 protons and 2 neutrons: mass number 238 − 4 = 234, atomic number 92 − 2 = 90. Atomic number 90 is thorium, so the result is thorium-234.
Half-Life
- Half-life is the time it takes for half of a radioactive sample to decay; it stays constant no matter how much of the sample is present.
- After n half-lives, the amount remaining is the original amount times (1/2)ⁿ.
An 80 g sample has a half-life of 3 days. How much remains after 9 days?
Solution. 9 days ÷ 3 days per half-life = 3 half-lives. Remaining = 80 × (1/2)³ = 80 ÷ 8 = 10 g.
Balancing Nuclear Equations
- In a nuclear equation, the mass numbers (top) must balance on both sides, and the atomic numbers (bottom) must balance on both sides.
- Fission splits a heavy nucleus into smaller nuclei, releasing energy (used in nuclear power plants); fusion combines light nuclei into a heavier one, releasing even more energy (powers the sun).
Balance this fission reaction: ²³⁵₉₂U + ¹₀n → ¹⁴¹₅₆Ba + 3¹₀n + ?
Solution. Mass numbers: 235 + 1 = 236 on the left; 141 + 3(1) = 144 on the right, leaving 236 − 144 = 92 for the missing nuclide. Atomic numbers: 92 + 0 = 92 on the left; 56 + 3(0) = 56 on the right, leaving 92 − 56 = 36. Mass 92, atomic number 36 is krypton-92.
Redox Reactions & Electrochemistry Basics
- Oxidation is the loss of electrons (oxidation number increases); reduction is the gain of electrons (oxidation number decreases) — "OIL RIG": Oxidation Is Losing, Reduction Is Gaining.
- In a galvanic (voltaic) cell, a spontaneous redox reaction generates electricity: electrons flow from the anode, where oxidation occurs, through an external wire to the cathode, where reduction occurs.
In Zn + Cu²⁺ → Zn²⁺ + Cu, which species is oxidized and which is reduced?
Solution. Zn goes from a charge of 0 to +2, losing 2 electrons — it is oxidized. Cu²⁺ goes from +2 to 0, gaining 2 electrons — it is reduced.
Check yourself
2 of this module's 5 quiz questions. Pick your answers, then check them in the app, with an explanation for every question.
1. What type of decay decreases an atom's mass number by 4 and atomic number by 2?
2. A 100 g sample has a half-life of 5 years. How much remains after 20 years?